Getting Mono Lake to rise

Answers to common questions we’re hearing about the low lake

From the shore of Mono Lake to the streets of Los Angeles, the Committee had a busy summer answering questions about the current low lake level, its causes, and what the future holds.

In 1994, nearly 30 years ago now, the California State Water Resources Control Board issued its landmark water rights decision after extensively reviewing the devastating impacts of excessive stream diversions by the Los Angeles Department of Water & Power (DWP), which began in 1941 and continued for decades. As a result, Mono Lake dropped more than 45 vertical feet, lost half its volume, doubled in salinity, and motivated a citizen-driven protection effort led by the Mono Lake Committee.

After extensive scientific analysis and consideration, the State Water Board required that Mono Lake be restored to a level that protects its Public Trust values—6392 feet above sea level—and that its tributary streams and wetlands be restored. The Board mandated that level to protect the lake ecosystem from collapse, safeguard millions of birds, prevent predators from accessing nesting islands, and cover the dusty exposed lakebed that generates some of the worst particulate air pollution in the nation. To achieve the sustainable lake level, the Board reduced DWP’s stream diversions and expected that in about 20 years Mono Lake would be higher and healthier, with the capacity to endure droughts.

The Board acknowledged that its plan to raise the lake could be thrown off schedule by actual climate and hydrology in the years following the decision. The Board wisely said that if the lake had not risen as expected, a hearing would be held to see if the stream diversions allocated to DWP might need adjusting to solve the problem.

Today Mono Lake is only 25% of the way to the 6392-foot Public Trust lake level; therefore, the State Water Board will hold a hearing to look at changes to DWP’s annual stream diversion amounts.

The Committee has been modeling projections of the lake’s possible levels under different stream diversion scenarios. If stream diversions continue unchanged, Mono Lake will not rise to 6392 feet in the next 30 years. The same is true if diversions are reduced by half—Mono Lake will not rise enough.

However, pausing diversions would ensure that Mono Lake rises to its healthy management level.

After years of waiting and watching to see if Mono Lake would reach the Public Trust lake level under the current State Water Board rules, it is time to take a new course. We have heard from many members and friends that you have questions about this new path, so we have set out to answer the most common ones here.

Is Mono Lake’s low level due to the drought or DWP’s diversions?

Mono Lake is at an artificially low level today because of past decades of unrestrained DWP water diversions. It is true that Mono Lake naturally goes up and down in response to wet and dry years, and wetter-than-average years are required to meaningfully raise the lake. However, if DWP had never extended the Los Angeles Aqueduct to the Mono Basin, hydrologic models show that Mono Lake would be nearly identical in size and volume to its pre-diversion level, despite recent droughts and climate change.

We have recently seen DWP statements in the press suggesting its water exports have no impact on lake elevation, which seems to be an attempt to ignore history.

The State Water Board already reduced DWP’s stream diversions—are the remaining exports really affecting lake level?

Yes. In 1994 the State Water Board reduced DWP’s stream diversions to a level that, models at the time projected, would provide water to the city while the lake rose to the ecologically healthy level of 6392 feet in elevation—the Public Trust lake level. Since 1994, DWP has diverted 381,000 acre-feet of surface water under those rules, supplemented by 151,000 acre-feet of Mono Basin groundwater captured in the Mono Craters tunnel—a total that is enough to supply the City of Los Angeles for an entire year. Lake level model projections at the time suggested this would all work out in 20 years, but when we look back to the actual pattern of wet and dry years we see that these exports have prevented Mono Lake from rising as expected.

For comparison, if DWP had diverted no stream water since 1994, the city still would have benefitted from its export of Mono Basin groundwater and, at the same time, our model projections show Mono Lake would have risen substantially to around 6388 feet before the recent drought—almost nine feet higher than it is today, creating significant benefits for wildlife, birds, nesting gulls, and air quality.

What will happen to Mono Lake if the drought continues?

Mono Lake will fall lower if the drought, now in its third consecutive year, continues. We expect the current dry year to result in an April 1, 2023 lake level of around 6379 feet—about a foot lower than the same date in 2022.

If the drought continues next year, Mono Lake could fall below the 6377-foot “emergency” level that causes stream diversions to be halted. If the drought continues another year, by the end of 2024 the lake could be a foot or two lower than that. This is a serious problem that would significantly increase the landbridge size, imperiling the gull colony; increase salinity to worrying levels that violate the Clean Water Act; and expose 2,000 more acres of dry dusty lakebed, likely increasing the severity of already hazardous dust storms that violate air quality health standards.

If the lake had entered the drought at the Public Trust lake level, consecutive dry years would pose less of a problem because the lake would be more productive, more resilient, and better buffered from these dire ecological consequences.

Does groundwater captured in the Mono Craters tunnel count toward DWP’s export maximums?

No. The State Water Board rules only apply to the diversion and export of water flowing in the tributary streams. The Mono Basin groundwater captured in the tunnel is unregulated. However, groundwater export deprives Mono Lake of inflow, thus lowering lake level, which in turn affects the rules for stream diversions. If stream diversions were paused while the lake rises to the management level, DWP would still benefit from the export of an average of 5,500 acre-feet of Mono Basin groundwater annually.

Since the State Water Board decision has DWP ever taken steps to boost the lake level toward the requirement, like voluntarily reducing diversions?

No, DWP has taken the maximum stream diversions allowed each year since 1994. The Committee has discussed voluntary reductions with DWP a number of times—extremely wet years, like 2017, when LA’s supply is plentiful are obvious times to divert less—but DWP has chosen to divert the maximum allowed each year.

Will I see Mono Lake rise to the ecologically healthy level of 6392 feet in my lifetime?

We certainly hope so. The future pattern of wet and dry years is, of course, unknown, but the Committee’s hydrologic model shows that the lake will rise much faster if it’s not held back by DWP diversions.

Wet years are the key. The abundant snowmelt from the mountains can add multiple feet to the lake’s surface elevation relatively quickly—that’s what happened in 2017 when the lake rose 4.5 feet in nine months. Preserving the gains made in wet years is essential, and history has shown that the current stream diversion rules allow the progress made to be eroded. The lake will rise—but only if we get new stream diversion rules in place and ensure that DWP follows them.

This post was also published as an article in the Fall 2022 Mono Lake Newsletter. Top photo by Ryan Garrett: Robbie and Maureen waded out to get an accurate lake level measurement on March 30, 2022.