Sunrise light on a grove of tufa towers emerging from the water of Mono Lake with soft green and dusty-red wild grasses in the foreground, Canada geese in the shallow water with reflections of the rocky towers, and desert hills in the distance.

Hydrology update: 88% runoff forecast

The Mono Lake Committee found out this past week that the Los Angeles Department of Water & Power (DWP) is forecasting 88% of average runoff for the 2009 Runoff Year (April 1, 2009–March 31, 2010) for the Mono Basin. Currently, the Mono Basin snowpack ranges from 89–102% of average.

Last year, DWP forecasted 86% of average runoff, but actual runoff turned out to be much less—about 64% of average based on preliminary numbers. This huge discrepancy was due to the lack of a May 1st forecast update and also due to the unusual pattern of last year’s snowpack. There was less snow at high elevations—seemingly stripped away by high winds during windier-than-usual snowstorms. So after the middle-elevation snow melted in early summer, the late-summer runoff was much less: June–July runoff was 80% but August–September was only 50%.

The result of 64% of average runoff coming into Grant Lake Reservoir but 86% of average operations in releasing water from the reservoir has emptied the reservoir to the lowest levels since 1977 (although by next week the rising reservoir will reach 1988 levels). If the forecast had been updated in May, the Mono Lake Committee believes 75% would have been a likely outcome, resulting in operations that would have brought Grant Lake Reservoir about 5,000 acre-feet higher today. That is still much lower than it was last year, and still in the range where problems are caused both above and below the dam. In particular, warm water temperatures and high turbidity caused by the low reservoir level threaten the health of the trout.

In order to get the reservoir as high as possible before the warm summer water temperatures threaten to kill fish, the State Water Resources Control Board has issued a temporary change in its minimum flow for Rush Creek below the dam from 47 cubic feet per second (cfs) to 22 cfs. This change, requested by the independent scientists in charge of the stream restoration program, keeps about 50 acre-feet per day—or about 1,500 acre-feet per month—in the reservoir instead of being released down Rush Creek to Mono Lake. DWP refused to defer export of its annual 16,000 acre-feet of water—it plans to export about 20 cfs during April–June but stop during July–September for a construction project. In many years when the reservoir was low DWP deferred spring-summer water exports, and it is mysterious why DWP is not deferring springtime export this year, when the reservoir is much lower—with potentially dire consequences for the fishery downstream. The Mono Lake Committee continues to push this question—why defer exports for a construction project, but not to protect the Rush Creek fishery?