Photo credit: Theo Willis

Casco Bay is an estuary – a place where rivers meet the sea. Casco Bay was named an “estuary of national significance” (a designation under the Clean Water Act) in 1990 because of its richness and diversity of marine life.

Casco Bay supports 850 species of marine life, and contains about 8,200 acres of eelgrass beds. This nationally significant embayment is home to important commercial species such as clams, lobsters, and numerous fish species.

The waters and islands of Casco Bay cover nearly 200 square miles from Cape Elizabeth to Cape Small in Phippsburg. From the top of the watershed, in the western mountains near Bethel, to the coast, all the waters that flow to the seas are part of the Casco Bay Watershed.

Twelve significant lake and river systems feed the Bay, including Sebago Lake and four major freshwater tributaries: the Fore, Presumpscot, Royal and Stroudwater Rivers. Casco Bay is dotted with roughly 785 islands, islets, and exposed ledges.


Hover your mouse over the map to link to the various subwatershed areas.

Royal & Cousins Rivers Sebago/Upper Presumpscot Lower Presumpscot Fore River & Back Cove Broad Cove, Cumberland & Falmouth Foresides Harraseeket Islands Maquoit and Middle Bays Harpswell Sound/Quahog Bay New Meadows Southern Bay