The St. George River is a river in Maine with a watershed of 225 square miles in a unique and historic area of mountains, sea coast, lakes, tidal streams and inlets. The origin of the St. George River is the outflow of St. George Lake in Liberty. The river follows a winding course 45 miles south to Thomaston, where the river flows into the estuary and runs about …The St. George River is a river in Maine with a watershed of 225 square miles in a unique and historic area of mountains, sea coast, lakes, tidal streams and inlets. The origin of the St. George River is the outflow of St. George Lake in Liberty. The river follows a winding course 45 miles south to Thomaston, where the river flows into the estuary and runs about 12 miles southwest to Muscongus Bay, forming the border between Cushing and St. George.