Source of the St. Lawrence River: Lake Superior Headwaters

The St. Lawrence River's ultimate source is Lake Superior, the world's largest freshwater lake by surface area (82,100 km² / 31,700 sq mi). The St. Lawrence system functions as the Great Lakes' outflow to the Atlantic Ocean.

Hydrological Pathway

1. Lake Superior (headwater, elevation 183 m / 600 ft)
2. St. Marys River → Lake Huron (176 m)
3. St. Clair River → Lake St. Clair → Detroit River → Lake Erie (174 m)
4. Niagara River (Niagara Falls, 99 m drop) → Lake Ontario (75 m)
5. St. Lawrence River proper begins at Lake Ontario's northeast end (Kingston, ON / Cape Vincent, NY)
6. Flows 1,197 km (744 mi) to Gulf of St. Lawrence → Atlantic Ocean

Key Characteristics

• Drainage Basin: 1.34M km² (517,000 sq mi)—includes all Great Lakes
• Average Discharge: 16,800 m³/s (593,000 cu ft/s) at Quebec City
• Seaway: St. Lawrence Seaway (locks, canals) enables Great Lakes shipping to Atlantic
• Border: Forms Canada-US boundary for ~180 km

Alternative Headwater Definitions

• Hydrologic Source (furthest stream): Nipigon River (Ontario) → Lake Superior—~2,300 km total length
• Geographic Source: Often considered Lake Superior as the system's "headwater lake"

The Great Lakes-St. Lawrence system represents one of Earth's largest freshwater surface water systems—holding 21% of global surface fresh water.