Under-Ice Hydrography of the La Grande River Plume in Relation to a Ten-Fold Increase in Wintertime Discharge

A major environmental change to affect the east James Bay coast is hydroelectric development of the La Grande River, which increased winter flows ten-fold. When the river enters coastal waters covered by landfast ice, it forms a freshwater surface layer under the ice and spreads out over the underlying salty water forming an “under-ice river plume.” We describe the results of studies performed with the Cree Nation of Chisasibi during 2016–2017 on the under-ice plume of the La Grande River. We found that the area of the plume almost completely composed of river water (salinity 0–5) north of the river mouth was of similar size to that in the 1980s/early 1990s but had faster currents and higher turnover rates of freshwater. A similarly fresh area extended southward. The overall region of freshwater influence in northeast James Bay was also larger under the high flows of 2016–2017 than in the 1980s/early 1990s. The results provide new information about how the hydroelectric development has affected the James Bay coast, which contains eelgrass habitats and is a migration staging area for waterfowl. They also show that the downstream impacts of dams may extend beyond enclosed estuaries and well into downstream coastal marine environments.