Coastal river plumes: Collisions and coalescence

Plumes of buoyant river water spread in the ocean, and these plumes influence water quality, sediment dispersal, primary productivity, and circulation along the world’s coasts. Many investigations of river plumes focus on settings for which the plume spreading is assumed to be independent from other buoyant plumes. Here we investigate spreading patterns, collisions and coalescence of multiple plumes by investigating: (i) the relative likelihood of plume-to-plume interactions at different coastal settings using geophysical scaling, (ii) the diversity of plume frontal collision types using a two-dimensional hydrodynamic model, and (iii) the differences in plume spreading patterns for coasts with single and multiple rivers using a three-dimensional hydrodynamic model. Geophysical scaling suggests that coastal margins with numerous small rivers (watershed areas < 10,000 km^2), such as found along most active geologic coastal margins, are much more likely to have river plumes that collide than coastal settings with larger rivers. Our numerical modeling suggests that when two plume fronts meet, several types of collisions may occur, including reflection, subduction and occlusion. We find that the relative differences in pre-collision plume densities and thicknesses strongly influence the resulting collision types. The three-dimensional spreading of buoyant plumes is found to be influenced by the presence of additional rivers for all modeled scenarios, including those with and without wind and Coriolis forcing. Combined, these results suggest that plume-to-plume interactions are common phenomena for coastal regions near the world’s smaller rivers and for coastal settings with multiple river mouths in close proximity, and that the spreading and fate of river waters in these settings will be strongly influenced by these interactions. We conclude that new investigations are needed to characterize how plumes interact offshore of river mouths to better understand the transport and fate of terrestrial sources of pollution, nutrients and other materials in the ocean.