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Construction of the La Grande Complex in Eastern Canada called for complete or partial diversion of six rivers, the mean annual discharge of which varied from 60 to 1600m3 s-I, and the transfer of water from two adjoining watersheds (1586m3ss-') into La Grande Rivitre. Three of these rivers with reduced flow, Eastmain (from 700 to 95 m3 s-'), Opinaca (from 260 to 35 m3 s-I), and Caniapiscau (from 1708 to 960 m3 s-I) and the waterwa s with increased flows, Boyd-Sakami (from 1 to 81 1 m3 s-I) and La Grande Riviere (from 1760 to 34(X)m3 s-) were intensively monitored from 1978 to 1988, while the others were surveyed sporadically. Very few modifications were observed, compared to previous conditions, in banks stability, water quality, and aquatic fauna of the affected rivers; those which appeared can be explained by the water level fluctuations, the water quality of the tributaries of the residual drainage basin, and/or by the direct influence of the reservoirs. In James Bay, the areal extent of the Eastmain River and La Grand Rivikre plumes varied with discharge in an exponential relation, mostly under ice cover; resources from the coastal environment had not yet undergone any major changes. Because of the very small population dwelling in the area, less than 3000 inhabitants, modifications in the hydrological regime of these rivers did cause relatively few impacts, the main being greater banks instability in specific locations, reduction of the thickness and ice cover on La Grande Riviere, enhancement of navigation constraints on Eastmain River, and higher mercury levels in fish related to operation of the reservoirs.

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