Frontal circulation and sediment distribution in the Columbia River plume during the 2004 RISE cruise

The National Science Foundation-funded RISE (River-Influenced Shelf Ecosystems) project seeks to understand how the Columbia River plume affects the productivity of the Washington and Oregon shelves by redistributing and mixing nutrients in coastal waters. Based on observations from the first RISE cruise, we present a description of the frontal circulation and sediment distribution in the Columbia River plume. River-borne sediments may play an important role in mediating productivity in this system by transporting nutrients and regulating light availability in the water column. During the first RISE cruise on July 8-28, 2004, we sampled the upper water column of the plume area intensively with a Triaxus tow-fish aboard the R/V Pt Sur. The Triaxis was outfitted with three CTDs, a fluorometer, a transmissometer, a nitrate sensor, a Laser In-Situ Scattering Transmissometer (LISST-25), an upward looking acoustic Doppler current profiler (1200 kHz), and a laser-optical plankton counter (LOPC). We obtained additional information on the sediment class distribution in the plume with a LISST-100 mounted to a CTD array and deployed separately from the Triaxis. Preliminary results suggest that strong fronts are generated on the windward side of the plume and that fronts on the northern side of the plume are typically stronger than those on the south. These fronts create strongly convergent flow near the surface and penetrate well below the buoyant surface layer. In shallow areas the front-induced flow appears to re-suspend nutrient-rich bottom sediments.