Rainy Lake is fed by the waters of Rainy River flowing to the border of both countries. An international bridge, the Fort Frances International Falls International Bridge, crosses the US-Canada border on the Rainy River near Rainy Lake.
Geologically, the Rainy Lake, part of the Canadian Shield dating from the Precambrian and retains the characteristics associated with it, like an old caldera and a fault line which is clearly visible on satellite images of the lake. The many islands that emerge from the lake consist of ancient Precambrian rocks. This rock has been considerably affected by the glaciation that dominates much of the recent geological history of this region.