Editing Colorado River (section)

==Engineering and development==
[[File:HooverDamFrontWater.jpg|thumb|right|upright=0.8|alt=Front view of a dam in a narrow canyon, with water shooting out of the gates|[[Hoover Dam]] releasing water in 1998]]
<!--[[Image:Arizona, Lake Powell 01.jpg|thumb|right|alt=Side view of a large concrete dam impounding a lake surrounded by red-rock hills|[[Glen Canyon Dam]] (right) forms [[Lake Powell]], the second-largest reservoir on the Colorado River with a capacity of more than {{convert|24.3|e6acre.ft|km3}}.]]
[[Image:Imperial valley fields.jpg|thumb|right|alt=Aerial shot of farm fields with a large body of water in the background
|The Colorado River is the sole source of water for the [[Imperial Valley]] in southern California, one of the most productive agricultural regions in the U.S.]]-->
{{See also|International Boundary and Water Commission|Dams in the Colorado River system|Colorado River Compact}}
About 40&nbsp;million people depend on the Colorado River's water for agricultural, industrial and domestic needs.<ref name="Water Supply and Demand Study">{{cite web |url=https://www.usbr.gov/watersmart/bsp/docs/finalreport/ColoradoRiver/CRBS_Executive_Summary_FINAL.pdf |title=Colorado River Basin Water Supply and Demand Study |publisher=U.S. Bureau of Reclamation |date=December 2012 |access-date=May 18, 2022 |archive-date=May 20, 2022 |archive-url=https://web.archive.org/web/20220520223319/https://www.usbr.gov/watersmart/bsp/docs/finalreport/ColoradoRiver/CRBS_Executive_Summary_FINAL.pdf |url-status=live }}</ref> The Colorado irrigates 5.5&nbsp;million acres (2.2&nbsp;million hectares) of farmland,<ref name="Water Supply and Demand Study"/> and its hydroelectric plants produce 12&nbsp;billion [[kilowatt hour]]s (KWh) of hydroelectricity each year.<ref name="Controversy"/> Hydroelectricity from the Colorado is a key supplier of [[peaking power]] on the Southwest electric grid.<ref>{{cite web |url=http://www.circleofblue.org/waternews/2010/world/low-water-may-still-hoover-dam%E2%80%99s-power/ |title=Low Water May Halt Hoover Dam's Power |publisher=Circle of Blue |date=September 22, 2010 |access-date=October 8, 2012 |archive-url=https://web.archive.org/web/20130120034145/http://www.circleofblue.org/waternews/2010/world/low-water-may-still-hoover-dam%E2%80%99s-power/ |archive-date=January 20, 2013 |url-status=dead}}</ref><ref>{{cite web |url=http://www.gcdamp.gov/keyresc/hydropower.html |title=Hydropower |publisher=Glen Canyon Dam Adaptive Management Program |date=November 18, 2010 |access-date=May 12, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120522080526/http://www.gcdamp.gov/keyresc/hydropower.html |archive-date=May 22, 2012}}</ref> Often called "America's [[Nile]]",<ref>{{cite news |author=Waterman, Jonathan |url=http://newswatch.nationalgeographic.com/2010/08/11/restoring_floods_to_grand_caynon_river/ |title=Restoring floods to America's Nile |work=National Geographic |date=August 11, 2010 |access-date=March 2, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120324005737/http://newswatch.nationalgeographic.com/2010/08/11/restoring_floods_to_grand_caynon_river/ |archive-date=March 24, 2012}}</ref> the Colorado River is so intensively managed that each drop of its water is used an average of 17 times in a single year.<ref>{{cite web |author=Nabhan, Gary Paul |url=http://www.environment.nau.edu/water/ColoradoRiver.htm |title=The Beginning and the End of the Colorado River: Protecting the Sources, Ensuring its Courses |publisher=Northern Arizona University |date=January 16, 2007 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20111225000708/http://www.environment.nau.edu/water/ColoradoRiver.htm |archive-date=December 25, 2011}}</ref>{{sfn|Reisner|1993|p=120}} [[Southern Nevada Water Authority]] has called the Colorado River one of the "most controlled, controversial and litigated rivers in the world".<ref>{{cite web |url=http://www.snwa.com/ws/river_law.html |title=Colorado River Law |publisher=Southern Nevada Water Authority |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120426071547/http://www.snwa.com/ws/river_law.html |archive-date=April 26, 2012}}</ref>

{| class="wikitable sortable" style="float:left; width:200px; margin-right: 1em"
!colspan=3|Colorado River water allocations, in millions of [[acre-foot|acre-feet]]<ref name="CRCtext"/><ref name="1944Treaty"/><ref>{{cite web |url=http://awesome.good.is.s3.amazonaws.com/transparency/web/1009/colorado-river-2/flat.html |title=Colorado River |publisher=GOOD Magazine |access-date=February 26, 2012 |archive-date=July 20, 2012 |archive-url=https://web.archive.org/web/20120720180059/http://awesome.good.is.s3.amazonaws.com/transparency/web/1009/colorado-river-2/flat.html |url-status=live }}</ref>
|- style="font-size:9pt align=center"
!User
!<small>Amount</small>
!<small>Share</small>
|- style="font-size:9pt; text-align:left;"
| style="background:beige;"|'''United States'''
| style="background:beige;"|'''15.0'''
| style="background:beige;"|'''90.9%'''
|- style="font-size:9pt; text-align:center;"
|California
|4.4
|26.7%
|- style="font-size:9pt; text-align:center;"
|Colorado
|3.88
|23.5%
|- style="font-size:9pt; text-align:center;"
|Arizona
|2.8
|17.0%
|- style="font-size:9pt; text-align:center;"
|Utah
|1.72
|10.4%
|- style="font-size:9pt; text-align:center;"
|Wyoming
|1.05
|6.4%
|- style="font-size:9pt; text-align:center;"
|New Mexico
|0.84
|5.1%
|- style="font-size:9pt; text-align:center;"
|Nevada
|0.3
|1.8%
|- style="font-size:9pt; text-align:left;"
| style="background:beige;"|'''Mexico'''
| style="background:beige;"|'''1.5'''
| style="background:beige;"|'''9.1%'''
|- class="sortbottom" style="font-size:9pt; text-align:left; background:#b0e0e6;"
||'''Total'''
||'''16.5'''
||'''100%'''
|}
In 1922, six U.S. states signed the [[Colorado River Compact]], which divided half of the river's flow to both the Upper Basin (the drainage area above Lee's Ferry, comprising parts of Colorado, New Mexico, Utah, and Wyoming and a small portion of Arizona) and the Lower Basin (Arizona, California, Nevada, and parts of New Mexico and Utah). The Upper and Lower Basin were each allocated {{convert|7.5|e6acre.ft|km3}} of water per year, a figure believed to represent half of the river's annual flow at Lee's Ferry.<ref name="CRCtext">{{cite web |url=http://www.usbr.gov/lc/region/pao/pdfiles/crcompct.pdf |title=Colorado River Compact, 1922 |publisher=U.S. Bureau of Reclamation |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120915093352/http://www.usbr.gov/lc/region/pao/pdfiles/crcompct.pdf |archive-date=September 15, 2012}}</ref> The allotments operated under the premise that approximately 17.5 million acre-feet of water flowed through the river annually.<ref>{{Cite web |date=2022-08-15 |title=Big changes are coming for the Colorado River soon—and they could get messy |url=https://www.nationalgeographic.com/environment/article/big-changes-are-coming-for-the-colorado-river-soonand-they-could-get-messy |archive-url=https://web.archive.org/web/20220815162514/https://www.nationalgeographic.com/environment/article/big-changes-are-coming-for-the-colorado-river-soonand-they-could-get-messy |url-status=dead |archive-date=August 15, 2022 |access-date=2022-08-16 |website=Environment |language=en}}</ref>

Arizona initially refused to ratify the compact because it feared that California would take too much of the lower basin allotment. In 1944 a compromise was reached in which Arizona was allocated {{convert|2.8|e6acre.ft|km3}}, but with the caveat that California's {{convert|4.4|e6acre.ft|km3|adj=on}} allocation was prioritized during drought years.<ref name="UtahP" /> These and nine other decisions, compacts, federal acts and agreements made between 1922 and 1973 constitute what is now known as the Law of the River.<ref name="UtahP">{{cite web |author=Anderson, Larry |url=http://www.water.utah.gov/interstate/thecoloradoriverart.pdf |title=Utah's Perspective: The Colorado River |publisher=Utah Division of Water Resources |date=May 2002 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20140409152252/http://www.water.utah.gov/interstate/thecoloradoriverart.pdf |archive-date=April 9, 2014}}</ref><ref name="LoR">{{cite web |url=http://www.usbr.gov/lc/region/g1000/lawofrvr.html |title=The Law of the River |publisher=U.S. Bureau of Reclamation |date=March 2008 |access-date=February 18, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120626204917/http://www.usbr.gov/lc/region/g1000/lawofrvr.html |archive-date=June 26, 2012}}</ref>

In 1944, [[International Boundary and Water Commission#The border and water treaties|a treaty between the U.S. and Mexico]] allocated {{convert|1.5|e6acre.ft|km3}} of Colorado River water to Mexico each year.<ref name="1944Treaty">{{cite web |url=http://www.ibwc.gov/Files/1944Treaty.pdf |title=Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande: Treaty Between the United States of America and Mexico |publisher=International Boundary and Water Commission |date=February 3, 1944 |access-date=February 20, 2012 |archive-date=September 15, 2012 |archive-url=https://web.archive.org/web/20120915121837/http://www.ibwc.gov/Files/1944Treaty.pdf |url-status=dead}}</ref> [[Morelos Dam]] was constructed in 1950 to enable Mexico to utilize its share of the river. Water allocated to Mexico from the Colorado River is regulated by the [[International Boundary and Water Commission]], which also apportions waters from the [[Rio Grande]] between the two countries.<ref>{{cite web |url=https://www.usbr.gov/lc/yuma/facilities/dams/yao_dams_map.html |title=Dams Along the Lower Colorado River |publisher=U.S. Bureau of Reclamation |date= |access-date=May 18, 2022 |archive-date=May 17, 2022 |archive-url=https://web.archive.org/web/20220517135610/https://www.usbr.gov/lc/yuma/facilities/dams/yao_dams_map.html |url-status=live }}</ref>

===Transmountain diversions===
[[Image:GRAND DITCH OVERVIEW, VIEWING SOUTH - Grand Ditch, Baker Creek to LaPoudre Pass Creek, Grand Lake, Grand County, CO HAER COLO,25-GRLK.V,2-1.tif|thumb|right|The Grand Ditch, one of the earliest water diversions of the Colorado River, is still in use today.]][[File:Little Oso Diversion Dam.jpg|thumb|The Little Oso Diversion Dam diverts water from the [[Little Navajo River]] and transports it through a series of tunnels into the [[Rio Grande]] basin as part of the [[San Juan–Chama Project]].]]

Large-scale development of Colorado River water supplies started in the late 19th century, at the river's headwaters in La Poudre Pass. The [[Grand Ditch]], directing runoff from the river's headwaters across the [[Continental Divide]] to arid eastern Colorado, was considered an engineering marvel when completed in 1890.<ref>{{cite web |url=http://www.roaringfork.org/pub/collaborative/GDII%20Appendix%20I%20-%20Transmountain%20Diversion%20Projects.pdf |title=Appendix I: The Past, Present and Future of Transmountain Diversion Projects |publisher=Roaring Fork Conservancy |work=Roaring Fork Watershed Plan |access-date=March 18, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20130120034135/http://www.roaringfork.org/pub/collaborative/GDII%20Appendix%20I%20-%20Transmountain%20Diversion%20Projects.pdf |archive-date=January 20, 2013}}</ref> This was the first of twenty-four "transmountain diversions" constructed to draw water across the Rocky Mountains as the [[Front Range urban corridor|Front Range corridor]] increased in population.<ref>{{cite web |url=http://www.crwcd.org/media/uploads/20110719-policies_TMDs.pdf |title=Transmountain Water Diversions |publisher=Colorado River District |date=July 2011 |access-date=March 18, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20111201032049/http://www.crwcd.org/media/uploads/20110719-policies_TMDs.pdf |archive-date=December 1, 2011}}</ref> These diversions draw water from the upper Colorado and its tributaries into the [[South Platte River]], [[Arkansas River]] and [[Rio Grande]] basins.<ref name="Water Education CO">{{cite book |publisher=Water Education Colorado |title=Citizen's Guide to Colorado's Transbasin Diversions |year=2014 |url=https://issuu.com/cfwe/docs/cfwe_cgtb_web |access-date=May 19, 2022 |archive-date=May 19, 2022 |archive-url=https://web.archive.org/web/20220519013450/https://issuu.com/cfwe/docs/cfwe_cgtb_web |url-status=live }}</ref> Today, about 80&nbsp;percent of Colorado's population lives on the eastern slope of the Rockies, while 80&nbsp;percent of precipitation falls on the western slope.<ref>{{cite web |url=http://www.roaringfork.org/your-watershed/watershed-facts/transmountain-diversions/ |title=Transmountain Diversions |publisher=Roaring Fork Conservancy |date= |access-date=May 18, 2022 |archive-date=May 18, 2022 |archive-url=https://web.archive.org/web/20220518060240/http://www.roaringfork.org/your-watershed/watershed-facts/transmountain-diversions/ |url-status=live }}</ref>

While first planned at the same time as the Grand Ditch, construction on the [[Colorado–Big Thompson Project]] (C-BT) did not begin until the 1930s. Today, the C-BT is the largest of the transmountain diversions, delivering {{convert|230000|acre feet|m3}} per year from the Colorado River to cities north of [[Denver, Colorado|Denver]].<ref>{{cite web |url=http://www.usbr.gov/projects/Project.jsp?proj_Name=Colorado-Big+Thompson+Project |title=Colorado-Big Thompson Project |publisher=U.S. Bureau of Reclamation |date=October 18, 2011 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20111112085341/http://www.usbr.gov/projects/Project.jsp?proj_Name=Colorado-Big%20Thompson%20Project |archive-date=November 12, 2011}}</ref> Numerous other projects followed, with the largest including the Roberts Tunnel, which delivers water from the [[Blue River (Colorado)|Blue River]] to the city of Denver,<ref>{{cite web |url=http://blueriverwatershed.org/water-quantity/moving-water-in-the-brw/ |title=Moving Water in the Watershed |publisher=Blue River Watershed Group |access-date=May 18, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120329184230/http://blueriverwatershed.org/water-quantity/moving-water-in-the-brw/ |archive-date=March 29, 2012}}</ref><ref>{{cite news |author=Wahlstrom, Ernest E. |title=History of Geologic Investigations, Engineering Design, and Construction Methods of the Harold D. Roberts Tunnel, Colorado |publisher=U.S. Geological Survey |work=Geological Survey Professional Paper 831–A |url=http://pubs.usgs.gov/pp/0831a/report.pdf |year=1974 |access-date=May 18, 2012 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071446/https://pubs.usgs.gov/pp/0831a/report.pdf |url-status=live }}</ref> and the [[Fryingpan–Arkansas Project]], which diverts water from the [[Fryingpan River]] to the [[Arkansas River]] basin.<ref>{{cite web |url=http://www.usbr.gov/projects/Project.jsp?proj_Name=Fryingpan-Arkansas%20Project |title=Fryingpan–Arkansas Project |publisher=U.S. Bureau of Reclamation |date=May 10, 2011 |access-date=May 18, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20140827213909/http://www.usbr.gov/projects/Project.jsp?proj_Name=Fryingpan-Arkansas%20Project |archive-date=August 27, 2014}}</ref>

Combined, the transmountain diversions draw about {{convert|580000|acre feet|m3}} of water per year out of the Colorado River basin.<ref name="Water Education CO"/> Historically, most of the water has been used for irrigation, although water usage is increasing for urban water supply and for recreational purposes such as [[snowmaking]] and increasing Eastern Slope streamflows for boating and fishing. [[Denver Water]] receives about 50&nbsp;percent of its supply from the Colorado River basin. However, diversions have caused environmental harm to the upper Colorado River system by reducing streamflows in many tributaries.<ref name="Water Education CO"/> A number of reservoirs have been built to offset the impact of transmountain diversions by storing water for dry season release on the Western Slope, including [[Williams Fork Reservoir]] in 1959<ref>{{cite web |url=https://www.denverwater.org/recreation/williams-fork-resevoir |title=Williams Fork Reservoir |publisher=Denver Water |date= |access-date=May 18, 2022 |archive-date=May 22, 2022 |archive-url=https://web.archive.org/web/20220522195133/https://www.denverwater.org/recreation/williams-fork-resevoir |url-status=live }}</ref> and [[Wolford Mountain Reservoir]] in 1996.<ref>{{cite web |url=https://www.coloradoriverdistrict.org/wolford-mountain-reservoir-2/ |title=Wolford Mountain Reservoir |publisher=Colorado River District |date= |access-date=May 18, 2022 |archive-date=May 20, 2022 |archive-url=https://web.archive.org/web/20220520101522/https://www.coloradoriverdistrict.org/wolford-mountain-reservoir-2/ |url-status=live }}</ref>

===The Imperial Valley and the Salton Sea ===
{{main|Alamo Canal|Salton Sea}}
The Colorado River delta region in Mexico became a favored place for Americans to invest in agriculture in the late 19th century when Mexican President [[Porfirio Díaz]] welcomed foreign capital to develop the country. The Colorado River Land Company, formed by [[Los Angeles Times]] publisher [[Harry Chandler]], his father-in-law [[Harrison Gray Otis (publisher)|Harrison Gray Otis]], and others, developed the [[Mexicali Valley]] in [[Baja California]] as a thriving land company. The company headquarters was nominally based in Mexico, but its real headquarters was in Los Angeles, California. Land was leased mainly to Americans who were required to develop it. Colorado River water was used to irrigate the rich soil. The company largely escaped the turmoil of the [[Mexican Revolution]] (1910–20), but in the postrevolutionary period, the Mexican government expropriated the company's land to satisfy the demand for [[land reform in Mexico|land reform]].<ref>Dwyer, John J. ''The Agrarian Dispute: The Expropriation of American-Owned Rural Land in Postrevolutionary Mexico''. Durham: Duke University Press 2008</ref><ref>Kerig, Dorothy P. "Yankee Enclave: The Colorado River Land Company and Mexican Agrarian Reform in Baja California, 1902-1944." PhD diss. University of California, Irvine 1988.</ref><ref>Castillo-Muñoz, Verónica. ''The Other California: Land, Identities, and Politics on the Mexican Borderlands''. Oakland: University of California Press 2017.</ref>

In 1900, the [[California Development Company]] (CDC) envisioned irrigating the [[Imperial Valley]], a then dry basin on the California–Mexico border, using water from the Colorado River. Due to the valley's location below sea level, water could be diverted and allowed to flow there entirely by gravity. Engineer [[George Chaffey]] was hired to design the [[Alamo Canal]], which split from the Colorado near [[Pilot Knob (Imperial County, California)|Pilot Knob, California]] and ran south into Mexico, where it joined the [[Alamo River]], a dry arroyo which had historically carried overflowing floodwaters from the Colorado into the [[Salton Sink]] at the bottom of Imperial Valley. The scheme worked initially; by 1903, about four thousand people lived in the valley and more than {{convert|100000|acre|ha}} of farmland had been developed.{{sfn|Billington|Jackson|Melosi|p=140|2005}}<ref name="Alamocanal">{{cite journal |author=Sperry, Robert L. |url=https://sandiegohistory.org/journal/1975/january/imperial-2/ |title=When The Imperial Valley Fought For Its Life |publisher=San Diego History Center |journal=The Journal of San Diego History |date=Winter 1975 |volume=21 |access-date=June 25, 2023 |issue=1 |archive-date=June 26, 2023 |archive-url=https://web.archive.org/web/20230626013218/https://sandiegohistory.org/journal/1975/january/imperial-2/ |url-status=live }}</ref>

The Alamo Canal experienced continual problems due to the Colorado's high sediment content and its varying water levels. During low flows, the river often dropped below the level of the canal intake, while high flows silted up the intake, forcing the repeated excavation of new cuts. In early 1905, flooding destroyed the intake gates and water began to flow uncontrolled down the canal towards the Salton Sink. By August, the breach had grown large enough to swallow the entire flow of the river, which began to flood the bottom of the valley. The [[Southern Pacific Railroad]] attempted to dam the flow in order to protect their tracks which ran through the valley, but was hampered by repeated flooding.<ref name="Alamocanal"/> It took seven attempts, more than $3&nbsp;million, and two years for the railroad, the CDC, and the federal government to permanently block the breach and restore the river's original course – but not before part of the Imperial Valley was flooded under a {{convert|45|mi|km|adj=mid|-long}} lake, today's [[Salton Sea]]. The Imperial Valley fiasco demonstrated that further economic development of the region would require a dam to control the Colorado's unpredictable flows.<ref>Patten, McCaskie and Unitt, pp. 4–5</ref><ref>{{cite web |url=http://www.semp.us/publications/biot_reader.php?BiotID=505 |title=Origin of the Salton Sea |publisher=Suburban Emergency Management Project |archive-url=https://web.archive.org/web/20110718140458/http://www.semp.us/publications/biot_reader.php?BiotID=505 |archive-date=July 18, 2011}}</ref>{{sfn|Billington|Jackson|Melosi|2005|pp=141–142}}

===Boulder Canyon Project===
[[Image:HooverDamConstruction.jpg|thumb|right|Hoover Dam under construction, 1934]]
[[Image:IMPERIAL_DAM_TAKES_LAST_OF_COLORADO_RIVER_WATER_FOR_THE_UNITED_STATES._IT_DIVERTS_WATER_INTO_ALL-AMERICAN_CANAL...._-_NARA_-_548847.jpg|thumb|right|The Imperial Dam (bottom right) diverts water into the All-American Canal (center) running towards Imperial Valley.]]
A large dam on the Colorado River had been envisioned since the 1920s. In 1928, Congress authorized the Reclamation Service (today's [[U.S. Bureau of Reclamation]], or USBR) to build the Boulder Canyon Project, whose key feature would be a dam on the Colorado in Black Canyon {{convert|30|mi|km}} southeast of [[Las Vegas, Nevada]]. On September 30, 1935, [[Hoover Dam]] was completed, forming [[Lake Mead]], capable of holding more than two years of the Colorado's flow. Lake Mead was, and still is, the largest artificial lake in the U.S. by storage capacity.<ref name="LakeMead"/><ref>{{cite web |url=http://www.usbr.gov/projects/Project.jsp?proj_Name=Boulder%20Canyon%20Project%20-%20Hoover%20Dam |title=Boulder Canyon Project – Hoover Dam |publisher=U.S. Bureau of Reclamation |date=February 1, 2012 |access-date=October 9, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20121011123506/http://www.usbr.gov/projects/Project.jsp?proj_Name=Boulder%20Canyon%20Project%20-%20Hoover%20Dam |archive-date=October 11, 2012}}</ref> The construction of Hoover Dam stabilized the lower channel of the Colorado River, stored water for irrigation in times of drought, captured sediment and controlled floods. Hoover was the tallest dam in the world at the time of construction and also had the world's largest hydroelectric power plant.<ref>{{cite web |url=http://www.usbr.gov/lc/region/pao/brochures/faq.html |title=The Colorado River and Hoover Dam: Facts and Figures |publisher=U.S. Bureau of Reclamation |date=January 2012 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120514124746/http://www.usbr.gov/lc/region/pao/brochures/faq.html |archive-date=May 14, 2012}}</ref>

The Boulder Canyon Project Act also authorized the [[All-American Canal]],<ref>{{cite web |url=https://www.archives.gov/milestone-documents/boulder-canyon-project-act |title=Boulder Canyon Project Act (1928) |publisher=National Archives |date=September 21, 2021 |access-date=May 18, 2022 |archive-date=May 18, 2022 |archive-url=https://web.archive.org/web/20220518173242/https://www.archives.gov/milestone-documents/boulder-canyon-project-act |url-status=live }}</ref> which was built as a permanent replacement for the Alamo Canal and follows a route entirely within the U.S. on its way to the Imperial Valley. The canal's intake is located at [[Imperial Dam]], {{convert|20|mi|km}} above [[Yuma, Arizona]], which diverts the majority of the Colorado's flow with only a small portion continuing to Mexico. With a capacity of over {{convert|26000|cuft/s|m3/s}}, the All-American Canal is the largest irrigation canal in the world.<ref>{{cite web |url=http://earthobservatory.nasa.gov/IOTD/view.php?id=37078 |title=All-American Canal, California–Mexico border |publisher=National Aeronautics and Space Administration |work=Earth Observatory |date=February 23, 2009 |access-date=February 20, 2012 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071906/https://earthobservatory.nasa.gov/images/37078/all-american-canal |url-status=live }}</ref> Because the hot, sunny climate lends to a year-round growing season, the Imperial Valley has become one of the most productive farming regions in North America, providing much of the winter produce supply in the U.S.<ref name="ImperialEB">{{Cite encyclopedia |title=Imperial Valley |encyclopedia=Encyclopædia Britannica |year=1995}}</ref> The Imperial Irrigation District supplies water to {{convert|520000|acre|ha}} south of the Salton Sea.<ref>{{cite web |url=https://www.iid.com/water/about-iid-water |title=About IID Water |publisher=Imperial Irrigation District |date= |access-date=May 18, 2022 |archive-date=May 6, 2022 |archive-url=https://web.archive.org/web/20220506235625/https://www.iid.com/water/about-iid-water |url-status=live }}</ref> The [[Coachella Canal]], which branches northward from the All-American Canal, irrigates another {{convert|78000|acre|ha}} in the [[Coachella Valley]].<ref>{{cite web |url=https://www.usbr.gov/projects/index.php?id=492 |title=CRBSCP - Coachella Canal Unit - Title I |publisher=U.S. Bureau of Reclamation |date= |access-date=May 18, 2022 |archive-date=May 19, 2022 |archive-url=https://web.archive.org/web/20220519013457/https://www.usbr.gov/projects/index.php?id=492 |url-status=live }}</ref>

[[Parker Dam]] was initially built as the diversion point for the [[Colorado River Aqueduct]], planned by the [[Metropolitan Water District of Southern California]] to supply water to [[Los Angeles, California|Los Angeles]].<ref>{{cite web |url=http://www.usbr.gov/projects/Facility.jsp?fac_Name=Parker+Dam&groupName=General |title=Parker Dam |publisher=U.S. Bureau of Reclamation |work=Parker-Davis Project |date=June 29, 2009 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120927195113/http://www.usbr.gov/projects/Facility.jsp?fac_Name=Parker+Dam&groupName=General |archive-date=September 27, 2012}}</ref> The construction of the dam was opposed by Arizona, which feared that California would take too much water from the Colorado; at one point, Arizona sent members of its National Guard to stop work on the dam. Ultimately, a compromise was reached, with Arizona dropping its objections in exchange for the USBR constructing the Gila Project, which irrigates {{convert|110000|acre|km2}} on the Arizona side of the river.<ref>{{cite web |url=http://www.usbr.gov/projects//ImageServer?imgName=Doc_1305042259136.pdf |title=Gila Project |author=Marie Bell, Tina |publisher=U.S. Bureau of Reclamation |year=1997 |access-date=February 28, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120224024443/http://www.usbr.gov/projects//ImageServer?imgName=Doc_1305042259136.pdf |archive-date=February 24, 2012}}</ref> By 1941, the {{convert|241|mi|km|adj=on}} long Colorado River Aqueduct was completed, delivering {{convert|1.2|e6acre.ft|km3}} of water west to Southern California. The aqueduct enabled the continued growth of Los Angeles and its suburbs, and provides water to about 10&nbsp;million people today.<ref>{{cite web |url=http://wsoweb.ladwp.com/Aqueduct/historyoflaa/coloradoriver.htm |title=The Colorado River: A Regional Solution |publisher=Los Angeles Department of Water and Power |access-date=February 28, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120304000641/http://wsoweb.ladwp.com/Aqueduct/historyoflaa/coloradoriver.htm |archive-date=March 4, 2012}}</ref> The [[San Diego Aqueduct]], which branches off from the Colorado River Aqueduct in [[Riverside County, California]], opened in stages between 1954 and 1971 and provides water to another 3&nbsp;million people in the [[San Diego]] metro area.<ref>{{cite web |url=http://www.usbr.gov/projects/Project.jsp?proj_Name=San+Diego+Project |title=San Diego Project |publisher=U.S. Bureau of Reclamation |date=January 11, 2012 |access-date=March 16, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120316220454/http://www.usbr.gov/projects/Project.jsp?proj_Name=San+Diego+Project |archive-date=March 16, 2012}}</ref>

The Las Vegas Valley of Nevada experienced rapid growth after Hoover Dam, and by 1937 Las Vegas had tapped a pipeline into Lake Mead. Nevada officials, believing that groundwater resources in the southern part of the state were sufficient for future growth, were more concerned with securing a large amount of the dam's power supply than water from the Colorado; thus they settled for the smallest water allocation of all the states in the Colorado River Compact.<ref>{{cite web |url=http://www.crwua.org/colorado-river/member-states/nevada |title=Nevada |publisher=Colorado River Water Users Association |access-date=November 2, 2013 |archive-url=https://web.archive.org/web/20131104165703/http://www.crwua.org/colorado-river/member-states/nevada |archive-date=November 4, 2013 |url-status=dead}}</ref> In 2018, due to declining water levels in Lake Mead, a second pipeline was completed with a lower intake elevation.

===Colorado River Storage Project===
{{main|Colorado River Storage Project}}
In the first half of the 20th century, the Upper Basin states, with the exception of Colorado, had developed very little of their water allocations from the Colorado River Compact. By the 1950s, however, water demand was rapidly increasing in Utah's [[Wasatch Front]] ([[Salt Lake City]] metro area) and the [[Rio Grande Valley (landform)|Rio Grande Valley]] of New Mexico, which both began exploring ways to divert water from the Colorado Basin.{{sfn|Fradkin|p=191|1996}} The Upper Basin states were concerned that they would not be able to use their full Compact allocations due to increasing water demands in the Lower Basin. The Compact requires the Upper Basin to deliver a minimum annual flow of {{convert|7.5|e6acre.ft}} past Lee's Ferry (measured on a 10-year rolling average). Without additional reservoir storage, the Upper Basin states could not utilize their allocations without impacting water deliveries to the Lower Basin in dry years.<ref name="CRSP"/>

[[Image:Glen_Canyon_Dam_AZ.jpg|thumb|left|upright|Glen Canyon Dam, the largest dam of the Colorado River Storage Project]]
In 1956 Congress authorized the USBR to construct the Colorado River Storage Project (CRSP), which planned several large reservoirs on the upper Colorado, Green, Gunnison and San Juan Rivers.<ref name="CRSP">{{cite web |url=https://www.usbr.gov/uc/rm/crsp/index.html |title=Colorado River Storage Project |publisher=U.S. Bureau of Reclamation |date= |access-date=May 18, 2022 |archive-date=November 8, 2019 |archive-url=https://web.archive.org/web/20191108020419/https://www.usbr.gov/uc/rm/crsp/index.html |url-status=live }}</ref> The initial blueprints for the CRSP included two dams on the Green River within [[Echo Park (Colorado)|Echo Park Canyon]] in [[Dinosaur National Monument]] – a move criticized by both the [[U.S. National Park Service|National Park Service]] and environmental groups such as the [[Sierra Club]]. The controversy received nationwide media attention, and the USBR dropped its plans for the Dinosaur dams in exchange for increasing the size of a proposed dam at Glen Canyon.<ref>{{cite web |url=http://www.nps.gov/history/history/online_books/colorado/chap9.htm |title=Chapter IX: Dinosaur National Monument |publisher=U.S. National Park Service |work=A Survey of the Recreational Resources of the Colorado River Basin |date=September 6, 2004 |access-date=February 20, 2012 |archive-date=November 3, 2013 |archive-url=https://web.archive.org/web/20131103155731/http://www.nps.gov/history/history/online_books/colorado/chap9.htm |url-status=live }}</ref>

The controversy associated with Glen Canyon Dam did not build momentum until construction was well underway. Due to Glen Canyon's remote location, most of the American public did not even know of its existence; the few who did contended that it had much greater scenic value than Echo Park. The [[environmental movement]] in the American Southwest has opposed the damming and diversion of the Colorado River system due to negative effects on the ecology and natural beauty of the river and its tributaries. During the construction of [[Glen Canyon Dam]] (1956–66), environmental organizations{{which|date=September 2022}} vowed{{clarify|date=September 2022}} to block any further development of the river, and a number of later dam and aqueduct proposals were defeated by citizen opposition. Sierra Club leader [[David Brower]] fought the dam both during the construction and for many years afterwards until his death in 2000. Brower believed that he was personally responsible for the failure to prevent Glen Canyon's flooding, calling it his "greatest mistake, greatest sin".<ref>{{cite web |url=http://www.drainit.org/campaigns/drought/facts3.cfm |title=Historic Opposition to Glen Canyon Dam |publisher=Colorado Riverkeeper |work=The End of Lake Powell Campaign |date=July 20, 2004 |access-date=November 2, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131228020203/http://www.drainit.org/campaigns/drought/facts3.cfm |archive-date=December 28, 2013}}</ref><ref>{{cite web |url=http://archive.audubonmagazine.org/fieldnotes/fieldnotes0607.html |title=Resurrection: Buried Treasure |publisher=National Audubon Society |year=2006 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20130120034155/http://archive.audubonmagazine.org/fieldnotes/fieldnotes0607.html |archive-date=January 20, 2013}}</ref>

In addition to Glen Canyon Dam, the CRSP includes the [[Flaming Gorge Dam]] on the Green River, the [[Blue Mesa Dam|Blue Mesa]], [[Morrow Point Dam|Morrow Point]] and [[Crystal Dam]]s on the Gunnison River, and the [[Navajo Dam]] on the San Juan River. A total of 22 "participating projects" (of which 16 have been constructed) were later authorized in order to develop local water supplies at various locations across the Upper Basin states.<ref name="CRSP"/> These include the [[Central Utah Project]], which delivers {{convert|102000|acre feet|m3}} per year from the Green River basin to the Wasatch Front, and the [[San Juan–Chama Project]], which diverts {{convert|110000|acre feet|m3}} per year from the San Juan River to the Rio Grande Valley. Both are multi-purpose projects serving a variety of agricultural, municipal and industrial uses.<ref>{{cite web |url=https://www.doi.gov/cupcao/Overview |title=The Central Utah Project - An Overview |publisher=U.S. Department of the Interior |date= April 3, 2019|access-date=May 18, 2022 |archive-date=May 20, 2022 |archive-url=https://web.archive.org/web/20220520093204/https://www.doi.gov/cupcao/Overview |url-status=live }}</ref><ref>{{cite web |url=https://www.usbr.gov/projects/pdf.php?id=186 |title=San Juan-Chama Project |publisher=U.S. Bureau of Reclamation |author=Glaser, Leah S. |date= |access-date=May 18, 2022 |archive-date=March 8, 2022 |archive-url=https://web.archive.org/web/20220308132051/https://www.usbr.gov/projects/pdf.php?id=186 |url-status=live }}</ref>

===Pacific Southwest Water Plan===
[[Image:Arizona_cap_canal.jpg|thumb|upright|The main canal of the [[Central Arizona Project]], crossing the [[Sonoran Desert]]]]
By the middle of the 20th century, planners were concerned that continued growth in water demand would outstrip the available water supply from the Colorado River. After exploring a multitude of potential projects, the USBR published a study in January 1964 known as the Pacific Southwest Water Plan, which proposed diverting water from the northwestern United States into the Colorado River basin.<ref name="Pacific Southwest Water Plan">{{cite web |url=https://www.usbr.gov/lc/region/programs/crbstudy/PSWPRptJan64.pdf |title=Pacific Southwest Water Plan |publisher=U.S. Bureau of Reclamation |date=January 1964 |access-date=May 18, 2022 |archive-date=September 4, 2021 |archive-url=https://web.archive.org/web/20210904064353/https://www.usbr.gov/lc/region/programs/crbstudy/PSWPRptJan64.pdf |url-status=live }}</ref> Arizona's water allocation was a significant focus of the plan, due to the growing concern that its water supply could be curtailed due to California's [[Prior-appropriation water rights|seniority of water rights]]. In addition, the plan would guarantee full water supplies to Nevada, California and Mexico, allowing the Upper Basin states to utilize their full allocations without risking reductions in the Lower Basin.<ref name="Pacific Southwest Water Plan"/>  The project would cost an estimated $3.1 billion.<ref name="NYT PSW">{{cite news |url=https://www.nytimes.com/1964/03/15/archives/us-plan-for-dams-on-colorado-under-fire-by-conservationists.html |title=U.S. Plan for Dams on Colorado Under Fire by Conservationists; Interior Department Insists Projects Are Vital to Southwest's Development —National Battle Expected |work=The New York Times |date=March 15, 1964 |access-date=May 18, 2022 |archive-date=May 19, 2022 |archive-url=https://web.archive.org/web/20220519013449/https://www.nytimes.com/1964/03/15/archives/us-plan-for-dams-on-colorado-under-fire-by-conservationists.html |url-status=live }}</ref>

The first stage of this plan would divert water from Northern California's [[Trinity River (California)|Trinity]], [[Klamath River|Klamath]] and [[Eel River (California)|Eel River]]s to Southern California, allowing more Colorado River water to be used, by exchange, in Arizona. A canal system, which ultimately would become the [[Central Arizona Project]] (CAP), would be constructed to deliver Arizona's Colorado River allocation to Phoenix and Tucson, both located far away from the Colorado River in the middle of the state. At this point, central Arizona was still entirely dependent on local water supplies, such as the 1911 [[Theodore Roosevelt Dam]],<ref>{{cite web |url=http://www.usbr.gov/lc/region/programs/Roosevelt/story.html |title=Reclamation Helps Celebrate Theodore Roosevelt Dam's 100th Year |publisher=U.S. Bureau of Reclamation |date=March 2011 |access-date=March 16, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120925050455/http://www.usbr.gov/lc/region/programs/Roosevelt/story.html |archive-date=September 25, 2012}}</ref>  and was quickly running out of surplus water.<ref name="Pacific Southwest Water Plan"/>

In order to supply the massive amount of power required to pump Colorado River water to central Arizona, two hydroelectric dams were proposed in the Grand Canyon ([[Bridge Canyon Dam]] and [[Marble Canyon Dam]]), which while not directly located in [[Grand Canyon National Park]], would greatly impact flows of the Colorado River through the park.<ref name="Pacific Southwest Water Plan"/> With the controversy over Glen Canyon Dam still ongoing, the public pressure against these dams was immense.<ref name="NYT PSW"/> As a result, the two Grand Canyon dams were omitted from the final CAP authorization in 1968.<ref>{{cite web |url=https://grcahistory.org/sites/colorado-river-corridor/bridge-canyon/ |title=Bridge Canyon |publisher=Arizona State University |date= |author=Gerke, Sarah Bohl |access-date=May 18, 2022 |archive-date=February 5, 2021 |archive-url=https://web.archive.org/web/20210205222136/https://grcahistory.org/sites/colorado-river-corridor/bridge-canyon/ |url-status=live }}</ref> In addition, the boundaries of Grand Canyon National Park were redrawn to prevent future dam projects in the area. The pumping power was replaced by the building of the coal-fired [[Navajo Generating Station]] near Page, Arizona, in 1976.<ref name="CAP"/><ref>{{cite web |last=Murdock |first=J. Neil |url=http://www.gcrg.org/bqr/10-1/bcd.html |title=Bridge Canyon Dam |publisher=Grand Canyon River Guides |work=from Early History of the Colorado River Storage Project May 1971 U. S. Department of the Interior, Bureau of Reclamation |year=1996 |access-date=July 15, 2013 |archive-url=https://web.archive.org/web/20170525095548/http://www.gcrg.org/bqr/10-1/bcd.html |archive-date=May 25, 2017 |url-status=dead}}</ref><ref>{{cite web |url=http://www.nps.gov/history/history/online_books/colorado/chap7.htm |title=Chapter VII: The Grand Canyon |publisher=U.S. National Park Service |work=A Survey of the Recreational Resources of the Colorado River Basin |date=September 6, 2005 |access-date=July 15, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131106001019/http://www.nps.gov/history/history/online_books/colorado/chap7.htm |archive-date=November 6, 2013}}</ref> In 2019, the Navajo Generating Station ceased operation.<ref>{{cite web |url=https://www.srpnet.com/about/stations/ngs/default.aspx |title=Navajo Generating Station, Project Information |publisher=Salt River Project |access-date=May 18, 2022 |archive-date=May 19, 2022 |archive-url=https://web.archive.org/web/20220519080022/https://www.srpnet.com/about/stations/ngs/default.aspx |url-status=live }}</ref>

The CAP was constructed in stages from 1973 to 1993, ultimately extending {{convert|336|mi|km}} from the Colorado River at Parker Dam to [[Tucson, Arizona]]. It delivers {{convert|1.4|e6acre.ft|km3}} of water per year, irrigates {{convert|830000|acre|km2}} of farmland and provides municipal water to about 5&nbsp;million people.<ref name="CAP">{{cite web |url=http://www.usbr.gov/projects/Project.jsp?proj_Name=Central+Arizona+Project |title=Central Arizona Project |publisher=U.S. Bureau of Reclamation |date=April 18, 2011 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20110613215932/http://www.usbr.gov/projects/Project.jsp?proj_Name=Central+Arizona+Project |archive-date=June 13, 2011}}</ref> Due to environmental concerns, most of the facilities proposed in the Pacific Southwest Water Plan were never built (though a smaller version of the Trinity River project was constructed as part of the unrelated [[Central Valley Project]]),<ref>{{cite web |url=https://www.usbr.gov/mp/ncao/trinity-river.html |title=Trinity River |publisher=U.S. Bureau of Reclamation |date=May 18, 2022 |access-date=May 19, 2022 |archive-date=May 19, 2022 |archive-url=https://web.archive.org/web/20220519013457/https://www.usbr.gov/mp/ncao/trinity-river.html |url-status=live }}</ref> leaving Arizona and Nevada vulnerable to future water reductions under the Compact.<ref>{{cite web |url=https://sgp.fas.org/crs/misc/R45546.pdf |title=Management of the Colorado River: Water Allocations, Drought, and the Federal Role |publisher=Federation of American Scientists |author=Congressional Research Service |date=August 16, 2021 |access-date=May 18, 2022 |archive-date=May 11, 2022 |archive-url=https://web.archive.org/web/20220511173029/https://sgp.fas.org/crs/misc/R45546.pdf |url-status=live }}</ref>

===Indigenous water rights===
Water rights of Native Americans in the Colorado River basin were largely ignored during the extensive water resources development carried out on the river and its tributaries in the 19th and 20th&nbsp;centuries. The construction of dams has often had negative impacts on tribal peoples, such as the Chemehuevi when their riverside lands were flooded after the completion of [[Parker Dam]] in 1938. Ten Native American tribes in the basin now hold or continue to claim water rights to the Colorado River.<ref>{{cite web |url=http://www.crwua.org/colorado-river/ten-tribes |title=Ten Tribes Partnership |publisher=Colorado River Water Resources Association |access-date=November 2, 2013 |archive-url=https://web.archive.org/web/20131104165709/http://www.crwua.org/colorado-river/ten-tribes |archive-date=November 4, 2013 |url-status=dead}}</ref> The U.S. government has taken some actions to help quantify and develop the water resources of Native American reservations. The first federally funded irrigation project in the U.S. was the construction of an irrigation canal on the Colorado River Indian Reservation in 1867.<ref>{{cite web |author=Voggesser, Garrit |url=http://www.usbr.gov/history/ProjectHistories/INDIAN%20PROJECTS%20OVERVIEW.pdf |title=The Indian Projects |publisher=U.S. Bureau of Reclamation |year=2001 |work=Bureau of Reclamation History Program |access-date=April 10, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120224015718/http://www.usbr.gov/history/ProjectHistories/INDIAN%20PROJECTS%20OVERVIEW.pdf |archive-date=February 24, 2012}}</ref> Other water projects include the [[Navajo Indian Irrigation Project]], authorized in 1962 for the irrigation of lands in part of the Navajo Nation in north-central New Mexico.<ref>{{cite web |url=http://www.usbr.gov/projects/Project.jsp?proj_Name=Navajo%20Indian%20Irrigation%20Project |title=Navajo Indian Irrigation Project |publisher=U.S. Bureau of Reclamation |date=May 11, 2011 |access-date=April 10, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120925123839/http://www.usbr.gov/projects/Project.jsp?proj_Name=Navajo%20Indian%20Irrigation%20Project |archive-date=September 25, 2012}}</ref> The Navajo continue to seek expansion of their water rights because of difficulties with the water supply on their reservation; about 40&nbsp;percent of its inhabitants must haul water by truck many miles to their homes. In the 21st&nbsp;century, they have filed legal claims against the governments of Arizona, New Mexico and Utah for increased water rights. Some of these claims have been successful for the Navajo, such as a 2004 settlement in which they received a {{convert|326000|acre foot|ML|adj=on}} allotment from New Mexico.<ref>{{cite news |author=Jenkins, Matt |title=In Navajoland, a Contentious Water Deal Divides the Tribe |work=High Country News |location=Paonia, Colorado |url=http://www.hcn.org/issues/43.3/in-navajoland-a-contentious-water-deal-divides-the-tribe |date=February 21, 2011 |access-date=July 1, 2013 |archive-date=January 25, 2021 |archive-url=https://web.archive.org/web/20210125015648/https://www.hcn.org/issues/43.3/in-navajoland-a-contentious-water-deal-divides-the-tribe |url-status=live }}</ref>

===Post-2000 water supply ===
{{See also|Southwestern North American megadrought}}
{{rquote|right|[The Colorado is] a 'deficit' river, as if the river were somehow at fault for its overuse.|[[Marc Reisner]], in ''[[Cadillac Desert]]''{{sfn|Reisner|p=121|1993}}}}
When the Colorado River Compact was drafted in the 1920s, it was based on barely 30 years of streamflow records that suggested an average annual flow of {{convert|17.5|e6acre.ft|km3}} past Lee's Ferry.{{sfn|Reisner|p=126|1993}} Modern studies of [[tree ring]]s revealed that those three decades were probably the wettest in the past 500 to 1,200 years and that the natural long-term annual flow past Lee's Ferry is probably closer to {{convert|13.5|e6acre.ft|km3}},<ref>{{cite journal |author1=Stahle, David W. |author2=Fye, Falko K. |author3=Therrell, Matthew D. |title=Interannual to Decadal Climate and Streamflow Variability Estimated from Tree Rings |doi=10.1016/S1571-0866(03)01023-6 |journal=Developments in Quaternary Science |date=2003 |volume=1 |pages=491–504 |series=Developments in Quaternary Sciences |bibcode=2003DevQS...1..491S |isbn=9780444514707}}</ref>{{refn|The discrepancy between the natural flow at Lee's Ferry (13.5&nbsp;million acre-feet or 16.65&nbsp;km<sup>3</sup>) and the gauged flow between 1922 and 2020 (10.58&nbsp;million acre-feet or 13.05&nbsp;km<sup>3</sup>)<ref name="LeesFerryMonthly"/> is mostly due to water diversions above Lee's Ferry and evaporation from reservoirs, especially Lake Powell.<ref>{{cite web |url=http://www.usbr.gov/uc/library/envdocs/reports/crs/pdfs/cul2006-2010prov.pdf |title=Upper Colorado River Basin Consumptive Uses and Losses Report 2006-2010 |publisher=U.S. Bureau of Reclamation |date=August 2012 |access-date=June 29, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20121010175929/http://www.usbr.gov/uc/library/envdocs/reports/crs/pdfs/cul2006-2010prov.pdf |archive-date=October 10, 2012}}</ref>|group=n}} with a natural flow at the mouth around {{convert|16.3|e6acre.ft|km3}}.<ref name="Nowak">{{cite web |author=Nowak, Kenneth C. |title=Stochastic Streamflow Simulation at Interdecadal Time Scales and Implications to Water Resources Management in the Colorado River Basin |publisher=University of Colorado |work=Center for Advanced Decision Support for Water and Environmental Systems |url=http://cadswes.colorado.edu/sites/default/files/PDF/Theses-PhD/Nowak-PhD.pdf |date=April 2, 2012 |page=114 |access-date=July 11, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20140501143218/http://cadswes.colorado.edu/sites/default/files/PDF/Theses-PhD/Nowak-PhD.pdf |archive-date=May 1, 2014}}</ref> This has resulted in more water being allocated to river users than actually exists in the Colorado.<ref name="Gelt">{{cite journal |author=Gelt, Joe |url=http://worldcat.org/arcviewer/2/WCA/2009/12/14/H1260824596711/viewer/file2.html |title=Sharing Colorado River Water: History, Public Policy and the Colorado River Compact |journal=Arroyo |publisher=University of Arizona, Water Resources Research Center |volume=10 |date=August 1997 |issue=1 |access-date=February 25, 2014 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071625/http://worldcat.org/arcviewer/2/WCA/2009/12/14/H1260824596711/viewer/file2.html |url-status=live }}</ref> Droughts have exacerbated the issue of water over-allocation.<ref>{{cite web |url=http://www.riverguides.org/Confluence/27/27DroughtWeisheit.pdf |title=Persistent Drought in the Colorado River Basin |author=Weisheit, John |work=Confluence |volume=27 |access-date=May 15, 2012 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071530/http://www.riverguides.org/Confluence/27/27DroughtWeisheit.pdf |url-status=live }}</ref><ref>{{cite web |url=http://www.fs.fed.us/psw/cirmount/policy/pdf/Paleo_Arc.pdf |title=Paleoenvironmental Archives as a source of Climate Information for Natural Resource Management: An Example from Tree Rings and Colorado Water Management |publisher=U.S. Forest Service |author1=Woodhouse, Connie |author-link1=Connie Woodhouse |author2=Webb, Robert S. |author3=Lukas, Jeff |access-date=May 19, 2012 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071545/https://www.fs.fed.us/psw/cirmount/policy/pdf/Paleo_Arc.pdf |url-status=live }}</ref>

[[File:Lake Mead October 2010.jpg|thumb|left|alt=View of a reservoir where the water level has dropped, showing white deposits on the surrounding mountains|Lake Mead in 2010, showing the "bathtub ring" left behind by low water levels]]
The most severe drought on record, the [[southwestern North American megadrought]], began in the early 21st&nbsp;century, in which the river basin has produced above-average runoff in only five years between 2000 and 2021.<ref>{{cite web |url=https://www.usbr.gov/uc/water/crsp/cs/gcd.html |title=Glen Canyon Dam Water Operations |publisher=U.S. Bureau of Reclamation |year=2022 |access-date=May 18, 2022 |archive-date=September 28, 2020 |archive-url=https://web.archive.org/web/20200928134447/https://www.usbr.gov/uc/water/crsp/cs/gcd.html |url-status=live }}</ref> The region is experiencing a warming trend, which is accompanied by earlier snowmelt, lower precipitation and greater [[evapotranspiration]]. A 2004 study showed that a 1–6&nbsp;percent decrease of precipitation would lead to runoff declining by as much as 18&nbsp;percent by 2050.<ref>{{cite web |url=http://wwa.colorado.edu/climate_change/docs/Hoerling%20v2%20WWA%20Front%20Range%20Water.pdf |author1=Hoerling, Martin |author2=Eischeid, Jon |title=Outlook for Water in the West at 2050 |publisher=Western Water Assessment |work=NOAA Earth System Research Laboratory, Climate Change Workshop |date=November 17, 2006 |archive-url=https://web.archive.org/web/20100609222623/http://wwa.colorado.edu/climate_change/docs/Hoerling%20v2%20WWA%20Front%20Range%20Water.pdf |archive-date=June 9, 2010}}</ref>

Since 2000, reservoir levels have fluctuated greatly from year to year, but have experienced a steady long-term decline.<ref>{{cite web |url=http://www.crwcd.org/media/uploads/CO_River_Uncertain_Future.pdf |title=The Colorado River's Uncertain Future: How Climate Change May Affect Future Planning Decisions on the Colorado River |publisher=Colorado River Water Conservation District |author=Kuhn, Eric |work=CRWCD 49th Annual Convention |date=January 26, 2007 |access-date=February 21, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20111201024238/http://www.crwcd.org/media/uploads/CO_River_Uncertain_Future.pdf |archive-date=December 1, 2011}}</ref>  The particularly dry spell between 2000 and 2004 brought Lake Powell to just a third of capacity in 2005, the lowest level on record since initial filling in 1969.<ref>{{cite web |author=Vernieu-Presenter, William S. |url=http://www.gcrg.org/docs/gtslib/vernieu-05.pdf |title=Current Status and Trends of Lake Powell and Glen Canyon Dam Release Water Quality |publisher=Grand Canyon River Guides |work=U.S. Geological Survey, Grand Canyon Monitoring and Research Center |year=2005 |access-date=May 15, 2012 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071038/http://www.gcrg.org/docs/gtslib/vernieu-05.pdf |url-status=dead}}</ref> In late 2010, Lake Mead was approaching the "drought trigger" elevation of {{convert|1075|ft|m}}, at which water supplies to Arizona and Nevada would be reduced in accordance with the Colorado River Compact.<ref>{{cite news |url=http://www.azcentral.com/arizonarepublic/news/articles/2010/10/19/20101019lake-mead-water-level-new-historic-low.html |author=McKinnon, Shaun |title=Lake Mead Sinks to a New Historic Low: If Lake Falls 8 More Feet, Thirsty Ariz. Could Experience Water Restrictions |newspaper=The Arizona Republic |location=Phoenix, Arizona |date=October 19, 2010 |access-date=September 6, 2012 |archive-date=January 17, 2013 |archive-url=https://archive.today/20130117144207/http://www.azcentral.com/arizonarepublic/news/articles/2010/10/19/20101019lake-mead-water-level-new-historic-low.html?nclick_check=1 |url-status=live }}</ref> Due to Arizona and Nevada's water rights being junior to California's, their allocations can legally be cut to zero before any reductions are made on the California side.<ref name="Gelt"/><ref>{{cite news |author=Hiltzik, Michael |title=Water war bubbling up between California and Arizona |url=http://www.latimes.com/business/hiltzik/la-fi-hiltzik-20140620-column.html#page=1 |work=Los Angeles Times |date=2014-06-20 |access-date=2014-09-23 |archive-date=June 24, 2021 |archive-url=https://web.archive.org/web/20210624134813/https://www.latimes.com/business/hiltzik/la-fi-hiltzik-20140620-column.html#page=1 |url-status=live }}</ref>

A wet winter in 2011 temporarily raised lake levels,<ref>{{cite news |url=http://www.azcentral.com/news/articles/2011/04/18/20110418lake-mead-replenished-by-snowfall.html |author=McKinnon, Shaun |title=Lake Mead Replenished by Snowfall |work=azcentral.com |date=April 19, 2011 |access-date=February 21, 2012 |archive-date=January 17, 2013 |archive-url=https://archive.today/20130117112702/http://www.azcentral.com/news/articles/2011/04/18/20110418lake-mead-replenished-by-snowfall.html?nclick_check=1 |url-status=live }}</ref><ref>{{cite news |url=http://www.lasvegassun.com/news/2011/aug/17/lake-meads-water-level-rises/ |title=Lake Mead's water level rises 30 feet after wet winter |author=Shine, Conor |newspaper=Las Vegas Sun |date=August 17, 2011 |access-date=February 21, 2012 |archive-date=July 25, 2016 |archive-url=https://web.archive.org/web/20160725022127/http://lasvegassun.com/news/2011/aug/17/lake-meads-water-level-rises/ |url-status=live }}</ref> but dry conditions returned in the next two years.<ref>{{cite news |author=Perry, Tony |title=As Colorado River drought worsens, water officials parley |work=Los Angeles Times |date=May 27, 2013 |url=https://www.latimes.com/local/lanow/la-xpm-2013-may-27-la-me-ln-as-drought-worsens-along-colorado-river-water-officials-discuss-moving-forward-20130527-story.html |access-date=July 9, 2013 |archive-date=July 24, 2013 |archive-url=https://web.archive.org/web/20130724233655/http://articles.latimes.com/2013/may/27/local/la-me-ln-as-drought-worsens-along-colorado-river-water-officials-discuss-moving-forward-20130527 |url-status=live }}</ref> In 2014, the Bureau of Reclamation cut releases from Lake Powell by 10&nbsp;percent —the first such reduction since the 1960s, when Lake Powell was being filled for the first time.<ref>{{cite news |author=Johnson, Terrell |title=Dwindling Colorado River Forces First-Ever Cuts in Lake Powell Water Releases |url=http://www.weather.com/news/science/environment/drought-lake-powell-lake-mead-climate-change-20130818 |work=The Weather Channel |date=August 20, 2013 |access-date=January 16, 2014 |url-status=dead |archive-url=https://web.archive.org/web/20140113020305/http://www.weather.com/news/science/environment/drought-lake-powell-lake-mead-climate-change-20130818 |archive-date=January 13, 2014}}</ref> This resulted in Lake Mead dropping to its lowest recorded level since 1937, when it was first being filled.<ref>{{cite news |author=Brean, Henry |title=Lake Mead sinks to a record low |work=Las Vegas Review-Journal |url=http://www.reviewjournal.com/news/nevada/lake-mead-sinks-record-low |date=2014-07-11 |access-date=2014-10-02 |archive-date=January 3, 2015 |archive-url=https://web.archive.org/web/20150103032005/http://www.reviewjournal.com/news/nevada/lake-mead-sinks-record-low |url-status=live }}</ref>

Water year 2018 had a much lower-than-average snowpack.<ref>{{cite news |url=http://knau.org/post/colorado-river-water-shortage-could-be-declared-2020 |title=Colorado River Water Shortage Could be Declared in 2020 |author=Sevigny, Melissa |work=KNAU |date=2018-05-17 |access-date=2018-05-24 |archive-date=August 18, 2021 |archive-url=https://web.archive.org/web/20210818042930/https://www.knau.org/post/colorado-river-water-shortage-could-be-declared-2020 |url-status=live }}</ref><ref>{{cite news |url=https://www.ecowatch.com/colorado-river-water-shortage-2569729456.html |title=Unprecedented Colorado River Water Shortage Could Be Declared in 2020 |author=Chow, Lorraine |work=EcoWatch |date=2018-05-17 |access-date=2018-05-24 |archive-date=May 24, 2020 |archive-url=https://web.archive.org/web/20200524233910/https://www.ecowatch.com/colorado-river-water-shortage-2569729456.html |url-status=live }}</ref> In July 2021, after two more extremely dry winters, Lake Powell fell below the previous low set in 2005. In response, the Bureau of Reclamation began releasing water from upstream reservoirs in order to keep Powell above the minimum level for [[hydropower]] generation.<ref>{{cite news |url=https://www.sltrib.com/news/environment/2021/07/22/lake-powell-level-about/ |title=Lake Powell level about to hit a historic low as West's water crisis deepens |author=Brian Maffly |work=Salt Lake Tribune |date=2021-07-22 |access-date=July 31, 2021 |archive-date=August 18, 2021 |archive-url=https://web.archive.org/web/20210818095410/https://www.sltrib.com/news/environment/2021/07/22/lake-powell-level-about/ |url-status=live }}</ref><ref>{{cite news |url=https://www.cpr.org/2021/07/24/lake-powell-lowest-water-level-on-record-heres-what-that-means-for-colorado-and-states-that-rely-on-it/ |title=Lake Powell Just Hit Its Lowest Level On Record. Here's What That Means For Colorado And States That Rely On It |work=Colorado Public Radio |author=Michael Elizabeth Sakas |date=2021-07-24 |access-date=July 31, 2021 |archive-date=August 15, 2021 |archive-url=https://web.archive.org/web/20210815042611/https://www.cpr.org/2021/07/24/lake-powell-lowest-water-level-on-record-heres-what-that-means-for-colorado-and-states-that-rely-on-it/ |url-status=live }}</ref> Lake Mead fell below the {{convert|1075|ft|m|adj=on}} level expected to trigger federally mandated cuts to Arizona and Nevada's water supplies for the first time in history, and is expected to continue declining into 2022.<ref>{{cite news |url=https://www.theguardian.com/us-news/2021/jul/13/hoover-dam-lake-mead-severe-drought-us-west |title=Severe drought threatens Hoover dam reservoir – and water for US west |work=The Guardian |author=Oliver Milman |date=2021-07-13 |access-date=July 31, 2021 |archive-date=August 25, 2021 |archive-url=https://web.archive.org/web/20210825214104/https://www.theguardian.com/us-news/2021/jul/13/hoover-dam-lake-mead-severe-drought-us-west |url-status=live }}</ref>

On August 16, 2021, the Bureau of Reclamation released the Colorado River Basin August 2021 24-Month Study, and for the first time declared a shortage and that because of "ongoing historic drought and low runoff conditions in the Colorado River Basin, downstream releases from Glen Canyon Dam and Hoover Dam will be reduced in 2022 due to declining reservoir levels."<ref name="Aug16">{{cite web |url=https://www.usbr.gov/newsroom/#/news-release/3950 |title=Reclamation announces 2022 operating conditions for Lake Powell and Lake Mead |date=Aug 16, 2021 |website=usbr.gov |publisher=[[United States Bureau of Reclamation]] |access-date=2021-08-19 |archive-date=August 17, 2021 |archive-url=https://archive.today/20210817005912/https://www.usbr.gov/newsroom/%23/news-release/3950 |url-status=live }}</ref>
The Lower Basin reductions will reduce the annual apportionments – Arizona's by 18 percent, Nevada's by 7 percent, and Mexico's by 5 percent.<ref>{{cite web |url=https://www.msn.com/en-us/news/politics/overnight-energy-federal-government-announces-first-ever-water-shortage-in-lake-mead-colorado-river-biden-administration-releases-guidance-limiting-international-financing-for-fossil-fuels-biden-administration-to-review-federal-coal-leasing/ar-AANocbi |title=OVERNIGHT ENERGY: Federal government announces first-ever water shortage in Lake Mead, Colorado River &#124;Biden administration releases guidance limiting international financing for fossil fuels &#124;Biden administration to review federal coal leasing |last=Budryk |first=Zack |date=Aug 17, 2021 |website=msn.com |publisher=[[MSN]] |access-date=2021-08-19 |archive-date=August 28, 2021 |archive-url=https://archive.today/20210828075952/https://www.msn.com/en-us/news/politics/overnight-energy-federal-government-announces-first-ever-water-shortage-in-lake-mead-colorado-river-biden-administration-releases-guidance-limiting-international-financing-for-fossil-fuels-biden-administration-to-review-federal-coal-leasing/ar-AANocbi |url-status=live }}</ref>

On June 14, 2022, Bureau of Reclamation Commissioner Camille Touton told the Senate Committee on Energy and Natural resources that additional cuts of 2–4 million acre-feet were required to stabilize reservoir levels in 2023. Touton warned that if states were unable to negotiate the requisite cuts the Interior department may use its legal authority to cut releases.<ref>{{cite web |title=Full Committee Hearing To Examine Short And Long Term Solutions To Extreme Drought In The Western U.S. |url=https://www.energy.senate.gov/hearings/2022/6/full-committee-hearing-to-examine-short-and-long-term-solutions-to-extreme-drought-in-the-western-u-s |website=Senate Committee on Energy and Natural resources |date=June 14, 2022 |publisher=United States Senate |access-date=16 December 2022 |archive-date=December 16, 2022 |archive-url=https://web.archive.org/web/20221216053044/https://www.energy.senate.gov/hearings/2022/6/full-committee-hearing-to-examine-short-and-long-term-solutions-to-extreme-drought-in-the-western-u-s |url-status=live }}</ref> When the states were unable to come to an agreement about how to share the proposed cuts, Reclamation began the legal steps to unilaterally reduce releases from Hoover and Glen Canyon Dams in 2023.<ref name="Reuters">{{cite news |last1=Trotta |first1=Daniel |title=U.S. warns western states it may impose Colorado River water cuts |url=https://www.reuters.com/world/us/us-warns-western-states-it-may-impose-colorado-river-water-cuts-2022-10-28/ |access-date=16 December 2022 |work=Reuters |date=28 October 2022 |archive-date=December 16, 2022 |archive-url=https://web.archive.org/web/20221216053042/https://www.reuters.com/world/us/us-warns-western-states-it-may-impose-colorado-river-water-cuts-2022-10-28/ |url-status=live }}</ref> As of December 2022 the lower basin states of Nevada, Arizona, and California had not agreed on how to reduce water use by the approximately 30% required to keep levels in lakes Mead and Powell from crashing.<ref name="Las Vegas Review Journal">{{cite news |last1=Lochhead |first1=Colton |title=Las Vegas water boss urges states to take action to keep lakes from crashing |url=https://www.reviewjournal.com/news/politics-and-government/las-vegas-water-boss-urges-states-to-take-action-to-keep-lakes-from-crashing-2695166/ |access-date=16 December 2022 |agency=Las Vegas Review Journal |date=15 December 2022 |archive-date=December 16, 2022 |archive-url=https://web.archive.org/web/20221216053042/https://www.reviewjournal.com/news/politics-and-government/las-vegas-water-boss-urges-states-to-take-action-to-keep-lakes-from-crashing-2695166/ |url-status=live }}</ref> The Bureau of Reclamation had projected that water levels at Lake Powell could fall low enough that by July 2023 Glen Canyon Dam would no longer be able to generate any hydropower, a scenario referred to as "dead pool".<ref>{{cite news |last1=Partlow |first1=Joshua |title=Officials fear 'complete doomsday scenario' for drought-stricken Colorado River |url=https://www.washingtonpost.com/climate-environment/2022/12/01/drought-colorado-river-lake-powell/ |access-date=16 December 2022 |agency=The Washington Post |date=1 December 2022 |archive-url=https://web.archive.org/web/20221209152836/https://www.washingtonpost.com/climate-environment/2022/12/01/drought-colorado-river-lake-powell/ |archive-date=December 9, 2022}}</ref> Arizona proposed a plan that severely cut allocations to California, and California responded with a plan that severely cut allocations to Arizona, failing to reach consensus. In April 2023, the federal government proposed cutting allocations to Nevada, Arizona, and California evenly which would cut deliveries by as much as one-quarter to each state, rather than according to senior water rights.<ref>{{cite news |url=https://www.nytimes.com/2023/04/11/climate/colorado-river-water-cuts-drought.html |title=White House Proposes Evenly Cutting Water Allotments From Colorado River |newspaper=[[The New York Times]] |author=Christopher Flavelle |date=April 11, 2023 |access-date=April 13, 2023 |archive-date=April 13, 2023 |archive-url=https://web.archive.org/web/20230413192525/https://www.nytimes.com/2023/04/11/climate/colorado-river-water-cuts-drought.html |url-status=live }}</ref>

In May 2023, the states finally reached a temporary agreement to prevent a "dead pool", reducing allocations by 3 million acre-feet over three years (until the end of 2026). 700,000 acre-feet were to be negotiated later among California, Arizona, and Nevada.<ref name="deal">{{cite news |url=https://www.sltrib.com/news/nation-world/2023/05/22/do-or-die-talks-reach-deal-keep/ |title=Do-or-die talks reach deal to keep Colorado River from going dry, for now |author=Christopher Flavelle |newspaper=[[The New York Times]] |date=May 22, 2023 |access-date=June 10, 2023 |archive-date=June 10, 2023 |archive-url=https://web.archive.org/web/20230610013841/https://www.sltrib.com/news/nation-world/2023/05/22/do-or-die-talks-reach-deal-keep/ |url-status=live }}</ref> The cuts were less than the federal government had demanded, and so further cuts will be needed after 2026.<ref name="deal" /> Fewer cuts were needed in the short term because the Colorado River Basin experienced an unusually rainy and snowy weather in early 2023.<ref name="deal" /><ref>{{cite news |url=https://www.latimes.com/environment/story/2023-04-08/colorado-river-snowpack-california |title=Drought-ravaged Colorado River gets relief from snow. But long-term water crisis remains |author=Ian James |newspaper=[[The Los Angeles Times]] |date=8 April 2023 |access-date=April 13, 2023 |archive-date=April 13, 2023 |archive-url=https://web.archive.org/web/20230413210949/https://www.latimes.com/environment/story/2023-04-08/colorado-river-snowpack-california |url-status=live }}</ref>

The agreement also became easier to negotiate because many cuts are being offset by one-time federal funding.<ref>{{Cite web |url=https://www.reuters.com/world/us/us-states-reach-colorado-river-water-conservation-deal-interior-dept-2023-05-22/ |title=Western states reach 'historic' deal to help save Colorado River |website=[[Reuters]] |access-date=June 10, 2023 |archive-date=June 10, 2023 |archive-url=https://web.archive.org/web/20230610013840/https://www.reuters.com/world/us/us-states-reach-colorado-river-water-conservation-deal-interior-dept-2023-05-22/ |url-status=live }}</ref> Billions of dollars in funding for programs in the Colorado River Basin to recycle water, increase efficiency, and competitive grants to pay water rights holders not to use water from the river are being provided by the [[Infrastructure Investment and Jobs Act]] and [[Inflation Reduction Act]], and other programs funded through the [[United States Environmental Protection Agency]] and [[United States Department of the Interior]]. These are projected to reduce demand by hundreds of thousands of acre-feet per year.<ref>{{Cite web |url=https://bidenwhitehouse.archives.gov/briefing-room/statements-releases/2023/04/06/fact-sheet-biden-harris-administration-announces-new-investments-to-protect-the-colorado-river-system/ |title=FACT SHEET: Biden-Harris Administration Announces New Investments to Protect the Colorado River System |first=The White |last=House |date=April 6, 2023 |website=The White House |access-date=April 13, 2023 |archive-date=April 13, 2023 |archive-url=https://web.archive.org/web/20230413231804/https://www.whitehouse.gov/briefing-room/statements-releases/2023/04/06/fact-sheet-biden-harris-administration-announces-new-investments-to-protect-the-colorado-river-system/ |url-status=live }}</ref>