PNNL Researchers Assess Impacts of Dredging on Columbia River Crabs

The Columbia River, which serves as a waterway for export commerce from the West and Midwest, must be dredged yearly to keep the river bottom deep enough for safe navigation. During dredging, Dungeness crabs are taken up with the sediment and other dredged materials. The Portland, Oregon, District of the U.S. Army Corps of Engineers is concerned about the effects of dredging on the crab population, and is sponsoring an assessment by scientists at the Battelle-operated Pacific Northwest National Laboratory’s (PNNL) Marine Sciences Laboratory.

Pacific Northwest National Laboratory researchers and U.S.Army Corps of Engineers cadets count crabs that have been entrained during dredging on the Columbia River for an assessment the Laboratory is doing for the Portland District of the U.S. Army Corps of Engineers.

The Corps’ concerns are twofold. First, regular maintenance means that each summer vessels dredge up huge amounts of sediment—and the marine life in it—along a three-mile stretch of river from the ocean inland. The second concern involves the Corps’ plan to enable larger ships to call at ports on the Columbia by deepening the channel from 40 to 43 feet in the stretch from Portland to about three miles from the ocean. Currently, large ships carrying grain and other cargo cannot be fully loaded because the water is not deep enough for them to return to the ocean safely.

PNNL scientists are assessing how the dredging and disposal of dredged materials will affect the crab population. “We’re providing scientific information that can be used to make decisions that minimize the impact,” said Walt Pearson, director of the PNNL project. The researchers are looking at crab entrainment—how many crabs are trapped by dredges. In the vessels used by the Corps, pipes bring sediment and crabs from the river bottom and pass them into a hopper aboard the dredge. When the hopper is full, its contents are released at a dumpsite. Scientists are studying how many crabs are trapped as the dredge fills up and what happens to crabs that get buried by sediment when the hopper is emptied.

Working with a University of Washington statistician, PNNL researchers developed a field sampling and analysis plan designed to determine the rates at which crabs are being entrained. Researchers have already counted trapped crabs in an area slated for deepening, and are now examining the materials collected by a Corps vessel during maintenance dredging to determine how many crabs are being entrained. The rates will be used in a crab dredge impact model. The unique statistical design enables the scientists to calculate confidence levels for their forecasts about the potential impact of the deepening project on the crab population.

For more information, contact Walt Pearson at (360) 681-3661 or walter.pearson@pnl.gov.