Extremes likely to continue
The water year 2022-2023 will certainly be a wet year. Lake Tahoe and the Truckee River basin both are near or above 200% of snow pack water content for this time of the year. The question is, how much of this exceptional snow pack and water will flow downstream to Pyramid Lake?
Lake Tahoe: critical for the Truckee River flows
Lake Tahoe is the beginning of the Truckee River. Over the millennia Lake Tahoe was a natural regulating reservoir. Today a dam constructed on the river artificially raises the level of the Lake and controls how much water from the Lake flows into the Truckee River. No water flows into the Truckee River unless the elevation of the water surface elevation of the Lake rises above the outlet’s rim at elevation 6,223 feet. The dam allows the level of the water in Lake Tahoe to rise 6.1 feet higher and store up to 740,000 acre-feet of water. The rise in the Lake’s water surface elevation is most obvious to us when stored water covers up a significant part of beaches such as Sand Harbor State Park or Zephyr Cove. The Federal Water Master determines the amount of water to release into the Truckee River at any given time.
Today (3/29/23) as I write this, only 46 CFS (cubic-feet-per-second) flows out of Lake Tahoe through the gates of the dam and into the Truckee River. Under natural conditions at the Lake’s current water surface elevation of 6,225.4 ft (2.4 feet above its natural rim), much more water would be flowing into the Truckee River. The dam allows farmers and other downstream water users to delay flows into the Truckee River to better suit their needs.
Dams delay and reduce spring flows of the Truckee River
Fish and wildlife throughout the Truckee River basin evolved to take advantage of high spring time river flows that now are controlled by the 6 major dams on the river and its tributaries. Lake Tahoe is the largest storage reservoir on the river. Together, the reservoirs can store nearly 1.1 million acre-feet of water – or nearly two years of the average long-term flow volume of the Truckee River. In early March 2023 about 360,000 acre-feet of water was stored in the 6 reservoirs – most of it in Lake Tahoe. Once snow melt commences, the all the reservoirs will likely be filled as close to their capacity as possible.
From Dry to Wet to Dry to Wet?
Dry years or below “average” years since 2000 dominated the weather for much of the southwestern USA including California and Nevada. Well above average winters benefitting flows in rivers and storage in Lakes and reservoirs allow us all to breathe a sigh of relief hoping that the “drought is over”. Dry or drought conditions appear to be getting drier and perhaps wet conditions are likewise getting wetter. Extremes may be increasing on both ends. Climate change drives both very likely.
Above average or “wet” (water) years occurred in 2016-17 and 2018-19. Dry years occurred in 2019-20, 2020-21, and 2021-22. The graph shows that up to the middle of March 2023, Lake Tahoe has once again begun to rise. With a significant snowpack now at the end of March, it will likely continue to rise. What isn’t clear is how that will translate into flows for the Truckee River below the dam at Lake Tahoe.
The 5 year drought that proceeded the very wet 2016-17 water year (from October 1 to September 30) left Lake Tahoe nearly a foot below its rim. With significant rain and snow that year Lake Tahoe rapidly rose to near its maximum water surface elevation. Over 314,000 acre-feet of water flowed that year into the Truckee River and the following two years saw an additional 525,000 acre-feet of water. The next 2 years of drought and hot summers emptied Lake Tahoe’s stored water. 2021-22 water year started off wet, but the following January 2022 was the driest on record followed by very dry February and March. Lake Tahoe was quickly drained again.
However, this year the snowpack is nearly double average and colder weather has suppressed the snow melt. As of today, 163 inches of snow stands at Ward Creek on the west side of Lake Tahoe. That snow contains the equivalent of 70 inches of water – just under 6′ of water content. The official “end of winter” measurement of comes in 2 days.
Starting in mid-October 2023, Lake Tahoe fell below its outlet (rim) for just over 2 months. Atmospheric river rain and snowstorms rapidly raised its level by over two feet.
With the approach of April, below average temperatures are expected to persist for the next week or so with more snow also possible over the weekend. Presently, the watershed contains 226% of average snow water equivalent.
Under purely natural conditions all the water in Lake Tahoe above its rim flows into the Truckee River and downstream to Pyramid Lake each year. Pyramid Lake’s water surface elevation stands significantly below its historical level and well below it level in 2000.
How much of this year’s abundant snowpack’s water will flow to Pyramid Lake?