The Saguenay-Lake-Saint-Jean hydrographic system located within the south- central part of the Province of Quebec (Canada), has been since 1926 an important source of hydro-electric power. From the late twenties to the end of the fifties six hydro-electric power dams were built on the Peribonca and the Saguenay Rivers for a total installed capacity of 2,700 MW. These are owned and operated by the Aluminium Company of Canada (ALCAN) which uses this energy for the electrolysis of alumina in its three nearby aluminium smelters having a total annual metal production of 606,000 t. The whole hydrographic system covers a total area of 85,000 km2 within the northern latitudes of 47!|0' to 52!|1' and the western longitudes of 69!|1' to 74!|0'. The catchment, of typical Canadian Shield landforms, is made up of three distinct physiographic units. The forested head water unit, varying in altitude from a maximum of 1,500 m to a minimum of 180 m, drains 80% of the whole system. In its northern part, it is drained by several large rivers (Peribonca, Mistassini, Mistassibi, Chamouchouane, Ouasienmsca, Samaqua) all running in a southern direction through nearly parallel glacier-carved valleys. The southern smaller part of the Highlands is drained by the Metabetchouane, the Ouiatchouane and the Ouiatchouaniche. All these converge toward Lake Saint- Jean which acts as the main central reservoir for power dams located on the head of the Saguenay River.

Although the Saguenay Fiord physiographic unit is very rugged, it is of majestic beauty and this has recently lead our governments to create national parks within that part of the country. Moreover, it is the most southerly fiord of the North Hemisphere and has been sustaining since postglacial times a typical arctic inland-entrapped fauna within its cold water which reaches a depth over 350 m. The white whale (Delphinapterus leucan) has been, till lately, the most important species representative of this northern fauna. Presently, because of industrial pollution originating since half a century largely from the aluminium smelters, their numbers are rapidly diminishing. From its asymmetric population age structure, it is only a matter of time before being completely wiped out from that peculiar range habitat.

The climate of the area is humid continental, with cold summer but without dry season. At near the lake level (Roberval), July mean temperature of the area is 17!| while January average is -17!|. The growing season lasts from May 18th to October 4th, for a period of about 100 days without frost.

Because of the large volume of low mineralized waters (1,457 m3 sec-1) originating from unpopulated forested areas and the short retention time (93 days) within the lacustrine basin, the water quality of the pelagic zone is almost unaffected by anthropogenic activities (Q).