Before 1850, “Carson Lake” was commonly a lake surrounded by extensive wetlands. That was not favorable for fairy shrimp. Due to occasional abandonment of the South Branch of Carson River in the 1860s, the lake might have dried up. That would be more favorable for fairy shrimp. However, an 1885 map of the Carson Desert by I.C. Russell of the U.S. Geological Survey shows the lake and wetlands much the same as depicted on the General Land Office survey plats of the 1870s (Kerley and others, 1993, Figure 2 on p. 8).
With the Newlands Project, river flows were rerouted into canals and spread across large areas of farm land to water crops or evaporate. That would likely have dried up “Carson Lake” and its wetlands and opened up areas for ephemeral ponds. But then water-logged soils became a problem so an extensive network of drains was constructed. Some drains were routed to “Carson Lake” for evaporation of the irrigation returns and that made things wet again. In 2023, the potential for fairy shrimp on “Carson Lake” Playa is seemingly no better than before 1850.
Before the relatively recent water purchases, essentially all of the surface water at “Carson Lake and Pasture” was drain water. Drains of the Newlands Project can inundate “more than 90 percent of the CLP [“Carson Lake and Pasture”]” (U.S. Bureau of Reclamation, 2009, p.12). This allows for a mix of irrigated pasture, marsh, and open water over most of the area. However, the accumulation of the dissolved solutes from evaporating drain waters over time could kill off much of the vegetation. To prevent that, “Each of the water units can be separately managed and drained – for water quality management – to the South Sump, where the water evaporates each summer and the resultant salts are blown out of the area by the prevailing southwest winds” (U.S. Bureau of Reclamation, 2009, p.13). Such winds would miss Fallon but could impact the Fallon Paiute Shoshone Reservation and Stillwater National Wildlife Refuge.
By design, a terminal sump for irrigation return flows accumulates all the constituents of the solids dissolved in the irrigation water. Like the saying goes for Las Vegas, whatever solutes reach “Carson Lake and Pasture” stay in “Carson Lake and Pasture”. Although strong winds commonly raise dust clouds over Nevada’s playas, quantitative removal of a large proportion of the precipitates from evaporated water as Bureau of Reclamation suggested is questionable. Some of the highly concentrated waters are likely to seep into the ground before the last drop evaporates. The accumulation of such waters could explain why shallow ground water has concentrations of many solutes that are 100 times or more higher than those in irrigation water. Another proportion of the constituents is taken up by plants and animals and is recycled into the water and soil upon decomposition. Finally, occasionally extensive flooding in and around “Carson Lake”, such as in 2017 and 2023, redissolves the constituents and transports them to wherever the flood waters go. The submergence of the dikes in 2023 (personal observation from US 95) allowed the redissolved constituents to literally cover all of “Carson Lake”. Such redistribution of the accumulated solutes could eventually have a negative impact on the vegetation.
Some of the constituents dissolved in the irrigation return flows are not benign. “Studies in the 1980s indicated that samples of water, bottom sediment, and biota in areas affected by irrigation drainage contained concentrations of arsenic, boron, mercury, and selenium that exceeded naturally occurring levels or Federal and State criteria” (U.S. Bureau of Reclamation, 2009, p. 13). Potentially harmful concentrations of arsenic, boron, and selenium could have been dissolved from the sediments of the area or concentrated by evaporation of Carson and Truckee river waters. Mercury is a different story.
Mercury was spilled during mining activities in the latter 1800s and has been spread by flooding ever since. The Carson River Mercury Superfund Site stretches from the New Empire/Morgan Mill about 6.5 km (4 miles) east of Carson City, down the Carson River, through “Lahontan Reservoir”, and all the way to Stillwater and Fallon National Wildlife Refuges (U.S. Environmental Protection Agency, 2020; Nevada Division of Environmental Protection, 2012, slide 2).
Consequently, the water of “Carson Lake and Pasture” is subject to a confluence of damaging processes: dissolution of mercury from irregularly distributed, mercury-contaminated mill tailings of the Carson River Mercury Superfund Site, leaching of potentially harmful trace elements from sediments, and evaporative enrichment of all dissolved solutes to potentially harmful levels.
Evaporation starts at “Lahontan Reservoir” with TDS averaging about 120 mg/L in 1966-1971 (n=14 at 4 sites with averages of 118, 118, 116, and 119 mg/L, range at all 4 sites 151-238 mg/L; samples collected during spring and summer from depths less than 31 cm, 1 foot) (Glancy and Katzer, 1976, Table 34, p. 82). The average of 4 samples collected from the Carson River below Lahontan Dam in May, August, and September of 1986-1988 was higher at 213 mg/L (range 169-242 mg/L) (Hoffman and others, 1990; Rowe and others, 1991). The apparent increase may be due to the non-representativeness of the small sample sets as the collection of samples during the same seasons should reduce the effects of evaporation. It could also be due to annual variations in water quality for weather-related or other reasons. Evaporation from canals, flood-irrigated crop lands, and drains has surely affected the water by the time it gets to “Sprig Pond” at “Carson Lake”. 3 samples collected there in August and September of 1986-1988 averaged 783 mg/L TDS (range 484-971 mg/L) (Hoffman and others, 1990; Rowe and others, 1991). 3 March samples had higher TDS concentrations of 1,730-4,630 mg/L because the water in drains at that time of year is exclusively ground water seepage.
The ground water under “Carson Lake and Pasture” is much worse than the surface water. 14 wells were drilled in the northeastern corner of “Carson Lake and Pasture” in 4 clusters of 2-3 wells each to sample the ground water. Each cluster had wells of different depths, ranging from 4.6 m to 10.7 m (15-35′). TDS does not consistently increase or decrease with depth and the minimum TDS differs from the maximum by only 10% or less. The TDS averages of the 4 clusters were 31,340, 36,900, 41,160, 41,670, and 48,900 mg/L (wells 1-14 in Township 17 North, Range 29 East, Section 12 in Whitney, 1993). Typical seawater has a TDS of about 35,000 mg/L. Such high TDS could be due to decades of evapotranspiration near the surface and to flushing in wet years of solids left behind by water dried up in the unsaturated zone during dry years. It seems less likely that the high-TDS ground water flowed laterally to beneath “Carson Lake and Pasture” from somewhere else or that it flowed upward from a deeper zone. Ground water flowing upward cannot have received recharge from the Newlands Project and ground water flowing laterally would likely have been recharged on hills or alluvial fans west of “Lahontan Reservoir”. The very high TDS would then be a long-term natural feature and that would be inconsistent with the relatively fresh character and thriving fish populations of pre-1880 “Carson Lake”.
As the end destination for irrigation water distributed over some 230 square km (56,800 acres), “Carson Lake and Pasture” receives rather poor quality water and makes it worse.
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