Wednesday May 10, 2023

US Army Corps of Engineers

Due to record-high snowpack over the winter, the USACE Sacramento District is preparing for above-average inflows to and outflows from USACE-managed reservoirs throughout California’s Central Valley, including Isabella Lake.

The district expects Isabella Lake to exceed gross pool of 568,000 acre-feet this summer, which will cause the dam’s service spillway to release water for the first time since 1983. Based on the rate and timing of snowmelt, this spill could last several weeks, beginning sometime between late May and mid-June.

There is no expectation that water levels will reach the top of the new labyrinth weir or spill into the emergency spillway. The emergency spillway is designed to be used in a 4,000-year flood situation, meaning that in any given year there is a 1 in 4,000 chance of such an event. Current snowmelt projections are far below this level and are not expected to get there.  

The district has received approval to implement a plan to fill Isabella Lake to capacity this year following the substantial completion of Phase 2 of the Isabella Dam Safety Modification Project, but execution of the fill plan will be adjusted to accommodate this year’s record snowpack.

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