The Saint Marys River begins at the downstream end of Whitefish Bay in Lake Superior. East of Sault Ste. Marie, the river divides multiple times, creating a series of islands and pools.
Over its course, the river runs approximately 77 nautical miles before passing through De Tour Passage into Lake Huron.
At Sault Ste. Marie, the Soo locks allow ships to bypass an otherwise unnavigable stretch of the river. The Poe Lock, the largest lock at the Soo Locks is 1,200 ft (365.8 m) long, 110 ft (33.5 m) wide, and 32 ft (9.8 m) deep. The Soo Locks Visitor Center is a favorite stop for ship spotting enthusiasts.
East of Sault Ste. Marie, the river divides, forming Sugar Island. The northern fork flows through Little Lake George, Lake George, the Saint Joseph Channel, around Saint Joseph Island and into the North Channel of Georgian Bay.
The southern fork contains a maintained channel as well as a ship anchorage. At Lake Nicolette the channel divides around Neebish Island. Below Neebish Island, a section of the river is known as Munuscong Lake. The river flows south of Saint Joseph Island towards the De Tour Passage and into Lake Huron.
In 1987, a portion of the St. Marys River was designated an Area of Concern by the EPA. In 2013, a Great Lakes Restoration Initiative (GLRI) project was launched to re-establish a portion of the historic Little Rapids.
A partnership of government organizations entities, cooperated in the construction of a new bridge which allowed free flow of the Little Rapids section of St. Marys River.
The Saint Marys River is designated as a Canadian Heritage River by the Canadian Heritage Rivers System (CHRS). The river forms part of the International Boundary separating Canada and the United States.
The DeTour Passage Underwater Preserve includes waters around Drummond Island, all of DeTour Passage and adjacent sections of Lake Huron and the St. Mary’s River. The Preserve features a variety of dive sites.
Several U.S. and Canadian important bird areas (IBAs) are located along the river. On the American side are Sault Sainte Marie Clay Plain IBA, Lower St. Mary’s River and Potagannissing Bay IBA, and Helmet Shoal and Saddlebag Island IBA. On the Canadian side is St. Mary’s River Complex (Echo Bay ON to Maxton Bay MI) IBA.
The Little Rapids were historically some of the most productive spawning grounds in the Great Lakes for sportfish. The restored rapids provide habitat for trout, salmon, walleye, lake herring, lake sturgeon, whitefish, aquatic invertebrates, and forage fish.
The Saint Marys River is one of the few Great Lakes waterways where opportunities exist for anglers to catch Atlantic salmon. The Michigan Department of Natural Resources (DNR) stocks Atlantic salmon annually in the St. Marys River and other waterways in the state.