Water-quality characteristics and nutrient and suspended-sediment loads, Carson River and Truckee Canal, western Nevada, water year 1980

Abstract

Lahontan Reservoir, which stores water for irrigation in the nearby Fallen area, is a major recreational site in the Carson River basin of western Nevada. The reservoir is fed primarily by the Carson River and the Truckee Canal. The latter allows water from the adjacent Truckee River basin to flow into the reservoir. Water quality in both basins is strongly influenced by agriculture and urbanization. In the Carson River during water year 1980, specific conductance tended to increase in a downstream direction, with the highest mean value occurring below the reservoir. Values of pH generally ranged from 7.0 to 8.8. Water temperature followed seasonal and diel cycles in response to meteorological conditions. Dissolved-oxygen concentrations were greater than 5 milligrams per liter. Among the major ions, calcium and sodium were the dominant cations and bicarbonate the dominant anion. Inorganic nitrogen and phosphorus concentrations in the river increased in Carson Valley and between the sampling stations near Carson City and at Deer Run Road. The same nutrients, however, were reduced between Deer Run Road and the station near Fort Churchill. The inorganic nitrogen input to Lahontan Reservoir from the Truckee Canal was greater than that from the Carson River. Measured suspended-sediment concentrations in the river ranged from 3 to 1,790 milligrams per liter during water year 1980. Excluding flow resulting from the January storm, about 95 percent of the concentrations were less than 500 milligrams per liter. The maximum ultimate carbonaceous biochemical oxygen demand determined in the Carson River and Truckee Canal during water year 1980 occurred in January, with concentrations ranging from 10.4 to 19.6 milligrams per liter. All other values ranged from 1.2 to 10.8 milligrams per liter. Average carbonaceous deoxygenation rates ranged from 0.10 to 0.14. The carbonaceous biochemical oxygen-demand load decreased in the Carson River between Carson City and Lahontan Reservoir. The load entering the reservoir was approximately two times greater than the output from the reservoir. The codominant algal groups in the river above the reservoir were diatoms and blue-green algae, whereas those in the Truckee Canal were diatoms and green algae. An algal bloom caused by Aphanizomenon occurred in the reservoir during the summer of 1980. This blue-green algae was also the dominant genera below the reservoir during that summer. -1Background nutrient loads (those upstream from most agriculture and urbanization) in the river during 1980 were about 490 tons for total nitrogen and about 130 tons for total phosphorus. An increase to about 670 tons of total nitrogen and 230 tons of total phosphorus was observed between the background load and the Carson City station. The total nutrient loads remained nearly constant between the Carson City and Fort Churchill stations. The background suspended-sediment load for the river in 1980 was about 200,000 tons, increasing to about 210,000 tons at the Carson City station. The load decreased to 170,000 tons at the Deer Run Road station and then increased to about 230,000 tons at the Fort Churchill station. During water year 1980, the total nitrogen load to Lahontan Reservoir was about 980 tons, the total phosphorus load was about 300 tons, and the suspended-sediment load was about 300,000 tons. The estimated long-term average loads of total nitrogen and total phosphorus were about 680 and 180 tons, respectively. Loads to the lower Carson River from the reservoir during 1980 were about 820 tons for total nitrogen, 180 tons for total phosphorus, and 26,000 tons for suspended sediment.