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Hydrology Update: Snowpack catches up to average

March 13th, 2009 by Greg, Information & Restoration Specialist
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SNOWPACK
Some snow sensors in the Mono Basin are reporting above-average snowpack! Gem Pass snow water content is 108% of the March 10th average! This is 96% of the April 1st average–the typical date of peak snowpack in the Sierra. Most other snow measurements in the area are still slightly below average. The Tioga Pass WebCam is showing a snow depth that looks about average for this time of year.

While the snowpack at high elevations is near-average, there is very little snow at the lower elevations of the Mono Basin. This is resulting in an abrupt transition from bare-ground to snow at about the 7,500′ elevation (depending on aspect).

RUNOFF
While near-average snowpack is very exciting for a winter season that started off so dry, this year follows two dry years, so all that snow won’t translate into runoff. A lot will soak into the ground and recharge depleted soil moisture. The April-July runoff forecast (issued on March 1st) for the Mono Basin is only 76% of average–and while that number should improve slightly due to last week’s wet weather, without a lot more wet weather we are still looking at a dropping level of Mono Lake this year. Currently Mono Lake is at 6,382.4 feet above sea level. Right now the forecast looks pretty dry for the next week, although seasonal outlooks show average chances for precipitation during Spring.

RESERVOIRS
Southern California Edison has been emptying Gem Lake Reservoir (later in winter than usual due to maintenance). Rush Creek flowing into Grant Lake Reservoir is running 117 cfs–very high for this time of year. Grant Lake Reservoir is rising fast–2,500 acre-feet of water has filled it in the last month–although it has only been this low two other times (1960 and 1977)  since the dam was built in 1940. If it keeps rising at that rate, it will reach 11,500 acre-feet of storage by mid-April, especially if the State Water Resources Control Board grants an urgent petition for a reduction in the lower Rush Creek minimum flow in order to increase storage in Grant Lake Reservoir. 11,500 acre-feet of storage is a level that triggers changes in the minimum flow–below that level LADWP (Los Angeles Department of Water and Power) is only required to release the inflow to the reservoir or the dry year baseflow, whichever is less.

Concerns about Grant Lake Reservoir’s level have arisen related to the condition this summer of the fishery in the creek below the reservoir. Very low reservoir levels result in very warm water temperatures which could cause fish kills in the creek. Analysis is ongoing to determine how to raise the reservoir as high as possible this spring prior to the warm season. Chances are that the reservoir will be lower than last year, but hopefully it will be above critical thresholds for fish survival in the outflow waters.

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