It is a tributary of the Saint John River, which flows to the Bay of Fundy (an arm of the Atlantic Ocean) in New Brunswick, Canada.

Fish River enters the south end of the lake 2 miles (3.2 km) downstream of the confluence of Fox Brook and Carr Pond Stream.

Fish River enters the western end of the lake near the western edge of town and overflows the northern end of the lake near the northern edge of town.

Tributary Mosquito Brook flows 8 miles (13 km) from township 14, range 7, into the northern part of the lake.

The Bangor and Aroostook Railroad approaches from the south and runs along the eastern shore of the lake to follow the Fish River from Portage Lake all the way downstream to Fort Kent.