Influence du couvert de glace sur l'étendue du panache de La Grande Rivière (baie James) [Influence of Ice Cover on the Extent of the Plume of La Grande Riviere (James Bay)]

Abstract

As a result of the La Grande Riviere hydroelectric complex development, the winter inflow of freshwater from La Grande Riviere into the ice-covered James Bay has increased from 500/cu m/s under natural conditions to more than 4000/cu m/s during peak power production. A field program to monitor the related changes on the freshwater plume into the coastal waters of James Bay for the 1986-87 winter season was undertaken. These data were combined with previous observations for lower discharge (1980 and 1984). Although the plume area increased markedly for the discharge values ranging from 500 to 1500 cu m/s, the data showed very little change in the extent of the coastal plume for discharge varying between 1500 and 4000 cu m/s. Contrary to earlier predictions, this is explained by the effect of the land-fast ice zone, beyond which intense mixing occurs, rapidly diffusing the freshwater. The study also showed the importance of fortnightly tidal variations, which are expected to have a larger effect on vertical mixing and plume dynamics than changes in runoff.

Key words: plurne, ice cover, La Grande Rivière, James Bay