The Great Lakes system is a chain of lakes and connecting channels descending like a series of steps toward the Atlantic Ocean. Lake Superior, located at the top of the chain, is about 183 metres (601.7 feet) above sea level, while Lake Ontario stands at about 74.7 metres (245.1 feet). (Select the adjacent image, courtesy of the U.S. Army Corps of Engineers, Detroit District, for a larger view of the Great Lakes-St. Lawrence system's profile.)
Water from Lake Superior flows into Lake Huron through the St. Marys River. Since Lake Michigan and Lake Huron are connected by the broad and deep Straits of Mackinac and stand at virtually the same elevation, they are often referred to as one lake hydrologically; that is, Lakes Michigan-Huron. From Lakes Michigan-Huron, water flows through the St. Clair River, Lake St. Clair and the Detroit River into Lake Erie. Lake Erie discharges through the Niagara River and the Welland Canal into Lake Ontario. The portion of flow diverted to Lake Ontario through the Welland Canal is relatively small (about 4 to 5 percent of the total Lake Erie outflow). Water from Lake Ontario flows to the Atlantic Ocean through the St. Lawrence River. The average St. Lawrence River flow, recorded at Cornwall, Ontario, during the period 1900-95, is 6,910 cubic metres (244,000 cubic feet) per second. This average outflow is relatively small (less than 1 percent per year) in comparison to the total volume of water contained in the system.
Only the outflows from lakes Superior and Ontario are regulated via control structures, and may be varied within limits in accordance with their respective regulation plans. The outflows from lakes Michigan-Huron and Erie are controlled exclusively by the hydraulic characteristics of their outlet rivers.
The immense storage capacities of the Great Lakes, in combination with their restricted outflow capacities, allows the lakes to absorb large variations in water supplies, while maintaining remarkably steady outflows compared with other large rivers. For example, the highest St. Lawrence River flow is only 2.3 times greater than its lowest rate. In contrast, the Mississippi River at St. Louis, Missouri, has a maximum flow about 30 times its minimum.
Because of the size of the Great Lakes and the limited discharge capacities of their outlet rivers, extremely high or low levels can persist for a considerable time, even when water supplies change significantly.
Charles F. Southam
Environment Canada-Ontario Region, Environmental Services Branch firstname.lastname@example.org
General Resources GLIN: Compare snow melt (snow water equivalent) for the upper Great Lakes basin: 1997-2000 From the summer of 1997 to today, water levels on the middle Great Lakes have fallen from near record highs to near record lows. This decline is nearly 3.5 feet on lakes Michigan, Huron, St. Clair and Erie. This trend is forecasted to continue for the foreseeable future due in part to the fact that the drainage basins for lakes Superior and northern Michigan and Huron (the headwaters for the Great Lakes) have had one of the driest winters on record. Snowmelt runoff is a key component in replenishing groundwater and tributary stream flows into the Great Lakes.
Significant Events U.S. Army Corps of Engineers (USACE), Detroit District The outflows of lakes Superior and Ontario for the last month, along with other significant events for the current month are updated daily.
Great Lakes Monthly Hydrologic Data (1860-1990) National Oceanic and Atmospheric Administration (NOAA), Great Lakes Environmental Research Laboratory (GLERL) Historical monthly flows for the connecting channels--St. Marys, St. Clair, Detroit and Niagara rivers--and the St. Lawrence River and diversion are available from various original sources.