A Free-Flowing Tuolumne River??Posted January 19, 2006 by Robyn Suddeth
I recently wrote a post referring to a short story from the LA Times about what the process would be like if Congress ever decided to restore Hetch Hetchy Valley. (See Restoring Hetch Hetchy and the Tuolumne River) This would mean removing the dam and a free-flowing Tuolumne River.
After writing the post I started wondering about the effect this would have, if any, on the state of things a little further downstream; on Cherry Creek and the lower section of the Tuolumne that so many people raft on every year.
After a bit of genious investigating, here are some of the more interesting things I found out:
Where Does the Tuolumne’s Water Come From?
Apparently, flows on the Tuolumne during summer months are courtesy of Lake Lloyd Reservoir, not Hetch Hetchy. Water is released from Lloyd through the Holm Powerhouse and then into Cherry Creek. From there it flows on down to the confluence with the Main T, giving us that familiar 1200 cfs we see during most of the summer.
(Look at this map of the Tuolumne and Hetch Hetchy Water System to get a better idea of how everything connects.)
What About Spring and High Water?
Here’s where a restored naturally-flowing river upstream would have the greatest effect. During Spring runoff, flows can already get pretty high in a year like last, but even these are tempered by Hetch Hetchy. With a lot of that water being diverted to San Francisco, and Lloyd releasing flows gradually depending on the amount of water in the lake, even the biggest flows we’ve seen in high water years are smaller than the river’s natural potential.
What would be gained in higher flows would be lost a little in consistency. With one less dam controlling snowmelt input, flows could vary quite a bit from day to day depending on weather.
How would these higher, fluctuating Spring flows affect other aspects of the river?
This is one thing I hadn’t thought of at all, but it was pointed out to me that much of the riparian vegetation along the river is unnaturally dominant. Without a dam upstream controlling flows, a lot of that thicker vegetation closer to shore would get cleaned out every year by peak flows, allowing other plant species a little more room.
This bodes well for beaches too– they would get a Spring cleaning every year!
What About the Section Above the Cherry Creek Confluence?
I had hopes and dreams that a free-flowing Tuolumne would mean another section like the one from Meryl’s Pool down to Don Pedro opening up in the ten or so miles from Yosemite to the confluence with Cherry Creek, but apparently that upper section is just as technical and difficult as Cherry Creek, and probably even a little more so. Maybe a two-day Cherry Creek trip in the future??
All In All?
We’ve got higher, but less consistent flows in Spring, swept beaches, and more Class V rapids as the river emerges from Yosemite. Guess I could have just said that in the first place, but I thought some people would enjoy the details. And just in case you still want more,…
For More information on the debate over Hetch Hetchy Valley, visit: