The Interaction Between Two Small Mountainous River Plumes Under Downwelling Wind Conditions in an Idealized Coastal Ocean Model

Small mountainous rivers are globally significant sources of sediment and nutrients in the coastal ocean. During downwelling wind conditions the combined plumes have been observed to form a buoyant coastal jet which can have an along-shore spatial coherence spanning hundreds of kilometers. This indicates the potential for one-way transport of suspended particulate from a specific river over long distances. Several studies have investigated the dynamics of buoyant coastal currents from combined river sources to understand the influences of wind and river flow. Using an idealized three-dimensional ROMS model, the impact of the interactions between two river plumes on the spatial scales and transport pathways is studied under a constant downwelling wind stress. The model is run for one- and two-river plume scenarios with a range of steady river flow rates. The coastal current is defined by a threshold salinity; to distinguish the contribution from individual rivers, they are each dyed with a unique passive numerical tracer. During downwelling favorable wind conditions the model results indicate a northward flowing buoyant coastal current with bulge regions near the river mouths that vary in size with river flow. Water from the southern river is deflected offshore by the northern bulge, indicating the possibility of a longer residence time in this region. The net transport downstream is relatively unaffected by the presence of the second river, but the shape of the coastal current changes when the buoyancy sources are distributed along the coastline. Under some conditions, there is evidence that water sourced from the southern river can also intrude into the river located to the north. These results further our understanding of the coastal current associated with small, distributed river plumes and the transport pathways of water properties, suspended nutrients, and larvae sourced from individual rivers in these coastal systems such as along the Oregon coast.