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Located in the Great Lakes region, Michigan is composed of two large peninsulas. The Lower Peninsula is bounded by Lakes Huron and Erie on the east and by Lake Michigan on the west. The Upper Peninsula is bounded by Lakes Michigan and Huron on the south and by Lake Superior on the north. The climate of Michigan is affected by the Great Lakes. The large surface area and volume of the Great Lakes have a stabilizing effect on temperature. Prevailing westerly winds cause milder winters and cooler summers than at similar latitudes farther west.
Surface-water supplies are constantly replenished by precipitation, which is evenly distributed throughout the year. About 55-60 percent of the annual precipitation of 31 inches is received during the growing season. Summer precipitation generally results from convective storms, whereas winter precipitation generally results from frontal systems. Annual snowfall in the Upper Peninsula is among the largest east of the Rocky Mountains.
Major flooding is not frequent in Michigan. When flooding does occur, it is most likely in late winter or early spring when sudden warming and rainfall combine with snowmelt, saturated or frozen soils, and decreased evapotranspiration to produce large quantities of runoff. Flooding can occur during any season. About 6 percent of the State is considered to be susceptible to flooding, and damage by flooding is $60-100 million annually. A major flood in the central Lower Peninsula in March 1904 was one of the first floods recorded in the State. The cause of this flood was rain, snowmelt, and frozen soil. Other memorable floods were recorded in April 1947, April-May 1960, and April 1975. The most extensive and damaging flood of record was in the central Lower Peninsula in September 1986. This flood caused damage of about $500 million.
Mild droughts are common in Michigan, but severe droughts are infrequent and generally of short duration. The even distribution of precipitation and moderate humidity are helpful in meeting the large demand for moisture by crops. Although rainfall is abundant during the summer, runoff is decreased because of increased evapotranspiration and absorptive capacity of soils. The most severe drought of record was during the 1930's, when only a few streamflow-gaging stations were in operation. The recurrence interval of the 1930-37 drought was 50-70 years, depending upon locality. The second most severe drought, that of the 1960's, was well documented. The recurrence interval for the 1960-67 drought ranged from 40 to 65 years. The effects of both droughts were statewide.
A flood-hazards program, which is both service oriented and regulatory, is operated by the State of Michigan. Coordination among Federal, State, and local agencies is an important component of
the program. About 650 communities or local units of govern-
ment that have flood-prone areas participate in the National Flood Insurance Program.
The National Weather Service (NWS) provides flood forecasts for 74 locations. Most of the data necessary for these forecasts are provided by a network of volunteer observers and numerous automated telemetering devices installed at streamflow-gaging stations operated by the U.S. Geological Survey. No fully automated flood-warning systems are operated within the State.
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