Freshwater Rivermouths Are Just Like Marine Estuaries, Just without All That Salt

We investigated rivermouths in the Laurentian Great Lakes to develop a better understanding of the biophysical processes that govern material processing, create habitat, and result in these areas supporting important fishery and wildlife resources. One surprising finding is that freshwater rivermouths share many attributes with marine systems. We found similarities in physical structure; while most rivers discharged directly, we also found embayments, bar-built lagoons, and drowned rivermouths. Fjords were the only estuary type not present. Surveys combining data from an acoustic Doppler profiler, sondes, and direct water quality samples revealed that rivermouths were substantial mixing zones with lake water influences extending landward, and riverine influences extending into the lakes for surprisingly long distances. Temperature profiles revealed thermally driven flow patterns that were similar to the salt wedges observed in marine estuaries. Chemical gradients were small compared to marine habitats but highly important in the context of discharge of nutrient laden river water into lake systems that have become highly oligotrophic. And while freshwater systems lacked tidal influences, we found that wind driven seiche activity resulted in frequent backflushing. However, we hypothesize potential differences between freshwater rivermouths and coastal estuaries. Freshwater rivermouths supported wetlands and depositional zones with deposits of deciduous leaves, but it is not known if these areas are functionally similar to salt marshes or if they export nutrients in a similar manner. Furthermore, chemical processing of riverine nutrients, particularly dissolved organic nitrogen, may be very different among freshwater and coastal systems. We believe strongly that these differences should be explored through comparative studies, especially given predictions that coastal systems may become less saline as a result of predicted increase in freshwater discharge via climate change.