Important granite cliffs line the Mattawa. Ten thousand years ago, the Great Lakes emptied through the Mattawa, into the Ottawa and then into the St-Lawrence. When the glaciers melted the land rebounded and eventually the Great Lakes starting draining south rather than northeast. This rebound eventually caused lake Nipissing to separate from Trout Lake. Today a 9 km land bridge separates these two lakes, with Trout Lake flowing east and lake Nipissing flowing south to Georgian Bay, Lake Huron and beyond, eventually reaching the St-Lawrence and the Atlantic Ocean.
Trout Lake is a good example of an oligotrophic lake.
This implies a cold, clean, deep lake with low nutrient levels and consequently low productivity. These kind of lakes usually support a cold water fishery such as lake trout, and in the case of Trout Lake, ouananiche. The Trout Lake watershed includes all lands that drain into Trout Lake. The long axis of Trout Lake runs east-west, and the topography of the northern portion of the watershed is very different from that of the southern portion of the watershed. The northern portion of the watershed is characterized by steep slopes.