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Largest rivers in Illinois by average flow

Active USGS stream gauges in Illinois, ranked by average flow. The Ohio River at Olmsted, IL gauge leads with 324,384 cfs.

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RankNameAverage flow
1Ohio River at Olmsted, IL324,384 cfs
2Mississippi River at Thebes, IL212,420 cfs
3Mississippi River at Chester, IL210,182 cfs
4Ohio River at Smithland Dam, Smithland, KY195,366 cfs
5Mississippi River at Grafton, IL114,150 cfs
6Wabash River at Mt. Carmel, IL27,178 cfs
7Illinois River at Valley City, IL23,196 cfs
8Illinois River at Kingston Mines, IL15,597 cfs
9Illinois River at Henry, IL13,929 cfs
10Illinois River at Marseilles, IL9,666 cfs
11Illinois River at Morris, IL8,548 cfs
12Rock River at Dixon, IL7,894 cfs
13Rock River at Byron, IL7,168 cfs
14Rock River near Joslin, IL6,849 cfs
15Rock River at Como, IL5,881 cfs
16Rock River at Rockton, IL4,528 cfs
17Kankakee River near Wilmington, IL4,047 cfs
18Kaskaskia River near Venedy Station, IL3,532 cfs
19Kaskaskia River at New Athens, IL3,504 cfs
20Des Plaines River at Route 53 at Joliet, IL3,376 cfs
21Sangamon River near Oakford, IL3,047 cfs
22Little Wabash River at Carmi, IL2,678 cfs
23Chicago Sanitary and Ship Canal near Lemont, IL2,494 cfs
24Kaskaskia River at Carlyle, IL2,079 cfs
25Kankakee River at Momence, IL2,068 cfs
26Pecatonica River near Shirland, Ill1,864 cfs
27Sangamon River at Petersburg, IL1,797 cfs
28Fox River at Yorkville, IL1,784 cfs
29Fox River at Dayton, IL1,765 cfs
30Embarras River at Lawrenceville, IL1,715 cfs
Average flowOhio River at Olmsted, …324,384 cfsMississippi River at Th…212,420 cfsMississippi River at Ch…210,182 cfsOhio River at Smithland…195,366 cfsMississippi River at Gr…114,150 cfsWabash River at Mt. Car…27,178 cfsIllinois River at Valle…23,196 cfsIllinois River at Kings…15,597 cfsIllinois River at Henry…13,929 cfsIllinois River at Marse…9,666 cfsIllinois River at Morri…8,548 cfsRock River at Dixon, IL7,894 cfsRock River at Byron, IL7,168 cfsRock River near Joslin,…6,849 cfsRock River at Como, IL5,881 cfs
Average flow

How the Average flow figures spread in Illinois

  • The ranking covers the 189 gauges in Illinois that have a recorded average flow in this dataset.
  • The highest is Ohio River at Olmsted, IL at 324,384 cfs; the lowest is Natalie Creek at Midlothian, IL at 1.0 cfs.
  • The median average flow is 99.5 cfs.
  • The gap between #1 Ohio River at Olmsted, IL and #2 Mississippi River at Thebes, IL is 111,964 cfs.
  • The page table lists the top 30.

Data: U.S. Geological Survey (USGS) (public domain, U.S. federal data).· Updated 2026-11-02 · Text generated from the data and checked against it.

Live values are provisional USGS data. Measurements only, not a safety assessment — for flood warnings see the National Weather Service.

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