Lakes / Illinois / Cass County / Swamp Slough
Swamp Slough, Illinois
31.63 acres
Surface Area
Where It Sits, and How Deep It Goes
Locator - Illinois
40.07 degrees N, 90.35 degrees W - Cass County. State and county outlines drawn from Census boundary data; the lake marker is placed by its USGS National Hydrography Dataset coordinates, not by eye. Schematic - not a survey map.
How Swamp Slough Stacks Up
- Larger than 14% of the 511 lakes counted in Illinois.
- About 42 miles from Springfield, as the crow flies.
Lakes Nearby
| Lake | County | Distance | Surface Area |
|---|---|---|---|
| Treadway Lake | Cass | 2.6 mi | 1,477.8 acres |
| Eagle Lake | Mason | 3.1 mi | 112.19 acres |
| Sugar Creek Lake | Schuyler | 3.2 mi | 83.77 acres |
| Mound Lake | Mason | 3.2 mi | 26.19 acres |
| Sangamon Bay | Schuyler | 3.3 mi | 122.07 acres |
| Little Lake | Schuyler | 3.5 mi | 171.49 acres |
Straight-line distance between lake centers.
Facts and Figures
Questions About Swamp Slough
How Big Is Swamp Slough?
Swamp Slough covers 31.63 acres in Cass County, Illinois.
What Is the Largest Lake in Cass County?
Treadway Lake is the largest lake in Cass County, at 1,477.8 acres.
Largest Lakes in Illinois
| # | Lake | County | Surface Area | Max Depth |
|---|---|---|---|---|
| 1 | Carlyle Lake | Clinton | 22,796.77 acres | - |
| 2 | Mine Number 21 Reservoir | Franklin | 17,062.04 acres | - |
| 3 | Upper Peoria Lake | Woodford | 15,460.64 acres | - |
| 4 | Lake Shelbyville | Shelby | 10,235.1 acres | - |
| 5 | Crab Orchard Lake | Williamson | 7,292.57 acres | - |
All 511 Illinois lakes ranked by surface area
Sources: USGS National Hydrography Dataset (The National Map) lake data (ID 154320657). Verified October 30, 2026.