Editing Colorado River (section)

===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>