Most major floods and droughts
described herein have large areal extent and substantial recurrence intervals--greater
than 25 years for floods and greater than 10 years for droughts. Numerous
other floods and droughts have occurred in Michigan that were of lesser
magnitude and that generally were less widespread than those described
but, nonetheless, had a significant impact. Major floods and droughts,
and those of a more local or less severe nature, are listed chronologically
in table 1; rivers and cities are shown
in figure
2.
A
record of stream response to precipitation extremes in a watershed is invaluable for water-resources planning. History indicates that streamflow maximums and minimums are continually surpassed; thus long-term, continuous streamflow monitoring is of great value. Streamflow data before 1931 are scarce, especially for unregulated streams. Before that time, most of the State's gaging stations were operated on regulated streams in conjunction with hydropower operations. The most useful streamflow data for this study began in 1931 when gaging stations on unregulated streams became more numerous. Data from 95 gaging stations were used to determine the areal extent and severity of historical floods in Michigan, and data from 40 stations were used for the drought analysis.
To depict floods (fig.
3) and droughts(fig.
4) graphically in Michigan, six streamflow-gaging stations were selected
from the statewide gaging-station network. The six gaging stations have
long periods of record, are located on unregulated streams, are representative
of hydrologic conditions in major areas of the State, and were operational
during water year 1988. Streamflow data are collected, stored, and reported
by water year (a water year is the 12-month period from October 1 through
September 30 and is identified by the calendar year in which it ends).
Because of the State's peninsular configuration,
rivers flow relatively short distances from their source areas to the
Great Lakes (fig.
2). Most of the basins (93 percent) are entirely within State boundaries
(Miller and Twenter, 1986, p. 277). The Great Lakes drain into the St.
Lawrence River and ultimately into the Atlantic Ocean. In this report,
the upper Grand, Maple, lower Grand, and Thornapple River basins are collectively
referred to as the Grand River basin. The Pine and Tittabawassee River
basins are denoted as the Tittabawassee River basin.
A discussion of floods and droughts in Michigan
would not be complete without mention of water levels in the Great Lakes.
The large storage capacity of the Great Lakes generally accommodates most
of the variations in water supply. However, water levels are subject to
seasonal and annual fluctuations. In the early 1950's and the early 1970's,
the average annual levels were record highs following record-low levels
in the mid-1930's and the mid-1960's. Record-high water levels occurred
again in the mid-1980's as a result of more than a decade of greater than
normal precipitation and less than normal air temperature, which translate
into less evaporation and transpiration. The greater than normal streamflow
that contributed to the rise of the lakes is graphically shown by positive
departures from normal in figure
4. Great Lakes diversions and damage caused by high water levels are
described by Hitt and Miller (1986).