Change in Sedimentation Following River Diversion in the Eastmain Estuary (James Bay), Canada

Abstract

Sedimentological changes that occurred in the Eastmain estuary after a 90% reduction in discharge following river diversion in July 1980 were studied during four consecutive summers. Before cut-off, the estuary was kept well flushed of river derived solids. The new sets of physical conditions led to progressive sediment retention. After July 1980, salt migration 8 km inland brought within one year the development of a turbidity maximum zone near the tip of the intrusion. This turbidity zone is unstable: suspended sediments trapped within it tend to be flushed downstream by large fluctuations in the residual flow which are caused by local precipitations or discharge control at the dam. Fine sediments accumulate at rates of 0.02 to 0.05m per year over the pre-cut-off surface. From sediment trap data and acoustic records, it appears that sediment movement takes place by means of dilute mobile lutite suspensions close to the sediment-water interface, particularly during periods of surge in runoff. Since 1981, there has been a statistically significant rise in turbidity near the bottom, but not in the surface layer, suggesting a general increase in the vertical turbidity gradient. The rates of suspended sediment delivery into James Bay are at least 25 times smaller than they were before 1980.