In additional to turbidity effects, the suspended sediments dredged from estuarine and coastal marine systems is generally high in organic matter and clay, both of which are biologically and chemically active. These contaminants cause toxicity to salmon and their prey. Dredging not only removes plants, but changes the entire physical, biological, and chemical structure of the ecosystem. The loss of eelgrass habitat can be either by direct dredging or by sediment deposition. The extent of the area affected by the dredging depends on the tidal range, current strength, and sediment texture in the area.
Sediments raised by dredging can bury plants away from the site.
The reduction in eelgrass density could result in an increased silt load, due to a reduction in sediment trapping, and increased erosion of bottom sediments. Concomitant with dredging is spoil disposal. Traditionally, disposal was on marshes, resulting in the loss of that habitat, but that practice has been halted. Dredge disposal occurs on upland sites and in open disposal sites. Four active dredge disposal sites occur within 2 to 6 miles seaward of the Columbia River mouth.