Trends in floods and related climate conditions in Illinois

The climate of Illinois (Midwest U.S.A.) has gradually become cooler and wetter since 1940, raising questions of possible effects on flooding. The frequency and duration of both winter and summer season floods during the 1921-80 period exhibited general up trends, peaking during 1971-80 in basins in the eastern two-thirds of Illinois. Heavy rain ( greater than or equal to 5.1 cm) events in the summer were found to have similar up trends in the same regions where summer flooding was increased. Summer heavy rain increased 27% from 1921-1980, as compared to 43% increase in flood durations and a 77% increase in flooding events. Winter temperatures have decreased 16% since 1930, whereas winter precipitation has increased in northern and eastern Illinois by 12% where winter flooding increased. The increase in winter precipitation apparently has been more important than the lowering temperatures in producing more winter floods, and particularly longer duration winter floods.