Hydropower 101: Reservoirs, The Region’s Storage Battery

Diablo Lake is a reservoir in the North Cascade Mountains of northern Washington.Just like animals in times of abundance gather and store food for the leaner months, hydropower operators store water during wet months in preparation for drier conditions. Though squirrels may opt for underground storage, hydropower operators store water behind hydroelectric dams in large pools called reservoirs.

Not all dams have reservoirs. But the hydropower system operators work together to make the best use of the available storage and release water to meet multiple needs year-round.

Serving Multiple Purposes, Providing Many Benefits

Stored water can be released through turbines to generate electricity. While operations change year to year depending on conditions, reservoirs are generally drawn down in winter and early spring to provide power and to make room for heavy spring runoff. In April through August, as the snowpack melts, water is stored to prevent flooding and keep communities safe. Stored water can also be released through spillways, rather than turbines, to increase river flows and help fish migrate downstream to the ocean.

Reservoirs Enable Reliability

Reservoirs act like giant batteries that provide energy when it’s needed. Hydropower operators use the stored water in the reservoirs behind the dam to adjust the amount of water flowing through the turbines to match electricity use. Power forecasters determine how much electricity will be needed during a given time period—and communicate that to hydropower operators.

The consistent availability of hydropower also helps support other, more variable, types of renewable energy sources, such as wind. Dams can quickly ramp up to provide more electricity when the wind drops and can scale back generation when the wind picks up again.

Snowmelt and runoff from upstream mountains in the spring enable reservoirs to be filled. This stored water comes in handy during the drier months of summer when water can be released to support energy demand and, in some instances, even reduce stream temperatures for the benefit of fish.

Storage, Spill and Salmon

Water stored in reservoirs can also help hydropower operators support the seasonal needs of both young and adult salmon. In winter and spring, operators help ensure salmon spawning grounds have enough water for spawning and to keep their nests (called redds) covered. In spring and summer, extra water is spilled from the dams to help young fish move quickly downstream to the ocean. Releasing stored water also helps other wildlife, such as lamprey eels, which are an important cultural resource for some Northwest tribes.

Operating the Columbia River hydropower system to support all these needs is quite the balancing act, and having the storage capacity of reservoirs is critical to its success.