The U.S. Army Corps of Engineers cautions Columbia River users that water levels from downstream of Bonneville Dam to Longview, Wash., will decrease starting Labor Day weekend and could remain low for several weeks.
Boaters are advised to exercise extreme caution when anchoring near the shore as river levels could drop considerably in a matter of hours. The decrease will result from multiple effects of natural seasonal low flow, flow reductions from upstream dams and the influence of low tide.
On Aug. 31, the Corps will shut off flows that provide for spill over Bonneville Dam and other Columbia River projects. The spill operations began in April and aid juvenile salmon as they travel downstream to the Pacific Ocean. Water will continue to pass through the powerhouse.
At the same time, Grand Coulee Dam, located on the Columbia River in north central Washington, will begin storing water for a variety of purposes, including for future winter power generation. As a result of these changes, by Sept. 2 river levels below Bonneville Dam could be as much as four feet lower than they were on Aug. 30. Low water levels will likely continue through September, and possibly into October if weather stays dry and warm.
In order to fulfill power generation needs with less water, Bonneville Dam will generate power in the evening and reduce flow during the day to conserve water. The cycling of flows for power production, together with tidal influence, may cause changes in water levels of up to four feet per day below Bonneville Dam. This means even at Portland river levels could drop by up to two feet within 24 hours.