Lake Level Watch
Annual Lake Decline Halted by Fall Storms
by Greg Reis
As usual, Mono Lake dropped at the end of the summer and began its annual rise as winter approached. Why does this pattern happen almost every year? Because Mono Lake is a terminal lake. This means that it has no outlet, and as a result it fluctuates over a wider range than most natural lakes, which have stable outlet elevations.
Within the course of a year, the lake rises and falls as a result of seasonal fluctuations in streamflow and evaporation. When inflow exceeds evaporation, the lake rises. When inflow is less than evaporation, the lake drops. Mono Lake typically rises from winter into spring and, during wet years, into summer. Then, as snowmelt slows, evaporation losses are not completely replaced by freshwater inflow, and the lake drops.
This year, with runoff at about 120% of average, Mono Lake peaked in August at 6,380.1 feet. It began dropping in late August, reached an October low of 6379.6, and, as of November 13, stands at 6,379.7. This year's winter rise in lake level has thus begun, a little earlier than usual thanks to several strong storms in October and November. Two feet of snow fell before Halloween, and a weather pattern with a tropical connection delivered over four inches of rain to Lee Vining in the first half of November (the lake's response to the rain hasn't yet registered in the lake level readings).
DWP began diverting water from the Mono Basin to Los Angeles on July 16, and the diversions ended in mid-November. The State Water Resources Control Board allows DWP to divert 4,500 acre-feet in years when Mono Lake's April 1 elevation is between 6,377 and 6,380 feet above sea level, as it was this year. To get a feel for how much water this is, consider this: if the diverted water had gone into the lake, the level would be a tenth of a foot higher (assuming that none of this water was lost along the way through evaporation, infiltration, or use by plants).
By April 1, 1997, Mono Lake should rise to at least 6,380 feet above sea level, according to Peter Vorster, the Mono Lake Committee's consulting hydrologist. Even during the driest winters, the lake has risen half a foot. DWP forecasts agree, predicting that with median precipitation through April, Mono Lake will stand at 6,380.3 feet above sea level, a level that will allow diversion of 16,000 acre-feet of water next year. What actually happens, of course, all depends on the weather; the wetter the winter, the higher the lake will rise!
Greg Reis is the Committee's Information Specialist. He has become a master of spreadsheets.
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