Located in the Appalachian geological province, Lake Magog consists of sedimentary rock, especially slate and sandstone.

The drainage area of the Magog River's watershed is 2,032 square kilometres (785 sq mi).

Magog Lake is a reservoir resulting from these dams, the most influential being that of Rock Forest built in 1911.

[4] These dams alter the rate of renewal of the lake by changing the flow of the Magog River.

At the southern end of the lake, a 65 hectares (160 acres) marsh is accessible to hikers.