Matilija Reservoir Drained

Thursday, July 9, 2020

Matilija Reservoir Drained

Matilija reservoir was drained over the 4th of July weekend.    According to county staff, this was done in response to safety concerns, as identified by the state Division of Safety of Dams (DSOD) as well as the liability resulting from recreational use of the dam site. The 12″ valve controlling the dam outlet was opened on the morning of Wednesday, July 1, releasing flows of approximately 30 cubic feet per second downstream.  The valve will remain open maintaining the reservoir in a drained state until the winter rains.

Images of the drained reservoir reveal the degree of additional sedimentation since the Thomas Fire, which has reduced the storage capacity to less

Matilija Reservoir 3-17-2020

than 150 acre feet.  As illustrated in the photos below, the initial flush released relatively clear water, but downstream water quality degraded as flows began to cut a channel through the sediment.  The channel evolution is documented in the overview photos of the reservoir.