The San Francisco Public Utilities Commission (SFPUC) and the National Park Service (NPS) are planning to conduct a "controlled flood" of water from Hetch Hetchy Reservoir May 27-29. The purpose of this pulse flow is to examine what pre-dam spring run-off conditions were like downstream of O'Shaughnessy Dam. This controlled flood will help SFPUC and NPS staff understand relationships between the Tuolumne River in flood and groundwater levels, extent of wetlands, and ecological responses to spring floods.

The Tuolumne River below the dam is presently running at about 80 cubic feet per second (cfs), but will begin to rise on Tuesday, May 27, peaking on Wednesday, May 28 at about 6,000 cfs. For comparison, the Merced River recently peaked at about 4,000 cfs on Saturday, May 17. By Friday, May 30 the river should be back down to about 80 cfs. Be aware of these elevated flows if planning hikes downstream of Hetch Hetchy Reservoir during this time.