Southern Indian Lake is a widening of the Churchill River located in northern Manitoba. It has a long complex shoreline with many islands, long peninsulas and deep bays. It is 145 km long and up to 19 km wide (2). The Churchill River rises in Alberta, drains central Saskatchewan and a portion of northern Manitoba into Southern Indian Lake.

Southern Indian Lake may be divided into seven regions using natural boundaries. Opachuanau Lake forms an eighth region.

The lake is situated on the Precambrian Shield. The dominant surface material is glaciolacustrine clays deposited by glacial Lake Agassiz. The relief is greater in the southern region. Prior to 1976 Southern Indian Lake drained into Hudson's Bay via the lower Churchill River. In that year Manitoba Hydro diverted the greater portion of the Churchill River flows from Southern Indian Lake at South Bay into the Nelson River drainage system and via that river into Hudson's Bay. This was accomplished by damming the Churchill River at Missi Falls, constructing a diversion channel at South Bay and then raising the lake level 3 m to permit gravity flow into the headwaters of the Rat River tributary of the Nelson River.

Before 1974 Southern Indian Lake was remote and accessible only by air and boat in the summer and by air and winter road in the winter. This remoteness permitted the lake to remain relatively immune from human influence except for a commercial whitefish fishery organized in 1942. Early in the 20th century people from the Nelson House Indian Reserve settled on Southern Indian Lake and established the community of Southern Indian Lake (population approximately 700 people in 1974). The present economy is composed of subsistence hunting and fishing, a severely damaged commercial fisheries and trapping supplemented by government services and compensation payments (Q).