Decatur Lake vs Mattoon Lake
Water quality, depth, fish species, and recreation comparison.
Decatur Lake has a higher water quality grade (D, Nutrient-rich) than Mattoon Lake (F, Algae-prone). Both are in Illinois.
Both Decatur Lake and Mattoon Lake sit in Illinois. A same-state comparison strips out the state-level water-quality regime as a variable: any grade differences here are about the lakes themselves, not the agencies grading them. The grades are close: Decatur Lake (D) and Mattoon Lake (F) are within one letter of each other on our A–F scale. The per-parameter sub-grades below will show where the small differences actually live.
With grades this close, the choice between the two lakes turns on non-water-quality factors: depth, fish species, public access, distance from home. The per-lake pages below cover all of those.
Decatur Lake
Very murky, less than 1.3 ft of visibility.
Mattoon Lake
Very murky, less than 1.3 ft of visibility.
Side-by-Side Metrics
| Metric | Decatur Lake | Mattoon Lake |
|---|---|---|
| Overall Grade | D (Nutrient-rich) Better | F (Algae-prone) |
| Water Clarity | 1.3 ft | 1.3 ft |
| Phosphorus | 103 µg/L Better | 294 µg/L |
| Chlorophyll-a (Algae) | 25 µg/L Better | 87.1 µg/L |
| Maximum Depth | - | - |
| Surface Area | 3,335 acres | 980 acres |
| Public Access | Yes | Yes |
| Fish Species | 0 | 0 |
| Trophic State | eutrophic | hypereutrophic |
A green ✓ marks the better value for that metric (lower phosphorus / chlorophyll = cleaner; higher Secchi / depth / species count = better). Surface area and trophic state carry no tick — a bigger lake is not a better lake.
Swap a lake
Compare Decatur Lake or Mattoon Lake against a different lake.
Verdict
Decatur Lake wins on overall water quality with a Grade D versus Mattoon Lake's Grade F. Water clarity: 1.3 ft vs 1.3 ft. For fishing diversity, Decatur Lake also leads with 0 species.
Source: EPA National Aquatic Resource Surveys, 2026.