Here's the latest update for Upper Tuolumne River flow conditions and Hetch Hetchy Project releases from Adam Mazurkiewicz, hydrologist for Hetch Hetchy Water and Power (HHWP).
Recently, thunderstorms have brought flows up quickly on the mainstem of the Tuolumne as measured by the Tuolumne at Grand Canyon gage. These thunderstorms have been over the Tuolumne and Merced watersheds, while Cherry and Eleanor have remained storm free. The additional precipitation and stunning longevity of the snowpack have caused some changes in the forecast and operations.
Starting August 1st, there will be additional water below Kirkwood Powerhouse (KPH), while releases below Holm Powerhouse (HPH) will be reduced to ~400-500 cfs throughout the day. Expect 700 cfs additional water below KPH. Given the spill occurring at Hetch Hetchy the flow at Lumsden on August 1st will be 2400 - 2600 cfs.
Tuesday August 2nd: Flows will be similar to Monday and the 2400-2600 cfs range should hold, but may fall to the lower end as spill from Hetch Hetchy may recede.
Wednesday August 3rd: Flows will be dependent on the inflow recession to Hetch Hetchy and any thunderstorms which may occur. Snowmelt should continue to be receding, but there will be flows for boating in a range of 2100 – 2600 cfs.
Thursday August 4th: Flows to allow for Cherry Creek run boating are right near the threshold given the uncertainty of spill at Hetch Hetchy. This estimate cannot be guaranteed but is based on the best current forecast and operation information so please check current flow conditions using the USGS gage stations or the Dreamflows estimate (these notices are also posted on the Dreamflows website). There will be boating flows at Lumsden between 2000 and 2500 cfs.
For the Cherry Creek run specifically, flows at the confluence are a total of what is below Hetch Hetchy, Eleanor and Cherry, HPH and additional water from KPH. Cherry and Eleanor are at minimum seasonal release (15 and 20 cfs) and are scheduled to remain at those rates unless thunderstorms generate significant inflows. Both these reservoirs are near their capacities. KPH is scheduled to contribute ~700 cfs. Spill from Hetch Hetchy will continue to follow natural recession as part of a UTREP flow experiment. It is expected spills from Hetch Hetchy will be between 800 and 1000 cfs on Thursday (total = approx. 1550-1750 cfs). Release from HPH between the hours of 0100 and 0900 hrs will be ~150 cfs, at 0900 to 1200 hrs releases will go up to ~250 cfs, at 1200 hrs releases will go up to 350 cfs. That brings the early day flow estimate to be between 1800 and 2000 cfs (which includes an estimate of accretions) going up to 1900 to 2100 cfs at 0900 hrs. Spill at Hetch Hetchy will control the variation from that estimate.
These estimates are subject to change due to mountain thunderstorms or unscheduled shutdowns. If you're out on the river, check gages for latest flow information.
