Connecting channel and St. Lawrence River flows are mainly determined by Great Lakes water levels. The flows increase in the spring and summer and decrease in the fall and winter in response to the seasonal rise and fall in lake levels. These flows also increase or decrease from year to year depending on the total annual water supply. Extended periods of above average precipitation and low evaporation can cause lake levels to rise and outflows to increase, while severe droughts can result in rapidly falling water levels and decreased outflows.
The river hydraulic characteristics determine the magnitude of flows in the St. Clair, Detroit and Niagara rivers in response to their respective upstream and downstream water levels. The influence of the downstream water level on river flows is referred to as a backwater effect. Because Lake Superior and Lake Ontario are regulated by physical structures, the flows in the St. Marys and St. Lawrence Rivers are controlled according to their respective regulation plans. However, the regulated flows are a function of upstream and downstream water levels and are constrained by channel capacity and other factors, such as winter river ice formation and breakup.
Other natural factors affecting flows are storms, ice and weeds, and the gradual rebounding of the earth's surface. Storms with strong winds that push water on shore and barometric pressure changes can result in short-term lake level fluctuations (called wind setup) that affect river flows. These fluctuations are most pronounced on Lake Erie, causing temporary increases and decreases in Niagara River inflows, and flow reversals in the lower Detroit River. River ice formation and transport during the winter and early spring, as well as weed growth in late summer, reduces flows. Isostatic rebound, a gradual rising of the earth's crust, affects flows over centuries by changing the river slopes.
Flows also are affected by human activity. These activities include channel navigation improvements, construction of bridge piers, shoreline modifications such as fills and seawalls, changes in ice accumulation due to ice booms and thermal input, and lake regulation. These activities alter the water level-flow relationships of the rivers. Diversions to and from the lakes can also affect flows because of their ultimate effect on lake levels. Human-related changes to date have been small in comparison to the long-term natural range of flow fluctuations; however, future impacts due to global climate change from an enhanced greenhouse effect may result in much larger changes.
Deborah H. Lee
Formerly with the National Oceanic and Atmospheric Administration, Great Lakes Environmental Research Laboratory
General Resources Detroit River Daily-Averaged Flows NOAA, Great Lakes Environmental Research Laboratory (GLERL) Data from an Acoustic Doppler Current Profiler (ADCP) that was deployed in the Detroit River near NOAA's Fort Wayne water level gage. This meter uses the doppler sonar principle to measure water velocity in a column at a point in the Detroit River.