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 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. 


Prior images on this blog:

In the news:
Matilija Reservoir Drained, Ojai Valley News, July 17, 2020 (see full article below)
How Oroville Is Changing Dam Safety in California, Public Policy Institute of California, March 28, 2018

Reference:




Matilija Reservoir, July 2, 2020

Matilija Reservoir, July 3, 2020

Matilija Reservoir, July 8, 2020


Matilija Reservoir, July 12, 2020


Matilija Reservoir, July 15, 2020


Matilija Reservoir, July 22, 2020

Matilija Reservoir, July 27, 2020

Matilija Reservoir, August 4, 2020

Matilija Reservoir, August 12, 2020


Matilija Reservoir, August 26, 2020

Matilija Reservoir, Sept 14, 2020





Matilija Dam and drained reservoir,
July 8, 2020

Matilija Dam and intake to the outlet pipe,
July 8, 2020

Matilija Dam intake to the outlet pipe,
July 8, 2020





Downstream of Matilija Dam,
July 17, 2020
(courtesy S. Zach Futujma)




Confluence of North Fork (foreground) and Matilija Creek 
July 8, 2020

Camino Cielo Bridge, July 2, 2020

Camino Cielo Bridge, July 6, 2020


Ventura River at Camino Cielo Bridge
7-2-2020


Ventura River at Camino Cielo Bridge
7-4-2020




Ventura River at Camino Cielo Bridge
7-6-2020



Ventura River at Camino Cielo Bridge
7-8-2020


Ventura River at Camino Cielo Bridge
7-12-2020



Ventura River at Camino Cielo Bridge
7-17-2020

Ventura River at Camino Cielo Bridge
7-22-2020

Ventura River at Camino Cielo Bridge
7-27-2020

Ventura River at Camino Cielo Bridge
8-4-2020

Ventura River at Camino Cielo Bridge
8-12-2020
Ventura River at Camino Cielo Bridge
8-26-2020






Ventura River looking downstream at Camino Cielo Bridge
7-8-2020

Ventura River looking downstream at Camino Cielo Bridge
9-14-2020




Ventura River at Oso trailhead crossing,
July 8, 2020



The long term effects of drawing down Matilija Reservoir are evident in the series of photos taken at Camino Cielo.  The slow downcutting and meandering of the stream channel through the sediment in the remnant reservoir mobilizes fine sediments and transports them downstream.  During the summer months flows are low and decreasing until the next rains come in November or December.    These "suspended sediments" are deposited on the streambed, with significant implications for the ecosystem.  Fish spawning and rearing habitat and the benthic organisms that form the foundation of the food chain are impacted by fine sediment.  This is why the dam removal planning analysis has been focused on timing sediment release with a large storm event so that the fine sediments will be transported to the ocean during high flows. 

The video below illustrates how the gravels deposited during winter rains became completely smothered in fine silt and clay in the reach immediately downstream of Camino Cielo bridge.  This was September 5, 2020, a full two months after drawdown; 









In the news:
Matilija Reservoir Drained, Ojai Valley News, July 17, 2020