During the 2004 water year, runoff was above normal in the northern and central parts of the State of Illinois, and below normal in the southern part of the State.  Annual runoff at the northern index station--Pecatonica River at  Freeport (05435500)--was 1,038 ft³/s or 110 percent of the 30-year (1961-90) average runoff of 947 ft³/s.  At the central index station--Sangamon River at Monticello (05572000)--, runoff was 600 ft³/s or 140 percent of the 30-year average of 430 ft³/s.  Annual precipitation in southern Illinois was nearly 8 inches below normal (from data map--Midwestern Regional Climate Center's Web site), which contributed to below-normal annual runoff in southern Illinois.  At the southern index station--Skillet Fork at Wayne City (03380500)--, runoff was 317 ft³/s or 78 percent of the 30-year average of 406 ft³/s.  A comparison of the 2004 daily discharges for the three index stations and the mean-daily discharges for the 30-year period (1961-90) is shown in figure 1.

Runoff at the northern-index station at Freeport was generally above normal during May -- July.  Northern Illinois experienced heavy rainfall and flooding near the end of May.  As a result, the upper reaches of the Des Plaines River had record flooding during May 23-24.  Above normal flows also occurred during parts of November, December, February, March, and August.  Below-normal flows occurred in October.  The central-index station at Monticello had events of above normal flows during parts of November - January and March - August.  Other months had near normal flows.  The southern-index station at Wayne City experienced near-normal flows most of the water year.  The largest runoff events occurred in May and August.  Due to below-normal rainfall in southern Illinois during October and parts of November, the gaging station at Wayne City recorded below-normal runoff during parts of those months.  Below-normal runoff also occurred at Wayne City during the months of March and April.

All index gaging stations  had above-normal runoff during May and June because of heavy rains throughout the State.  Below- to near-normal flows were observed at all stations during the months of October and September.