Editing Lake Tahoe (section)

===Water quality===
[[File:lake tahoe storm drain el dorado beach 2.jpg|thumb|An example of road runoff with fine sediment, failing to reach its sluice, at El Dorado Beach. This [[storm drain]] was removed during construction of a new beach, called Lakeview Commons, opened in mid-2012.]]
{{Image frame |caption={{center|Average Lake Tahoe [[water clarity|Clarity]]{{snd}}As Measured by [[Secchi disk|Secchi Depth]].<ref name="UC Davis">{{cite web|title=Average Lake Tahoe Secchi Depth|url=https://tahoe.ucdavis.edu/sites/g/files/dgvnsk4286/files/inline-files/Secchi_Averages_2023_final.pdf|website=Tahoe Environmental Research Center (TERC)|publisher=University of California, Davis|access-date=August 2, 2024}}</ref>}}|content=
{{Graph:Chart|width=400|height=100|xAxisTitle=Year|yAxisTitle=Average Secchi Depth [feet]|legend=|colors=red,lightblue,orange |y1Title=Annual|y2Title=Winter|y3Title=Summer|type=line
|x=1968,1969,1970,1971,1972,1973,1974,1975,1976,1977,1978,1979,1980,1981,1982,1983,1984,1985,1986,1987,1988,1989,1990,1991,1992,1993,1994,1995,1996,1997,1998,1999,2000,2001,2002,2003,2004,2005,2006,2007,2008,2009,2010,2011,2012,2013,2014,2015,2016,2017,2018,2019,2020,2021,2022,2023
|y1=102.4,93.8,99.1,94.2,89.9,85.6,89.2,85.6,89.9,91.2,85,87.6,81.4,89.9,79.7,73.5,74.8,79.4,79.1,80.7,81,77.4,77.4,73.5,78.4,70.5,74.1,70.5,76.8,64,65.9,68.9,67.3,73.5,78.1,70.9,73.5,72.2,67.6,70.2,69.6,68.2,64.3,68.9,75.1,70.2,77.8,73.2,69.2,60.4,70.9,62.7,63.0,61.0,71.9,68.2
|y2=109.6,119.1,99.4,109.9,85.6,96.8,97.4,94.5,90.6,91.2,87.6,95.1,90.9,81.7,90.6,95.1,72.2,89.6,88.3,76.1,77.4,87.6,84.6,70.9,72.5,84.6,71.5,75.1,88.3,65.6,76.1,81,70.5,77.8,78.4,70.9,83.3,80.4,76.8,82.3,85.3,81.4,72.8,85,88.3,77.8,79.1,71.5,83.3,78.7,73.5,81.0,64.0,71.9,72.2,91.9
|y3=94.2,74.8,93.5,86.3,91.2,75.1,83,77.8,84.6,92.8,82,81.7,74.8,97.8,64.6,57.1,74.5,72.5,74.1,85.6,91.9,75.5,75.5,72.8,82.7,65.3,77.8,58.1,69.2,62.7,59.7,63,64,72.8,81,69.2,73.2,66.9,57.4,65.3,50.5,59.1,51.8,51.5,64.6,63.6,76.8,73.2,56.4,53.5,61.7,52.8,59.1,54.8,68.9,53.5

<!--DATA IN METERS IF NEEDED IN FUTURE --i.e. toggle "radio button" to switch graph to meters
|y1=31.2,28.6,30.2,28.7,27.4,26.1,27.2,26.1,27.4,27.8,25.9,26.7,24.8,27.4,24.3,22.4,22.8,24.2,24.1,24.6,24.7,23.6,23.6,22.4,23.9,21.5,22.6,21.5,23.4,19.5,20.1,21,20.5,22.4,23.8,21.6,22.4,22,20.6,21.4,21.2,20.8,19.6,21,22.9,21.4,23.7,22.3,21.1,18.4,21.6,19.1,19.2,18.8,21.9,20.8
|y2=33.4,36.3,30.3,33.5,26.1,29.5,29.7,28.8,27.6,27.8,26.7,29,27.7,24.9,27.6,29,22,27.3,26.9,23.2,23.6,26.7,25.8,21.6,22.1,25.8,21.8,22.9,26.9,20,23.2,24.7,21.5,23.7,23.9,21.6,25.4,24.5,23.4,25.1,26,24.8,22.2,25.9,26.9,23.7,24.1,21.8,25.4,24.0,22.4,24.7,19.5,21.9,22.0,28.0
|y3=28.7,22.8,28.5,26.3,27.8,22.9,25.3,23.7,25.8,28.3,25,24.9,22.8,29.8,19.7,17.4,22.7,22.1,22.6,26.1,28,23,23,22.2,25.2,19.9,23.7,17.7,21.1,19.1,18.2,19.2,19.5,22.2,24.7,21.1,22.3,20.4,17.5,19.9,15.4,18,15.8,15.7,19.7,19.4,23.4,22.3,17.2,16.3,18.8,16.1,18.0,16.7,21.0,16.3
-->
}}
}}
Despite land-use planning and export of treated [[sewage]] effluent from the basin, the lake is becoming increasingly [[eutrophic]] (having an excessive richness of nutrients), with primary productivity increasing by more than 5% annually, and clarity decreasing at an average rate of {{convert|0.25|m|ft|order=flip}} per year.<ref>{{cite web|author=Lahontan Regional Water Quality Board|title=Lake Tahoe Basin Characterization & Assessment of Exemplary Programs for Water Quality Crediting and Trading Feasibility Analysis |url=http://www.enviroincentives.com/Tahoe%20Crediting_Trading%20Characterization%20and%20Cases%20draft%20v1.1.pdf |archive-url=https://web.archive.org/web/20120426023041/http://www.enviroincentives.com/Tahoe%20Crediting_Trading%20Characterization%20and%20Cases%20draft%20v1.1.pdf|archive-date=April 26, 2012|website=Water Quality Crediting and Trading Feasibility Study|publisher=Kieser and Associates|access-date=December 6, 2011 |url-status=dead}}</ref>  Until the early 1980s, nutrient-limitation studies showed that primary productivity in the lake was nitrogen-limited. Now, after a half-century of accelerated nitrogen input (much of it from direct atmospheric deposition), the lake is phosphorus limited. Theodore Swift et al.,<ref name=Swift>{{cite journal|first1=Theodore J|last1=Swift|first2=Joaquim|last2=Perez-Losada|first3=S. Geoffrey|last3=Schladow|first4=John E|last4=Reuter|first5=Alan D|last5=Jassby|first6=Charles R|last6=Goldman|title=Water clarity modeling in Lake Tahoe: Linking suspended matter characteristics to Secchi depth|journal=Aquat. Sci.|volume=68|year=2006|issue=1 |pages=1–15|doi=10.1007/s00027-005-0798-x|bibcode=2006AqSci..68....1S |s2cid=37395769}}</ref> concluded that "suspended inorganic sediments and phytoplanktonic algae both contribute significantly to the reduction in clarity, and that suspended particulate matter, rather than dissolved organic matter, are the dominant causes of clarity loss." The largest source of fine sediment particles to Lake Tahoe is urban stormwater runoff, constituting 72 percent of the total fine sediment particle load.<ref>{{cite web|title=Lake Tahoe Total Maximum Daily Load Report|url=http://www.waterboards.ca.gov/lahontan/water_issues/programs/tmdl/lake_tahoe/docs/tmdl_rpt_nov2010.pdf|publisher=California Regional Water Quality Control Board, Lahontan Region|year=2010}}</ref> Recent research has shown that the urban uplands also provide the largest opportunity to reduce fine sediment particle and phosphorus contributions to the lake. Historic clarity of approximately {{convert|30|m|ft|order=flip}} can be achieved with total reduction of approximately 75 percent from urban sources.<ref name=sahoo>{{cite journal|last=Sahoo |first=G. B. |author2=Schladow, S. G. |author3=Reuter, J. E. |journal=Water Resources Research |year=2010 |volume=46 |doi=10.1029/2009WR008447 |url=http://www.agu.org/pubs/crossref/2010/2009WR008447.shtml |title=Effect of sediment and nutrient loading on Lake Tahoe optical conditions and restoration opportunities using a newly developed lake clarity model |issue=10 |pages=n/a |bibcode=2010WRR....4610505S |s2cid=140172942 |url-status=dead |archive-url=https://web.archive.org/web/20120120034206/http://www.agu.org/pubs/crossref/2010/2009WR008447.shtml |archive-date=January 20, 2012 }}</ref>

Historically, the clarity of Lake Tahoe continued to decrease through 2010, when the average [[Secchi depth]], {{convert|64.4|ft|m}}, was the second lowest ever recorded (the lowest was {{convert|64.1|ft|m}} in 1997). This represented a decrease of {{convert|3.7|ft|m}} from the previous year.<ref name="sotl2011">{{cite web|url=http://terc.ucdavis.edu/stateofthelake/StateOfTheLake2011.pdf|title=Tahoe: State of the Lake Report|year=2011 |publisher=UC Davis Tahoe Environmental Research Center |archive-url=https://web.archive.org/web/20111208070952/http://terc.ucdavis.edu/stateofthelake/StateOfTheLake2011.pdf |archive-date=December 8, 2011 |url-status=dead}}</ref> However, the lake's clarity increased from 2011 to 2014, improving by nearly 20 percent.<ref>{{cite web|url=http://www.sacbee.com/2013/02/28/5224182/lake-tahoes-clarity-shows-gains.html|archive-url=https://web.archive.org/web/20130306111953/http://www.sacbee.com/2013/02/28/5224182/lake-tahoes-clarity-shows-gains.html|archive-date=March 6, 2013|title=Lake Tahoe's clarity shows gains for a second year}}</ref><ref>{{Cite web|url=https://www.rgj.com/story/tech/environment/2015/04/07/drought-helps-boost-lake-tahoes-clarity/25416091/|title=Drought helps boost Lake Tahoe's clarity|first=Jeff|last=DeLong|website=Reno Gazette Journal|access-date=February 15, 2022}}</ref>

A water quality study by the Lahontan Water Quality Control Board and the Nevada Division of Environmental Protection<ref>{{cite web|archive-url=https://web.archive.org/web/20150924063247/http://ndep.nv.gov/bwqp/file/LTTMDL_NDEP_Final.PDF|archive-date=September 24, 2015|url=http://ndep.nv.gov/bwqp/file/LTTMDL_NDEP_Final.PDF|title=Lake Tahoe Total Maximum Daily Load for Fine Sediment Particles, Nitrogen and Phosphorus|first=Colleen|last=Cripps|publisher=Nevada Department of Conservations and Natural Resources|date=August 3, 2011}}</ref> determined the largest source of fine [[sediment]] particles: 71 percent is developed area (urban) erosion and run-off, much of it associated with transportation infrastructure and services.<ref>{{cite web|url=http://ndep.nv.gov/bwqp/tahoe3.htm|archive-url=https://web.archive.org/web/20110916102925/http://ndep.nv.gov/bwqp/tahoe3.htm|archive-date=September 16, 2011|title=Final EPA Approved Lake Tahoe TMDL Report|publisher=Nevada Bureau of Water Quality Planning}}</ref>

Lake Tahoe is a tributary watershed drainage element within the Truckee River Basin, and its sole outlet is the Truckee River, which continues on to discharge to Pyramid Lake. Because of the sensitivity of Truckee River water quality (involving two protected species, the [[cui-ui]]<ref>{{cite iucn |author=NatureServe |date=2014 |title=''Chasmistes cujus'' |volume=2014 |page=e.T4586A3003395 |doi=10.2305/IUCN.UK.2014-3.RLTS.T4586A3003395.en |access-date=November 11, 2021}} Listed as Critically Endangered (CR B1+2b v2.3)</ref> [[Catostomidae|sucker fish]] and the [[Lahontan cutthroat trout]]), this drainage basin has been studied extensively. The primary investigations were stimulated by the [[United States Environmental Protection Agency|U.S. Environmental Protection Agency]], which funded the development of the [[DSSAM]] model to [[hydrology transport model|analyze water quality]] below Lake Tahoe.

Lake Tahoe never freezes.<ref>{{cite web |url=http://www.skiheavenly.com/lake-tahoe/tahoe-town-facts.aspx |title=Lake Tahoe Facts |access-date=October 3, 2010 |publisher=[[Heavenly Mountain Resort]] |archive-url=https://web.archive.org/web/20110106193151/http://www.skiheavenly.com/lake-tahoe/tahoe-town-facts.aspx |archive-date=January 6, 2011 |url-status=dead }}</ref> Since 1970, it has mixed to a depth of at least {{convert|1300|ft|abbr=on}} a total of six or seven times. Dissolved oxygen is relatively high from top to bottom. Analysis of the temperature records in Lake Tahoe has shown that the lake warmed (between 1969 and 2002) at an average rate of {{cvt|0.015|C-change|order=flip|}} per year. The warming is caused primarily by increasing air temperatures, and secondarily by increasing downward long-wave radiation. The warming trend is reducing the frequency of deep mixing in the lake and may have important effects on [[water clarity]] and nutrient cycling.<ref>{{Cite book |date=2022 |title=Scientific Investigations Report |chapter=Seasonal and long-term clarity trend assessment of Lake Tahoe, California–Nevada |language=en |doi=10.3133/sir20225070|last1=Naranjo |first1=Ramon C. |last2=Work |first2=Paul |last3=Heyvaert |first3=Alan |last4=Schladow |first4=Geoffrey |last5=Cortes |first5=Alicia |last6=Watanabe |first6=Shohei |last7=Tanaka |first7=Lidia |last8=Elci |first8=Sebnem }}</ref>

{{See also|List of Lake Tahoe inflow streams}}