Observed and urbanization-adjusted annual maximum peak discharge and associated urban fraction and precipitation values at USGS streamgage:
05533000 FLAG CREEK NEAR WILLOW SPRINGS, IL
| Water year | Segment | Discharge code | Cumulative reservoir storage (acre-feet) | Urban fraction | Precipitation (inches) | Observed peak discharge (ft3/s) | Adjusted peak discharge (ft3/s) | Exceedance probability |
| 1951 | 1 | -- | 0 | 0.495 | 1.450 | 540 | 870 | 0.429 |
| 1952 | 1 | -- | 0 | 0.520 | 0.978 | 398 | 663 | 0.697 |
| 1953 | 1 | -- | 0 | 0.545 | 1.268 | 306 | 534 | 0.851 |
| 1954 | 1 | -- | 0 | 0.570 | 1.679 | 770 | 1080 | 0.209 |
| 1955 | 1 | -- | 0 | 0.595 | 3.433 | 1300 | 1720 | 0.041 |
| 1956 | 1 | -- | 0 | 0.620 | 0.803 | 285 | 472 | 0.915 |
| 1957 | 1 | -- | 0 | 0.645 | 2.212 | 1350 | 1710 | 0.041 |
| 1958 | 1 | E | 0 | 0.670 | 1.506 | 388 | 559 | 0.824 |
| 1959 | 1 | -- | 0 | 0.695 | 1.239 | 1550 | 1870 | 0.029 |
| 1960 | 1 | -- | 0 | 0.720 | 1.926 | 800 | 987 | 0.285 |
| 1961 | 1 | -- | 0 | 0.730 | 1.773 | 2680 | 3090 | 0.006 |
| 1962 | 1 | E | 0 | 0.740 | 0.286 | 344 | 470 | 0.918 |
| 1963 | 1 | -- | 0 | 0.749 | 1.171 | 367 | 488 | 0.902 |
| 1964 | 1 | -- | 0 | 0.759 | 0.777 | 375 | 491 | 0.900 |
| 1965 | 1 | -- | 0 | 0.769 | 0.527 | 576 | 699 | 0.644 |
| 1966 | 1 | -- | 0 | 0.779 | 1.410 | 975 | 1120 | 0.189 |
| 1967 | 1 | D | 0 | 0.789 | 1.161 | 816 | 945 | 0.333 |
| 1968 | 1 | -- | 0 | 0.799 | 1.402 | 648 | 755 | 0.568 |
| 1969 | 1 | -- | 0 | 0.809 | 0.883 | 803 | 915 | 0.374 |
| 1970 | 1 | -- | 0 | 0.819 | 0.955 | 400 | 479 | 0.910 |
| 1971 | 1 | E | 0 | 0.825 | 0.302 | 188 | 239 | 1.000 |
| 1972 | 1 | -- | 0 | 0.831 | 1.293 | 830 | 923 | 0.363 |
| 1973 | 1 | -- | 0 | 0.837 | 0.851 | 786 | 871 | 0.428 |
| 1974 | 1 | -- | 0 | 0.843 | 0.560 | 696 | 770 | 0.549 |
| 1975 | 1 | -- | 0 | 0.849 | 1.152 | 1630 | 1740 | 0.038 |
| 1976 | 1 | D | 0 | 0.856 | 0.558 | 2230 | 2360 | 0.015 |
| 1977 | 1 | -- | 0 | 0.862 | 1.233 | 560 | 614 | 0.759 |
| 1978 | 1 | -- | 0 | 0.868 | 0.774 | 575 | 624 | 0.747 |
| 1979 | 1 | -- | 0 | 0.874 | 0.968 | 967 | 1020 | 0.251 |
| 1980 | 1 | -- | 0 | 0.880 | 0.594 | 439 | 477 | 0.912 |
| 1981 | 1 | -- | 0 | 0.882 | 1.929 | 823 | 869 | 0.431 |
| 1982 | 1 | -- | 0 | 0.884 | 0.316 | 826 | 870 | 0.430 |
| 1983 | 1 | -- | 0 | 0.886 | 1.507 | 1960 | 2030 | 0.022 |
| 1984 | 1 | D | 0 | 0.889 | 0.593 | 670 | 705 | 0.635 |
| 1985 | 1 | -- | 0 | 0.891 | 1.240 | 1390 | 1440 | 0.077 |
| 1986 | 1 | -- | 0 | 0.893 | 0.686 | 636 | 667 | 0.695 |
| 1987 | 1 | -- | 0 | 0.895 | 2.055 | 939 | 974 | 0.295 |
| 1988 | 1 | E | 0 | 0.897 | 0.783 | 759 | 789 | 0.526 |
| 1989 | 1 | E | 0 | 0.899 | 1.969 | 708 | 735 | 0.592 |
| 1990 | 1 | -- | 0 | 0.902 | 2.711 | 1910 | 1960 | 0.025 |
| 1991 | 1 | -- | 0 | 0.903 | 2.638 | 1470 | 1510 | 0.065 |
| 1992 | 1 | CE | 0 | 0.904 | 2.503 | 1100 | 1130 | 0.188 |
| 1993 | 1 | C | 0 | 0.905 | 2.611 | 791 | 815 | 0.496 |
| 1994 | 1 | C | 0 | 0.906 | 1.190 | 569 | 589 | 0.790 |
| 1995 | 1 | CE | 0 | 0.907 | 0.967 | 567 | 586 | 0.793 |
| 1996 | 1 | C | 0 | 0.909 | 3.610 | 2300 | 2340 | 0.015 |
| 1997 | 1 | C | 0 | 0.910 | 2.669 | 1540 | 1570 | 0.054 |
| 1998 | 1 | C | 0 | 0.911 | 2.310 | 1160 | 1180 | 0.166 |
| 1999 | 1 | C | 0 | 0.912 | 1.170 | 860 | 879 | 0.420 |
| 2000 | 1 | C | 0 | 0.913 | 1.767 | 633 | 648 | 0.719 |
| 2001 | 1 | C | 0 | 0.915 | 2.225 | 745 | 759 | 0.562 |
| 2002 | 1 | C | 0 | 0.917 | 1.982 | 962 | 976 | 0.293 |
| 2003 | 1 | C | 0 | 0.919 | 1.143 | 600 | 610 | 0.763 |
| 2004 | 1 | C | 0 | 0.921 | 2.350 | 790 | 800 | 0.514 |
| 2005 | 1 | CE | 0 | 0.922 | 1.099 | 692 | 700 | 0.644 |
| 2006 | 1 | C | 0 | 0.924 | 1.666 | 559 | 565 | 0.817 |
| 2007 | 1 | C | 0 | 0.926 | 1.851 | 1490 | 1500 | 0.067 |
| 2008 | 1 | C | 0 | 0.928 | 3.057 | 1240 | 1240 | 0.141 |
| 2009 | 1 | C | 0 | 0.930 | 1.282 | 1330 | 1330 | 0.108 |