Another benefit that stems directly from the unique nature of the Columbia Basin is irrigation. In fact, six percent of the Columbia River Basin’s yearly runoff is diverted to irrigate about 7.8 million acres of land. Much of the water that is diverted eventually finds its way back into the river system. Farmers in arid parts of eastern Washington, northeastern Oregon, and southern Idaho depend on irrigation to support crops such as wheat, corn, potatoes, peas, alfalfa, apples, and grapes.
The Columbia Basin Project, for instance, turned the high desert area of central Washington into another bread basket for America. The centerpiece is the Grand Coulee Dam, which was built primarily as an irrigation project by the Bureau of Reclamation. Water stored behind Grand Coulee Dam in Lake Roosevelt is pumped into Banks Lake. This lake was formed by damming both ends of Grand Coulee, which is one of the geological formations that dates back to ice age floods. The water then flows through a system of tunnels and canals to irrigate croplands. The project irrigates over 500,000 acres and has the potential to be expanded to irrigate over 1.1 million acres. The Grand Coulee Dam is also used as a hydroelectric project and has the capacity to generate more electricity than any other hydroelectric project in North America.
The Bureau of Reclamation, local irrigation districts and water companies are examples of authorities that regulate how much water is diverted from the river system to support irrigation. Although scheduled locally, the cumulative effect of utilizing water from the river system to support large and small irrigation projects can be quite significant. As a result, those responsible for coordinated management of the river system estimate the effect of such diversions when determining reservoir levels. through accurate estimates, reservoir levels can be properly maintained to meet flood control, hydroelectric generation and irrigation needs.
One should also note that some areas and stretches of river are much more affected by irrigation than others. The effects on the Snake River are more pronounced than on the main stem of the Columbia. For instance, the Minidoka Project along the Snake uses water from six storage and two diversion dams. Thousands of miles of distribution canals then provide irrigation service to more than 1.1 million acres of farmland.
For these reasons, the issue of “water rights” has been a source of contention for quite some time. In Idaho, for instance, the state constitution grants “the right to divert and appropriate the unappropriated waters of any natural stream to beneficial uses.” In other words, if you got there first you can keep diverting water for the “beneficial” purpose of irrigation. While contentious at a local and state level, the situation is made even more complex by recognizing that the Northwest as a whole is also affected. For example, because irrigation can cause less water to be available for reservoirs, flexibility in releasing water to support fish passage can be lost.