Strategic releases of reservoir water help keep the South Fork of the Tolt River cooler during the summer, which is good for our region’s salmon. The South Fork Tolt Hydroelectric (is one cool little) Project
In the Seattle area, most of the drinking water we use has generated power on its way to our faucets. The very same water serves a third purpose, too. The reservoir behind the South Fork Tolt (SFT) Hydroelectric Project on the South Fork of the Tolt River also plays a role in keeping the river cooler during the summer, which helps our region’s salmon.
A small but mighty project
Seattle Public Utilities has owned and operated a dam and reservoir on the South Fork of the Tolt River since the 1960s. These facilities provide about 30-40% of the drinking water supply for 1.6 million people across the greater Seattle area.
The City of Seattle realized that water being held in the reservoir for the region’s water supply could also be used to generate power. In 1995, the South Fork Tolt Hydroelectric Project was completed. Even though it supplies less than 1% of City Light’s power requirements, it is an important source of renewable energy. It powers the Tolt Water Treatment Facility and helps balance out power provided by other renewable sources in the region, so it fills gaps and increases resiliency within the local power grid.
The City’s other major water source, the Cedar River, also has a hydroelectric power project. If you want to impress your friends, you can tell them that the very same water that topped off your favorite water bottle also did its part to help keep the lights on.
How the project helps keep the river cool
The SFT reservoir does more than provide water to drink and produce energy, though. It also helps keep the Tolt River cooler during the hot summer months.
Seattle Public Utilities Senior Fish Biologist, Karl Burton, described how this works. A large pipe stands vertically within the lake above the dam. On this pipe are three valves at different heights under the water. Water released through the valves runs downstream through the South Fork Tolt River.
During much of the year, the water behind the dam is roughly the same temperature from top to bottom. But during the summer the water on top of the reservoir warms up while the water deeper down stays cooler. If you’ve ever jumped into a lake, you might have felt this effect. The water at your toes feels cooler than the water at your shoulders.
Karl pointed out that salmon need our rivers to be within a specific temperature range at different stages of their life, for example as juveniles in the summer or when they return in late summer as adults to spawn. So, project operators help counteract the heat of summer by strategically releasing cooler water from the deeper parts of the reservoir.
Deciding which valves to open is important work. Karl explained:
“I keep an eye on water temperatures on a daily basis, especially in the summer when the longer, warmer days begin to heat up our rivers. As the season progresses, I close the top intake valve and open the middle valve. By the end of the summer, we’re usually drawing entirely from the bottom valve.”
As summer turns to autumn, the days get shorter, the weather cools down, and windstorms mix up the lake. This activity makes the temperature at the top and bottom of the reservoir roughly the same again. By that time, the salmon have spawned, and their lifecycle is complete until it begins anew the next year.
About the South Fork Tolt Hydroelectric Project
The South Fork Tolt Hydroelectric Project is operated under a license administered by the Federal Energy Regulatory Commission (FERC). The current license continues through mid-2029. But relicensing any hydropower project, even a small one, takes many years. To do this, the City of Seattle is working closely with Tribes, federal and state agencies, local governments, non-governmental organizations, and other interested parties. (These groups are collectively called Licensing Participants or LPs.)
What’s next?
The City will continue to work with Tribes and other LPs over the next two summers to complete a series of studies outlined in the FERC Study Plan Determination.
Learn more about this project, the relicensing process, and access all the related documents – at: seattle.gov/tolt-relicensing. You can find a Public Documents Library under “Project Documents and Resources.”
Stay Informed and sign up for our mailing list to receive regular updates on the South Fork Tolt relicensing process.