Within the Northwest, hydroelectric projects can generally be placed in two major categories: storage and run-of-river projects.
The key to a storage project is the ability to adjust the river’s natural flow patterns to more closely mirror when electricity is in greatest demand. For example, more water is released to generate electricity in the morning hours when lights are turned on and hot showers are taken.
Storage projects also help adjust flow patterns because water does not flow through the river system at the same rate twelve months a year. In fact, in the Northwest 60% of runoff occurs from May through July. By storing water in reservoirs, water can be released when supply is less plentiful during the late summer, fall and winter months. Storing water is also helpful in meeting irrigation and flood control needs.
Run-of-river projects, on the other hand, allow water to pass through a facility at about the same rate the river is flowing. Generally, such projects are relatively small in size, generating 30 MW or less of electricity. For larger run-of-river projects, water that can back-up behind them is called pondage and can vary river elevation three to five feet during normal operation. With these more mild changes in river elevation, some run-of-river projects are also able to support barge navigation over rapids and other obstacles.
Source: U.S. Bureau of Reclamation, U.S. Army Corp of Engineers, and Bonneville Power Administration. The Columbia River System: The Inside Story, Portland OR, 1991, p.9
As the graphic of Storage and Run-of River Projects shows, storage projects offer greater flexibility in meeting the multiple needs of flood control, power generation, irrigation and navigation. As discussed in the environmental section, however, some of the most contentious environmental issues are associated with storage projects.
Although not common in the Northwest, pumped storage is another type of hydroelectric project. Here, some water that passes through a project is recaptured and pumped back up to a storage reservoir so it can be reused at another time.
Other distinguishing characteristics between projects are their generating capacity, location and structure. For instance, there are many more small projects (those that generate from 5 to 100 megawatts of electricity annually) than large projects (those that generate over 600 megawatts annually). Why projects differ so much in size and generating capacity largely depends on the flow rate of a river or tributary, whether a reservoir is used, the amount of elevation (head) that water is falling in order to spin a turbine, and environmental factors, such as fish passage, that affect daily operations.
In terms of a project’s physical structure, a link at the end of this section takes you to the hydro tours area of the home page. There, you can see both the major parts of a project and the specific parts that allow a turbine to spin and a generator to produce electricity. The turbine seen in the tour is a Kaplan design. Other designs, like the Francis turbine, which looks like a sideways paddle wheel, can be different. The specific type of turbine used at a project depends on the particular characteristics of that river.
Likewise, the number of turbines and generators used at a project relates to the size of a river or tributary and its potential to generate electricity. And finally, the environmental section reviews and provides graphics for how different projects incorporate different types of bypass facilities into their structures to assist upstream and downstream fish migration.
Because there is tremendous diversity between projects, people interested in learning more about a particular project or projects should contact the owner. At both federal and non-federal projects, tours, videos and print materials are often available.