The Deer River originates in a neighboring county, but a portion of its water flows through St. Lawrence County.
The Deer River originates in Franklin County, but the river then flows through northeastern St. Lawrence County before emptying into the St. Regis River at Helena. A canoe is the best way to fish the Deer River, which has good populations of smallmouths and walleyes. Muskellunge are also present, and catch and release of all muskies is encouraged. The Deer flows past the communities of Lawrenceville, North Lawrence, Brasher Iron Works, and Helena. Access is available from state land and road crossings.