The Town of Cobden lies on the shores of Muskrat Lake in a valley occupying an ancient fault line that was once a major channel of the Ottawa River. After the disappearance of the glaciers ten thousand years ago and removal of the weight of the Laurentian Ice Sheet, the elevation of the valley and surrounding lands gradually increased, and the flow of the Ottawa River declined. Eventually, water flow through the valley reversed, and a separate river – the Muskrat River – began to drain Muskrat Lake. The Muskrat River is the only major Ottawa River tributary in Renfrew County to flow in a northwest direction counter to the Ottawa. It was used by Aboriginal peoples, early European explorers (including Samuel de Champlain on his historic 1613 voyage) and later travellers to bypass the dangerous Rocher Fendu rapids on the Ottawa.
Muskrat Lake supports populations of pike, walleye, pickerel and lake trout; but is also known for annual outbreaks of blue-green algae. These are mainly attributed to excessive phosphorus loads in the Snake River, which flows into the lake from the south and carries nutrients from farming operations in drained wetlands. The southwestern shore of the long (15-km), narrow lake abuts a fertile clay plain with agricultural lands, while to the north and east, Sturgeon Mountain - a forested escarpment of Precambrian rock - rises more than 60 m, providing an impressive backdrop to the Town of Cobden. The Cobden Marsh, a provincially significant wetland at the southeast end of the lake, supports Sora, Virginia Rail, and Marsh Wren. The southwestern shore of the lake (including the Cobden municipal dock) is a good place for viewing ducks, shorebirds and other migrants, including rarities such as the Franklin’s Gull.