Dewey Marsh contains a large expanse of relatively undisturbed northern sedge meadow including smaller islands of tamarack and black spruce swamp, paper birch and white pine woods, and ericaceous bog. The marsh is dominated by blue-joint grass, steeple bush and wire-leaved sedges, with cat-tails and bur-reed. It forms the headwaters of Hay Meadow Creek. Scattered throughout the area are islands of aspen, birch, and white pine.