Managing the Supply
How the Truckee River Operates
From its headwaters high in the Sierra Nevada Mountains, the Truckee River flows through a 145-mile course starting at Lake Tahoe. Water meanders through two states and multiple counties before finally terminating at Pyramid Lake, which is under the jurisdiction of the Pyramid Lake Paiute Tribe.
From ensuring a vital ecosystem for fish and wildlife, water also serves as a source of vitality for the communities within the Truckee River Basin. Along its path, the river also generates hydro-power, irrigation supplies, and offers recreational opportunities benefitting residents and tourists alike.
How the Truckee River is managed is extremely complex, and is outlined in the Truckee River Operating Agreement (TROA). This agreement is balanced to serve multiple users and the legal demands placed upon the river.
The ability to use water from the Truckee River comes from water rights. These rights have been established through court decrees based on historic diversions of the river for power generation and irrigation use as far back as the late 1850’s. The number of Truckee River and surrounding creek water rights was established with the Orr Ditch Decree in 1944. No new water rights have been, or can be, created. The ownership and use of those rights are merely changing over time.
The chart below demonstrates the set number of water rights available on the Truckee River. The shifting colors reflect the transition of water rights categorization over time, mostly agricultural to municipal use.
Overview of Lakes and Reservoirs
Water supply in the Truckee Meadows primarily comes from the Truckee River. The source of the river comes from Lake Tahoe, North American’s largest alpine lake, which is fed by 63 tributaries. The Truckee River is Tahoe’s only outlet.
When Tahoe is higher than its natural rim (elevation 6223 ft.), water is retained by the Tahoe Dam. The volume of water released from the dam is highly regulated according to terms of the Truckee River Operating Agreement. See the history of Lake Tahoe levels from 1986 to 2019 in the graph below.
From Tahoe, water that flows downstream into the Truckee Meadows is stored in five locations that include two lakes (Donner and Independence) and three reservoirs (Prosser, Stampede and Boca). All water stored at Donner and Independence Lakes is owned by the Truckee Meadows Water Authority and serves as drinking water reserves for TMWA customers in Reno, Sparks, and Washoe County.
Downstream of Reno and Sparks, a diversion at Derby Dam also sends irrigation water through the Truckee Canal toward Lahontan Reservoir. The natural terminus of the Truckee River is Pyramid Lake. All water stored and flowing from the Truckee River system is federally managed.
Managing River and Groundwater Together
Using groundwater as a complimentary source of water is an effective way to add redundancy to the drinking water supply in the Truckee Meadows. It is an important strategy used by the Truckee Meadows Water Authority for water resource management.
Managing surface and groundwater together as one integrated resource is called Conjunctive Use Management. This strategy has resulted in increased groundwater levels in aquifers that had been in decline for decades. This happens through the distribution of surface water into areas formerly reliant upon wells.
In fact, conjunctive use can yield more water for collective management than trying to separately manage surface-water or groundwater systems.
As part of this approach, TMWA generally distributes as much surface water as possible year-round and then brings groundwater production wells online when customer demand increases during the summer.
When demand tapers down in the winter, this is typically when wells are recharged with treated surface water. Over time, this can improve groundwater levels and offset the effects of summertime pumping.
Recharging groundwater with treated surface water is also known as Aquifer Storage and Recovery (ASR). TMWA’s ASR program is monitored annually as part of its overall Conjunctive Use Management approach.
Learn more about Water Topics in Our Community: Recharge Program
Understanding Stored Reserves
In April of each year, the federal administrator of the Truckee River Operating Agreement (TROA) determines whether a season will be classified as 'normal', 'dry', or if a 'drought situation' exists. This determination directly sets capacity for reservoirs on the Truckee River.
When river flows cannot be maintained during drier years, Truckee Meadows Water Authority may release its privately owned water reserves into the Truckee River from Independence or Donner Lakes in coordination with the Federal Water Master, or it may augment the water supply with increased groundwater pumping.
During designated drought years, water stored in upstream reservoirs can carry over as additional stored water into the next year. In this way, water saved through conservation and not used for growth can greatly offset the impact of drought to the region.
In multi-year drought scenarios, TMWA can accumulate significant drought reserves, providing protection that simply did not exist until the 2015 passage of TROA. Read more about TMWA’s Drought Contingency Plan.
Meeting Future Water Demand
Future Water Demand
Estimating future demand is largely a function of projected population growth. For example, based on a continuation of population trends for the next 20 years, Truckee Meadows Water Authority projects a water demand increase of approximately 15%. When looking at these projections under multiple drought and climate scenarios, results show that the region's water supply is extremely resilient. These results are published in TMWA's 2020-2040 Water Resource Plan.
Future Water Supply
Modeling the most extreme drought on record under a range of future climate conditions, scenario results show that the water supply for the Truckee Meadows can withstand a repeat of any of the major droughts of record.
Below, an 80-year run scenario simulates more frequent and severe droughts, indicated by Lake Tahoe levels. The bar chart demonstrates how water demand (as projected by population growth) would be met with less reliance on the Truckee River by tapping into privately owned storage reserves and groundwater.
This is a clear demonstration of the importance of conjunctive use, and the resiliency offered in drought conditions under the Truckee River Operating Agreement (TROA) in the overall management outlook for our future water supply.