Tuesday, May 24, 2011

Update 2: O'Shaughnessy Dam releases

Cool weather continues to preserve the snowpack and moderate inflows to Hetch Hetchy Reservoir, which has been allowed to rise above 300,000 acre-feet to prepare for UTREP high flow releases this week.   Releases began yesterday (May 23rd) as part of a slightly modified schedule.

In order to confirm Poopenaut Valley wetland inundation flow thresholds, bench releases were made today at 3,300 cfs and 3,700 cfs. These will be followed by releases at 4,800 cfs (May 25 @ 08:00) and 5,700 cfs (May 25 @ 15:00). The peak release of 8,500 cfs will begin on May 26 @ 08:00 and begin to ramp down on May 28 @ 08:00. Following the peak, releases will be ramped down to 5,700 cfs to support Poopenaut Valley inundation.  These releases are being monitored by equipment in Poopenaut Valley and by NPS staff as part of Yosemite National Park's Looking Downstream Project.

Releases are currently 3,700 cfs from O’Shaughnessy Dam, 700 cfs from Kirkwood powerhouse, 1,900 cfs from Cherry Creek, 900 from the Middle and South Fork Tuolumne, and about 200 cfs of accretion flow. Dreamflows is a few hours behind recent changes (due to travel time) and is at 6,400 cfs and on its way to 7,400 cfs.  See realtime flow links at right for current flow information.

Flow estimates (plus or minus 500 cfs) for Lumsden are:

May 25: 7,400 cfs in the morning, rising through the day to 9,400 cfs around 6 pm
May 26: 9,400 cfs in the morning, rising to 12,200 cfs around 1 pm
May 27: 12,200 cfs all day
May 28: 12,200 cfs in the morning, decreasing to 10,100 cfs around 3 pm.
May 29: 10,100 cfs in the morning, decreasing to 9,400 cfs around 3 pm

Releases from O'Shaughnessy Dam will hold constant from May 30 through at least June 3, and the approximately 9,400 cfs at Lumsden will rise or fall based on the tributary contributions and snowmelt. If warmer weather follows the cold Memorial Day weekend, the ~9,400 cfs will continue to June 8.