Tuesday, May 10, 2011

Update: O'Shaughnessy Dam high flow releases

Due to cool temperatures and some highly variable weather, inflows to Hetch Hetchy Reservoir have dropped significantly.  In the last 4 days at 8,500 ft in the Tuolumne watershed, conditions have gone from 65F daytime highs at 8,500 feet to 3” of new snow at 8,500 ft.  Cool conditions are expected to persist through May 17th.  The low inflows mean that Hetch Hetchy operators will not have adequate levels of water storage in the reservoir to implement the "test drive" of ecological high flow releases as planned next week.  Instead, the planned high flow test drive will be delayed by a week; Hetch Hetchy operators plan to begin ramping on Sunday, May 22.

Here's the current plan for the releases at O'Shaughnessy Dam (OSD) and resulting flows at Lumsden:
  • May 12 - 22:  OSD release of 1,400 cfs.  Lumsden flows will be approximately 5200 when combined with Cherry, Kirkwood powerhouse, and Middle and South Forks flows;
  • May 23 -24:  OSD release of 3,300 cfs.  Lumsden flow will be approximately 7100 cfs;
  • May 25:  OSD release of 4,800 cfs.  8,800 cfs at Lumsden;
  • May 26:  OSD release of 5,700 cfs.  9700 cfs at Lumsden;
  • May 27 – 29:  OSD release of 8,500 cfs.  12,500 cfs at Lumsden;
The length of the 8,500 cfs high flow release will depend on weather and inflows.  Wetland inundation bench flows will be released after the peak, however the timing will again depend on weather.  High flows at Lumsden (>9,000 cfs) are likely to continue – assuming normal spring/warming weather – through at least June 8.  We'll provide updates here as the snowmelt season unfolds.