O'Shaughnessy Dam ecological pulse flow release underway

High capacity valves on the face of
O'Shaughnessy Dam release spring
snowmelt to the upper Tuolumne River.

With a relatively healthy snowpack in the upper Tuolumne watershed in Yosemite National Park and spring snowmelt well underway, the SFPUC Hetch Hetchy Water & Power division began ramping up releases from O'Shaughnessy Dam to the upper Tuolumne River on Sunday, May 8th, for the first ecological pulse flow since the California drought began in 2012.  Flows will reach 6,500 cubic feet per second (cfs) by Wednesday, May 11th.

The releases are designed to achieve a number of experimental ecological objectives, including managing foothill yellow-legged frog reproduction, inundating unique wetlands in the Poopenaut Valley, and mobilizing fine sediment, sand, and gravels to improve trout and foothill yellow-legged frog habitat.

Wetlands in the Poopenaut Valley that haven't seen significant flooding since 2011 will be fully inundated by the peak release of 6,500 cfs, providing valuable information on how wetlands may recover from several years of drought.

The sediment mobilizing releases will serve to move silt and sand that has increased in the upper Tuolumne River since the 2013 Rim Fire. Fine sediment can fill the spaces between gravels and cobbles, allowing stream side vegetation to encroach into the river channel and "fossilize" the loose gravel and cobble patches that trout, foothill yellow-legged frogs, and other aquatic life need to successfully reproduce.

Scientists from the SFPUC, Yosemite National Park, and UC Davis are monitoring the releases and will be studying their ecological effects throughout the spring and summer.  Data from this work will be used to update and refine analyses and river management strategies described in the Draft O'Shaughnessy Dam Instream Flow Management Plan.