A bi-directional river plume: The Columbia in summer

Freshwater plumes have important effects on marine ecosystems: in the presence of a plume, stratification, nutrient pathways, light and circulation patterns are significantly altered from patterns that occur under the influence of wind and ambient currents alone. The historical picture of the plume from the Columbia River is of a freshwater plume oriented southwest offshore of the Oregon shelf in summer and north or northwest along the Washington shelf in winter. Recent CTD data and new data from moored sensors support a picture quite different from the historical seasonal pattern. Specifically, the plume is frequently present up to 150 km north of the river mouth on the Washington shelf from spring to fall, even during periods of upwelling. The plume is frequently bi-directional, with branches both north and south of the river mouth. During a downwelling event, the southwest plume moves onshore over the Oregon shelf. At the same time, a new plume forms north of the river mouth over the Washington shelf, trapped within ∼20-30 km of the coast. This plume propagates and also is advected northward by inner shelf currents that reverse during downwelling. When winds return to upwelling-favorable, inner shelf currents reverse immediately to flow to the south and the shallow plume is advected offshore in the wind-driven Ekman layer. Once over the central shelf, the plume is advected farther south by the seasonal mean ambient flow. Overall, freshwater from the Columbia plume overlies the Washington shelf ∼50% or more of the summer. Capping of upwelling on the inner shelf by the Columbia freshwater plume is illustrated, where the “capping potential” is related to stratification and wind magnitude and duration. Evidence is also presented to suggest that the seaward front of the Columbia River plume may provide a barrier to the transport of harmful algal blooms to coastal beaches in summer and early fall.