Lake Michigan - Hydrology

Hydrology

The Milwaukee Reef, running under Lake Michigan from a point between Milwaukee and Racine to a point between Grand Haven and Muskegon, divides the lake into northern and southern basins. Each basin has a clockwise flow of water, deriving from rivers, winds, and the Coriolis effect. Prevailing westerly winds tend to move the surface water toward the east, producing a moderating effect on the climate of western Michigan. There is a mean difference in summer temperatures of 5 to 10 degrees Fahrenheit (2 to 5 degrees Celsius) between the Wisconsin and Michigan shores.

Hydrologically Michigan and Huron are the same body of water (sometimes called Lake Michigan-Huron), but are normally considered distinct. Counted together, it is the largest body of fresh water in the world by surface area. The Mackinac Bridge is generally considered the dividing line between them. Both lakes are part of the Great Lakes Waterway. In earlier maps of the region, the name Lake Illinois has been found in place of "Michigan".

Historic High Water

The lake fluctuates from month to month with the highest lake levels in October and November. The normal highwater mark is 2.00 feet (0.61 m) above datum (577.5 ft or 176.0 m). In the summer of 1986, Lakes Michigan and Huron reached their highest level at 5.92 feet (1.80 m) above datum. The high water records began in February 1986 and lasted through the year, ending with January 1987. Water levels ranged from 3.67 feet (1.12 m) to 5.92 feet (1.80 m) above Chart Datum. On February 21, the waters neared the all-time maximum.

Historic Low Water

Lake levels tend to be the lowest in winter. The normal lowwater mark is 1.00 foot (0.30 m) below datum (577.5 ft or 176.0 m). In the winter of 1964, Lakes Michigan and Huron reached their lowest level at 1.38 feet (0.42 m) below datum. As with the highwater records, monthly low water records were set each month from February 1964 through January 1965. During this twelve month period water levels ranged from 1.38 feet (0.42 m) to 0.71 feet (0.22 m) below Chart Datum.