USBR NEWS: Keeping Folsom Reservoir’s cold water in the bank until it’s needed

By Gary Pitzer, US Bureau of Reclamation

California’s winter storms have blasted the Sierra Nevada with a thick blanket of snow.

That ample snowpack has been subsequently pelted with rain, pushing some of it downhill as runoff through ravines, canyons, and creeks before feeding into the forks of the American River.

The water eventually fills Folsom Lake to the delight of the people throughout the Sacramento metropolitan area who flock there in the heat of summer. Beyond water supply, power generation, instream flow needs, and yes, summertime fun, Folsom has a key role to play in keeping the water temperature in the lower American River hospitable to the fish that return each year to spawn. That means having cold water on demand when it’s needed.

The dam employs an ingenious but simple design that relies on massive metal plates, or “shutters,” to orchestrate the flow of the cold snowmelt from the various levels of the reservoir. Like the layers of a cake, Folsom’s water stratifies, with warmer temperatures pooling at or near the surface and the frigid pool situated 100 feet below. Between the surface and bottom, the temperature varies by as much as 30 degrees Fahrenheit.

The shutters feed Folsom’s three penstocks and are situated at three vertical levels – “tops, mids and bottoms.” At the top of each unit is three rows of shutters, in the middle two, and the bottom four rows.

Thuy Washburn, hydraulic engineer with Reclamation Central Valley Operations, office, said Folsom’s shutters “are the key tool to manage the cold-water pool we have in Folsom each year.

That cold water is vital because Steelhead trout and Chinook salmon in the American River need cool water to survive during the warmer parts of the year. Young steelhead need water 65 degrees Fahrenheit or less and the ambient water temperatures coming out of the reservoir are higher than that for much of the year.  Chinook salmon arriving in the fall need temperatures dropping below 60 degrees Fahrenheit to spawn and for the eggs to survive.

“The temperature shutters enable us to release water from the reservoir from specific depths to provide temperatures closer to what the species need to survive,” said John Hannon, fish biologist with Reclamation’s Bay-Delta Office. “They also enable us to extend the duration of cool water releases out through the year by limiting the amount of cold water used all at one time. Without the shutters we have limited ability to manage water temperatures while maintaining power production”

The wet winter means Folsom’s storage and water level elevation will keep climbing and that the supply of cold water will be there when it’s needed the most.

“We will be able to preserve the cold water well into the year,” Washburn said. “It’s going to be quite cold so we’re going to just hopefully just take the water from the top until cold water is needed in the summer months”