Illinois HIGH and LOW

When people describe Illinois as being flat, what exactly do they mean? What is flat? Is there a way to measure flatness? In fact, there are two ways flatness can be measured. 1) It can be measured by the amount of relief, defined here as the vertical difference in elevation between the summits and the lowlands of a given region. 2) It can be measured by the average slope, which is measured with the equation (rise/run) x 100.

Because there are two ways to measure flatness, there are four possible combinations of slope and relief, as illustrated in the following example profiles:

Diagram of low relief and slope   Diagram of high relief and low slope

                  Low relief and low average slope                                               High relief and low average slope

Diagram of low relief and high slope   Diagram of high relief and high slope

                    Low relief and high average slope                                       High relief and high average slope

The landscape of Illinois resulted from the action of glaciers, streams, rivers, and wind. Glaciers eroded the previously existing landscape, and deposited the material elsewhere. Glacial moraines account for many of the high places in Illinois but the highest point in Illinois is a bedrock exposure in an area the glaciers never reached. The low places are generally in the river and stream valleys of the state and the lowest point in the state is at the southern tip, where the Ohio and Mississippi Rivers meet. The following list of county names contains links that will give you much more information about the specific county.

County Area Total Reliefsort descending Percent Slope
Douglas 412.83 square miles 100 feet 0.40%
Kendall 319.42 square miles 170 feet 0.89%
Moultrie 340.35 square miles 170 feet 0.55%
Cumberland 342.94 square miles 170 feet 0.73%
Jasper 492.09 square miles 180 feet 0.65%
Macon 579.62 square miles 185 feet 0.63%
Menard 311.94 square miles 185 feet 0.95%
Grundy 426.49 square miles 190 feet 0.54%
Edwards 220.38 square miles 190 feet 1.21%
Kankakee 674.62 square miles 190 feet 0.55%
Warren 557.82 square miles 198 feet 0.71%
Piatt 434.62 square miles 200 feet 0.46%
Dewitt 400.71 square miles 206 feet 0.63%
Perry 442.30 square miles 210 feet 0.73%
Macoupin 857.93 square miles 210 feet 0.86%
Clinton 497.76 square miles 210 feet 0.72%
Richland 358.02 square miles 210 feet 0.77%
Sangamon 867.37 square miles 210 feet 0.62%
Ford 481.40 square miles 215 feet 0.54%
Iroquois 1,108.09 square miles 215 feet 0.44%
Pope 201.20 square miles 220 feet 0.96%
Jefferson 577.55 square miles 220 feet 0.95%
Clay 464.22 square miles 225 feet 0.68%
Effingham 474.92 square miles 230 feet 0.75%
Wabash 225.04 square miles 230 feet 0.96%
Coles 504.87 square miles 230 feet 0.84%
Marion 569.69 square miles 230 feet 0.84%
Crawford 440.97 square miles 230 feet 0.91%
Champaign 987.74 square miles 230 feet 0.50%
Lawrence 369.94 square miles 235 feet 0.84%
Warren 538.32 square miles 240 feet 0.96%
Washington 707.76 square miles 240 feet 0.79%
Christian 707.56 square miles 240 feet 0.50%
White 496.49 square miles 240 feet 0.91%
Du Page 333.51 square miles 240 feet 0.88%
Livingston 1,035.63 square miles 245 feet 0.52%
Franklin 427.09 square miles 254 feet 0.85%
Montgomery 701.92 square miles 257 feet 0.84%
Fayette 717.64 square miles 260 feet 0.87%
Cass 379.55 square miles 260 feet 0.96%
Madison 732.09 square miles 260 feet 0.93%
Logan 612.32 square miles 261 feet 0.67%
Greene 541.04 square miles 270 feet 1.36%
Hancock 805.69 square miles 270 feet 1.17%
Rock Island 447.32 square miles 270 feet 1.50%
Scott 250.06 square miles 270 feet 1.32%
Morgan 566.72 square miles 275 feet 0.92%
Stark 285.67 square miles 275 feet 0.95%
Hamilton 431.24 square miles 280 feet 1.07%
Henderson 391.97 square miles 282 feet 1.04%
Massac 239.30 square miles 290 feet 1.26%
Mercer 562.12 square miles 290 feet 1.35%
Mason 557.44 square miles 295 feet 0.54%
Vermilion 891.67 square miles 300 feet 0.89%
Whiteside 691.50 square miles 300 feet 0.82%
Putnam 170.51 square miles 300 feet 1.77%
Winnebago 515.20 square miles 300 feet 1.73%
McDonough 583.92 square miles 300 feet 1.13%
Clark 499.36 square miles 305 feet 1.14%
DeKalb 629.22 square miles 310 feet 0.74%
Henderson 817.22 square miles 315 feet 1.11%
Bond 378.66 square miles 320 feet 0.93%
Will 841.22 square miles 320 feet 0.84%
Schuyler 436.62 square miles 320 feet 2.00%
Shelby 759.95 square miles 324 feet 0.88%
Boone 279.87 square miles 325 feet 1.07%
Edgar 616.99 square miles 325 feet 0.79%
Williamson 439.51 square miles 330 feet 1.19%
St. Clair 666.66 square miles 340 feet 0.99%
Fulton 873.68 square miles 345 feet 1.91%
Lee 722.21 square miles 345 feet 0.95%
Knox 712.37 square miles 350 feet 1.33%
McLean 1,174.05 square miles 350 feet 0.75%
Ogle 757.12 square miles 370 feet 1.71%
Cook 949.20 square miles 370 feet 0.64%
Lake 467.18 square miles 377 feet 1.03%
Randolph 588.68 square miles 380 feet 1.67%
Brown 303.95 square miles 385 feet 2.13%
Tazewell 650.28 square miles 390 feet 1.22%
Calhoun 280.66 square miles 395 feet 3.55%
Adams 861.86 square miles 395 feet 1.41%
Woodford 537.33 square miles 400 feet 1.19%
Peoria 623.69 square miles 410 feet 1.82%
Jackson 599.01 square miles 417 feet 2.25%
Monroe 393.49 square miles 425 feet 1.74%
Marshall 395.01 square miles 430 feet 1.47%
Kane 519.67 square miles 445 feet 1.06%
Pike 837.40 square miles 455 feet 1.90%
McHenry 606.12 square miles 459 feet 1.24%
La Salle 1,137.41 square miles 465 feet 1.03%
Jersey 373.43 square miles 465 feet 2.17%
Stephenson 560.06 square miles 470 feet 2.29%
Carroll 462.70 square miles 475 feet 2.54%
Bureau 865.96 square miles 500 feet 1.23%
Gallatin 324.70 square miles 520 feet 1.36%
Johnson 345.03 square miles 520 feet 3.01%
Hardin 179.48 square miles 530 feet 3.61%
Alexander 250.75 square miles 530 feet 1.97%
Jo Daviess 613.63 square miles 645 feet 4.25%
Saline 382.92 square miles 660 feet 1.51%
Union 417.84 square miles 680 feet 3.15%
Pope 370.46 square miles 749 feet 3.76%