PRE-1900 ST-CLAIR RIVER FLOW REGIME

The St. Clair River is the outlet channel for Lakes Michigan and Huron to the lower Great Lakes. The river's hydraulic characteristics natur ally regulate Lakes Michigan-Huron's water levels by controlling the a mount of water that flows out of the lakes. Accurate determinations of the outflows are necessary, in conjunction with the St. Marys River f lows and lake level data, to determine the water supplies to Lakes Mic higan-Huron. The hydraulic regime of the river has been changed many t imes since the mid-1800s primarily due to dredging for deeper draft na vigation and sand and gravel mining. In addition there have also been minor effects due to shipwrecks at the head of the river. For water re source studies of the Great Lakes system it would be highly desirable to determine Lakes Michigan-Huron's water supplies for the period 1860 -1900. Additional water supply values would extend the available perio d of study by 40 years or approximately 45 percent. This period is als o particularly important as it contains extreme high water supplies th at led to record Lakes Michigan-Huron water levels. This study compris ed an analysis of the existing discharge and dredging data for the 186 0-1902 period to determine if discharge equations could be developed w ith sufficient accuracy to compute monthly St. Clair River flows. The analysis demonstrated that it is not possible to quantify St. Clair Ri ver flows prior to 1900, thus limiting the period available for determ ining Lakes Michigan-Huron water supplies to this century. The study a lso confirmed the previously determined 0.18 m lowering of Lakes Michi gan-Huron between 1900 and present due to channel dredging in the uppe r river.