Sonoma Water Responds To Letter Questioning Releases From Lake Sonoma

The following letter to the editor from Sonoma Water Deputy Chief Engineer Don Seymour was submitted in response to a letter to the editor from Healdsburg resident Stanley Crane. The views expressed here are the author's own.

Dear Editor,

A recent letter raised several questions regarding recent releases of water from Lake Sonoma. If you imagine the lake like a layer cake, the bottom layer is the water supply pool and the top layer is the flood control pool. Sonoma Water controls releases from Warm Springs Dam when levels are in the water supply pool, which tops out at 245,000 acre feet. The US Army Corps of Engineers (USACE) controls releases when water levels rise about the water supply pool and enter the flood control pool (which tops out at 381,000 acre feet).

This year, thanks to minor deviation to flood control operations approved by USACE last December, we have some frosting between the layers, which allows extra storage for water supply at the discretion of USACE if forecasts indicate it is safe to retain the water. The deviation pool (the frosting) is equal to 2.5 percent of the total reservoir capacity for maximum additional storage of 9,500 acre feet until February 15th and 5 percent of total reservoir capacity for a maximum additional storage of 19,000 acre-feet starting March 1st.. Combined with the Forecast Informed Reservoir Operations (FIRO) pool in Lake Mendocino (11,650 acre feet), over 30,000 additional acre feet of water will be available to meet environmental and human needs.

Releases from the deviation pool at Lake Sonoma and the flood control pool are made by the USACE to reduce flood risks to downstream residents and to maintain the integrity of the dam. Using forecasting tools and computer models, the USACE makes releases during windows between storms in order to create space for new rainfall and runoff. A worst case scenario would be a nearly full reservoir which does not have capacity to store inflows from a major storm – which is what happened in 2017 when uncontrolled releases from Lake Oroville damaged the spillway.

In regard to the impact on Healdsburg residents, through a water supply agreement with Sonoma Water, Healdsburg can divert up to a maximum 425 acre-feet under Sonoma Water’s water rights permits. This includes water released from Lake Sonoma. .

For more information about FIRO, go to https://www.sonomawater.org/firo.

Sincerely,

Don Seymour
Deputy Chief Engineer