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: Dry July despite thunderstorms

Work in progress on the MGORD. Photo by Greg Reis.
Work progresses on the Grant Lake Reservoir outlet gate to Rush Creek. Photo by Greg Reis.

Mono Lake dropped a 1/4 foot in July, and stood at 6382.3 feet above sea level on August 1st. This is very similar to the forecasted July drop in lake level of 0.2 feet.

Lee Vining precipitation in July was below average, despite a month full of thunderstorms. Average rainfall is about 1/2 an inch—only 0.12 inches fell, total. This followed a wet May and June, so April-through-July precipitation is slightly above the 2-inch average.

At the end of June, total Rush Creek runoff was almost exactly as forecasted—about 101% of average—but the monthly pattern wasn’t. April and May had higher runoff than forecasted and June was lower than forecasted, mainly as a result of above-average temperatures in April and May followed by below-average temperatures in June.

By the end of July most of the snow had melted. July runoff for all the creeks was about 65% of average—slightly below the 74% of average forecast for the month. This brings the total April-through-July runoff for Rush, Parker, Walker, and Lee Vining Creeks to about 89% of average compared to the 91% of average that was forecasted.

Grant Lake Reservoir rose rapidly from its extremely low February level and peaked on August 4th, holding 37,700 acre-feet of water—another 10,000 acre-feet would cause it to spill. This is about 5,000 acre-feet more than predicted thanks to Southern California Edison (SCE) keeping Rush Creek Meadows (Waugh) Reservoir empty this summer for repairs. It also rose fast because there was no peak flow delivered downstream of Grant Reservoir, and because the Los Angeles Department of Water & Power deferred water export during reconstruction of the Grant Lake Reservoir outlet gate to Rush Creek. Export of the 16,000 acre-feet of water that is allowed to be taken out of the Mono Basin will resume when construction is finished at the end of August.

Grant Lake Reservoir is dropping since the snowmelt inflow has subsided, and that drop will quicken when water exports resume at the end of the month. The fall in level will slow down as SCE begins its annual draining of Gem Lake Reservoir in September, and will slow even more if the State Water Resources Control Board approves a planned temporary reduction in the minimum flow below Grant Reservoir that could start in November.