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The St. Marys River (sometimes spelled as St. Mary's River) is a river, approximately 22 mi (35 km) long, in southern Maryland in the United States. It rises in southern St. Mary's County, and flows SE through Great Mills, widening into a tidal estuary near St. Marys City, approximately 2 mi (3.2 km) wide at its mouth on Chesapeake Bay, near the mouth of the Potomac River.