Summary of Significant Floods in the United States, Puerto Rico, and the Virgin Islands, 1970 Through 1989

Illinois is located in the midcontinent interior and is affected by various weather patterns during the year. Moisture sources include the Gulf of Mexico, the Pacific Ocean, and, to a small degree, Lake Michigan. Frontal systems are quite active in Illinois as warm humid airmasses from the Gulf of Mexico are displaced by continental polar air from Canada or maritime polar air from the North Pacific Ocean. Winter storms usually become stronger as they cross the State and intensify as a result of temperature differences across the polar front. Mean annual precipitation ranges from 35 in. in the north to 46 in. in the south. Areas within 10 to 20 mi of Lake Michigan are affected by lake-effect snowfall in the winter and have greater annual values of precipitation. During the winter, floods can be caused by ice jams that force localized flooding. Thunderstorms associated with frontal systems and squall lines are responsible for flash flooding in the State during the summer months. Rainfall rates of 16 in. in 12 hours have been recorded from these systems. Persistent upper atmospheric patterns can result in repeating storm systems that can lead to regional flooding.

On the basis of regionalization procedures that relate flood characteristics to watershed and climatic characteristics, the magnitude of maximum discharges for streams in Illinois is dependent on drainage area, main-channel slope, and 2-year, 24-hour rainfall (Jennings and others, 1994).

Twenty percent of the streamflow-gaging stations in the State recorded significant discharges during the flood of December 1982. Rainfall in the Illinois River Basin totaling 3 to 5 in. on wet soil caused the flooding. This flood was responsible for eight deaths. During February to March 1985, excessive rain on snow in northern Illinois caused $10 million in damages. From September 20 to October 3, 1986, excessive rains in northern Illinois resulted in four deaths and $50 million in damages. Parts of Chicago were inundated by as much as 9.4 in. of rain on August 13 and 14, 1987, that resulted in four deaths and $77.6 million in damages (Paulson and others, 1991).

Jennings, M.E., Thomas, W.O., Jr., and Riggs, H.C., 1994, Nationwide summary of U.S. Geological Survey regional regression equations for estimating magnitude and frequency of floods for ungaged sites: U.S. Geological Survey Water-Resources Investigations Report 94-4002, 196 p.

Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, D.W., compilers, 1991, National water summary, 1988-89-Hydrologic events and floods and droughts: U.S. Geological Survey Water-Supply Paper 2375, 591 p.