The distribution and composition of suspended particulate matter in the waters over Willapa submarine canyon and the adjacent continental slope off the coast of Washington describe an apparently continuous bottom nepheloid layer ranging in thickness from 160 to 530 m and composed of particles supplied primarily by the nearby Columbia River. Near-bottom particulate concentrations, estimated by continuous vertical profiles of light-scattering intensity, systematically decrease from the continental shelf to the base of the slope. Within this trend, concentrations in Willapa Canyon are greater by a factor of as much as 3 than concentrations present at comparable locations on the adjacent open-slope areas.
Mineralogical analysis of the suspended sediment within the bottom nepheloid layer suggests that the increased concentrations found within Willapa Canyon result from a preferential accumulation of Columbia River-derived particles relative to the surrounding open-slope areas. Montmorillonite/chlorite ratios >1 characterize the Columbia River suspended load, and this ratio remains >0.7 in samples from upper Willapa Canyon while decreasing to <0.7 in samples from the surrounding open slope and lower Willapa Canyon. Chemical homogeneity of the suspended particulates from the Columbia River to the base of the slope is indicated by the nearly constant ratios of the major-element oxides to Al2O3.
The excess density provided by the intense nepheloid layer found in Willapa Canyon may be influential in generating a net downcanyon component of bottom flow such as observed in other submarine canyons. Predicted rates of net basinward transport of the bottom nepheloid layer sediments range from 2.8 cm sec−1 at 635 m to 1.5 cm sec−1 at 1,500 m, values that are in good agreement with long-term current measurements in other canyons.