NEVADA’S WATER RESOURCES
(Adapted from Nevada State Water Plan Part 1 - Background and Resources Assessment
March 1999 Nevada Division of Water Planning, Department of Conservation and Natural Resources, Carson City, Nevada)
Nevada is the driest state in the nation. It is also one of the fastest growing states in the nation. Water is one of the foremost concerns in the state, especially quantity and quality. The following overview of water resources in Nevada presents a general picture of the factors that influence Nevada’s climate and water resource distribution throughout the state.
The topography of Nevada and the surrounding areas makes for a unique and diversified climate. Nearly all of Nevada is in the Basin and Range Province of the Intermountain Plateaus, a rugged elevated area between the Rocky Mountains and the Pacific mountain system.
The topography of the Basin and Range province is characterized by isolated, long and narrow, roughly north-south trending, parallel mountain ranges and broad, intervening valleys. Internal drainage is a significant feature of the hydrology of much of Nevada. About 84 percent of the State is within the Great Basin in which drainage is to low areas in enclosed basins rather than to the sea.
The topography and related geology of the State has resulted in complex surface and ground water systems, complicating the management of these resources. The current hydrographic area map delineates 256 hydrographic areas within 14 major hydrographic regions and basins. Of the 14 hydrographic regions and basins, only the Snake River Basin and the Colorado River Basin drain to the sea.
Nevada is semi-arid to arid with precipitation and climate varying widely throughout the State. With temperatures that fall below -40°F in the northeast, and rise to over 120°F in the south, and precipitation that ranges from only three to four inches in Southern Nevada to over 40 inches (and over 300 inches of snowfall) in the Carson Range portion of the Sierra Nevada Mountains, Nevada is truly a land of constrasts
Three basic geographical characteristics are responsible for Nevada’s unusual and diverse climate:
Of the total annual precipitation falling in Nevada, approximately 10 percent results in stream runoff and groundwater recharge. The remaining 90 percent is lost through evaporation and transpiration. Like precipitation, evaporation is also widely variable. Average lake surface evaporation rates range from less than 36 inches per year in the west to over 80 inches per year in the south.
Surface water is a limited and precious resource in Nevada providing about 70 percent of the total water supply used in the state. Spring and summer snowmelt supplies most of the streamflow in Nevada. However, isolated summer convective storms probably cause a majority of the streamflow in southern Nevada’s low altitude basins.
Throughout the State, surface water flows can vary widely from year to year and from month to month, with maximum discharges generally in May and June as a result of snowmelt in the mountains. With the exception of the Humboldt Basin, most of the surface waters in Nevada’s rivers are the result of snowmelt occurring in other states such as California, Wyoming, Colorado, and Utah. Flows in the upper reaches of the larger rivers (Carson, Humboldt, Truckee, Walker) typically increase as one moves downstream. The larger rivers typically follow the flow pattern of a gaining stream in the well-watered mountain reaches and a losing stream in the lower-altitude reaches. Reductions in flow occur due to irrigation, public use, infiltration, and evapotranspiration.
Major Rivers, Lakes and Reservoirs
Nevada has very few large rivers and streams. With the exception of the Colorado River, Nevada’s perennial rivers are small. Rivers in the Snake River Basin and Colorado River Basin regions flow into the ocean, with the remaining streams systems discharging into terminal sinks and lakes with no outflow to the sea. The major river systems in Nevada are the Colorado, Walker, Carson, Truckee, and Humboldt. The Carson River flows from the eastern slopes of the Sierra Nevada in California and terminates in the Carson Sink. Waters of the Carson River are used predominately for agriculture from Carson Valley down to the Fallon area. Only a few regulating storage reservoirs exist in the basin, with Lahontan Reservoir being the largest. Lahontan Reservoir is used to store water from the Carson River, and water diverted from the Truckee River by Derby Dam and conveyed to Lahontan Reservoir via the Truckee Canal. Water released from Lahontan Reservoir is used predominately for agriculture, and wildlife purposes.
The Colorado River is the largest river in Nevada, flowing through Wyoming, Colorado, Utah, New Mexico, Arizona, California and Nevada. Along its 1,400 mile course to the Gulf of Mexico, the Colorado River Basin drains an area of about 240,000 square miles or about one-twelfth the area of the contiguous United States. The Colorado River and tributaries in Nevada provide a majority of the drinking water supply to the Las Vegas area, hydroelectric power and recreation opportunities at Lake Mead and Lake Mohave, and water for agricultural purposes.
Water Yields, Committed Resources and Droughts
The estimated average annual yield from Nevada’s surface water systems is approximately 3.2 million acre-feet per year. Generally, Nevada’s surface water sources, such as lakes, streams and springs, have been fully appropriated and used for many years. In some instances, water may be available from these sources during high water years, however storage facilities would be required to capture the surplus flows for later use.
Most priority rights for surface water in Nevada were established in the 1800s. Rights to use water for irrigation date back to the 1850s in streams draining the Sierra Nevada Mountains and to the 1870s and 1880s in the Humboldt River Basin. Nevada is a land of extremes, with droughts and floods common in our highly variable climate. Years of average streamflows are rarely experienced. Periods of high flows followed by low flows are more the norm in Nevada. BACK
Groundwater in Nevada is an important water supply source. The surface water resources in our state have been virtually fully appropriated and future development must rely on either ground-water sources or the reallocation of surface water supplies.
Groundwater provides about 40 percent of the total water supply used in Nevada and in some areas provides the entire supply. The extent to which groundwater is used may vary considerably from year to year. In many areas, groundwater is pumped to supplement surface water sources. As a result, groundwater usage in these areas increases during periods of low streamflow and decreases during high runoff periods.
Principal Ground-water Aquifers
Principal ground-water aquifers in Nevada are basin-fill aquifers, carbonate-rock aquifers, volcanicrock aquifers, and volcanic- and sedimentary-rock aquifers. The basin-fill aquifers, composed primarily of alluvial, colluvial and lacustrine deposits, are the major aquifers in the State. Virtually all major ground-water development has been in the basin-fill aquifers with the withdrawals from the upper 500 feet of these aquifers. In eastern and southern Nevada, thick sequences of carbonate rock underlie many of the alluvial basins forming a complex regional aquifer system or systems that are largely undeveloped and not yet fully understood. The carbonate-rock aquifer supplies water to numerous springs which are used for irrigation.
Volcanic-rock aquifers extend over hundreds of square miles but only one volcanic-rock aquifer in the Carson Desert (Churchill County) of west-central Nevada has been developed as a municipal water supply. Within the Basin and Range Province, aquifers are generally not continuous, or regional, because of the complex faulting in the region. Of the aquifer types discussed above, any or all may be in, or underlie, a particular basin and constitute separate sources of water. However in some instance, interconnection between the aquifers may exist.
Years of below average flows in rivers are not uncommon and many water users are prepared to cope with one year of low streamflow by resorting to supplemental sources such as reservoirs and groundwater. For most of Nevada’s water users, who depend mostly upon surface water, problems can begin to occur when below average flows are experienced for two or more consecutive years. Over time, reservoir and groundwater levels tend to decline due to increased uses and these supplemental sources may become depleted. Droughts can also create quality problems for both surface water and groundwater sources. The decreased flows experienced during a drought tend to result in diminished quality for the remaining water.
Nevada’s surface water quality is regulated by the Nevada Division of Environmental Protection (NDEP) and the State Environmental Commission (SEC). The quality of surface water in Nevada varies greatly from location to location and from month to month with changes in flows. In planning, both water quantity and quality need to be considered concurrently. In general, constituent concentrations vary with changes in streamflow. Similarly, lake water quality is impacted by water levels in the State’s terminal lakes. Total dissolved solids concentrations have increased in Walker and Pyramid lakes as the volume of flow to the lakes has decreased
The impacts on water quality from the municipal and industrial discharges have been greatly reduced over the last few years, with most point source polluters eliminated from direct discharges or stringently controlled. Nonpoint source pollution due mainly to agriculture, urban runoff and hydrologic modifications impacts various waters of Nevada. Water quality parameters of concern include nutrients, suspended solids, turbidity and bacteria which are being targeted in the State’s Nonpoint Source Program administered by NDEP. Water quality has been improving due to the removal of point sources and the implementation of more stringent standards. The Nonpoint Source Program helps to further improve water quality by promoting public awareness, improved grazing
and irrigation practices, erosion control measures and the implementation of best management practices.