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"Lake Superior: Does Water In = Water Out?" by George Hite. April, 1998

. We all know it's a big lake. With a surface area of about 31,700 square miles, it exceeds the combined size of Massachusetts, New Hampshire, Vermont and Connecticut. There is enough water in the lake, over 3200 billion gallons, to supply the freshwater needs of the 30 million residents in the lake's adjacent states and province for over three years. But how much new water does all the snow and rain add to the lake each year, and if over a period of several years the lake level is stable, as it is, where does all the new water go? Is all this new water flowing through the lake why the lake is so nice and clean? A Web search provided some surprises.

About 60% of the annual inflow to the lake is rain and snow falling directly on its surface, the remaining 40% is runoff from the land areas draining into the lake. Compared to most lakes, the percentage of total inflow representd by drainage from adjacent lands is quite low. The total inflow, which averages about 28 billion gallons a year, would raise the lake level about 4.2 feet or 50 inches....about 2.5 feet due to precipitation on the surface and 1.7 feet from runoff.

It's interesting to note that the annual inflow is less than 1% of the lake's water volume. It takes about 185 years for the lake to flush itself out. The quality of the lake's water is therefore little impacted by the quantity of the new water, but because pollution flushed in takes so long to flush out, the lake is extremely sensitive to pollution sources in the lake's relatively small drainage basin. Pollution sources such as the the stamp mill deposits still so evident in our area, the taconite tailing dumping along the Minnesota north shore a decade or two ago, or the current discharges from wood pulp mills and growing population areas around the lake, have long lasting and accumulative impact. In addition, the dominating winds of the lake are southwesterly in summer and northwest in winter, placing the lake downwind of the agricultural areas in the upper Midwestern United States and the prairie regions of Canada...a source of airborne pesticides. The lake has difficulty keeping itself's up to us to do the job!

So what happens to the 28 billion gallons of water added to the lake in an average year? Not surprisingly, most of it, about 64% , almost two-thirds,or about 18 billion gallons, is discharged into the lower Great Lakes through the sluices and rapids at the Sault. The rate of that discharge is regulated by the International Lake Superior Board of Control and is intended to mitigate high water shoreline damage on Lake Superior and downstream Great Lakes. Lake Superior levels are dropped during the fall and early winter, creating space to hold the snow pack melt and spring and early summer rain storms. Our highest water levels are generally in late summer and early fall.

What is surprising, is the large amount of the annual inflow that disappears through evaporation...about 36%, a little more than a third, or about 10 billion gallons. My guess is that this is due to the large surface area of the lake and the relatively dry air moving across its surface from the mid-continent of North America. In fact, over the western half of the lake, including the portion of the lake around Eagle Harbor, the evaporation is equal to or exceeds the the amount of inflow due to rain and snow falling into the lake (59% of total inflow). The eastern end of the lake usually receives more annual precipitation than the west half and by the time prevailing winds arrive there, they are less dry.

So, of the about 4.2 feet or 28 billion gallons of new water added to the lake in an average year, about 2,7 feet is discharged into the lower lakes, and the remaining 1.5 feet returned to the sky above through evaporation. The long term impact on lake level is nill, and the quantity of new water, less than 1% of the lake's total water volume, has negligible affect on the lake's water quality.

(The primary information source for this article is an academic paper modeling the deposition of a pesticide into and out of the Great Lakes based on data obtained from Eagle Harbor and Sleeping Bear Dunes...authored by J. Christensen, J. Kennedy and K. Kotimko. Your Eagle Harbor Web editor takes full responsibility for interpretation of data and calculations. The Lake Superior map can be purchased from Lake Superior Magazine.)

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