Oswego Lake is a mesotrophic lake in Kosciusko County, Indiana. Water quality is graded B based on 1 sampling years through 2021. Overall water quality is declining over recent sampling years. Limited fish-species data available. Public access is available. Compared to the 18 other monitored lakes in Kosciusko County, Oswego Lake ranks #3 for water quality.
Source: EPA Water Quality Portal sampling records, Indiana DNR, last sampled 2021-08-11. Grade methodology: LakeGrade methodology.
Watch Oswego Lake water quality
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Swimming Safety
Good for swimming, clear water with low algae levels
Water Quality Grade: B, Good
Good clarity, visible to about 9.9 ft. Trophic State Index: 44.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 9.9 ft | B |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 44 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay near Oswego Lake
Compare lakeside cabins, hotels, and vacation rentals near Oswego Lake in Kosciusko County, Indiana — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water.
Top-rated places to stay near Oswego Lake
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Fishing Oswego Lake
Invasive & Introduced Species
Red chips are ecologically harmful invasive species. Gray chips are stocked or introduced fish that aren't a current ecological concern.
Source: USGS Nonindigenous Aquatic Species (NAS) Database. Hover a chip to see scientific name, ecological context, status, and first-observed year.
Water Quality Trend: ↓ Declining
Based on 4 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.062 m/yr | 4 |
| Chlorophyll-a | ↓ Declining | +0.2 µg/L/yr | 4 |
Location & Depth Map
Free interactive depth-contour map of Oswego Lake, reaching a maximum surveyed depth of 40 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Indiana DNR.
County Ranking
Ranked #3 of 19 lakes in Kosciusko County
Nearby Lakes in Kosciusko County
Explore Oswego Lake across LakeGrade
Fish species and top-graded lakes across Indiana
Fish species and best lakes in Kosciusko County
Species activity outlook by water temperature and season
Top lakes and where to stay in the Northern Indiana Lake Country area
Moderate nutrients, good water quality
Indiana's cleanest and most-polluted lakes, ranked
More Indiana Lake Rankings
Other Grade B Lakes in Indiana
Mesotrophic Lakes in Indiana
More records, surveys & data sources
EPA Impairment Status
Oswego Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit INB06P1063_00 · Official waterbody report
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Regulated Pollution Sources Nearby
6 EPA-permitted facilities operate within about 5 miles of Oswego Lake under Clean Water Act (NPDES) discharge permits.
- Indiana University FoundationLeesburg · 0.1 mi away · NPDES INRA02846
- B & J Auto Salvage IncLeesburg · 1.9 mi away · NPDES INRM01333
- Leesburg Municipal StpLeesburg · 3.3 mi away · NPDES IN0040258
- Worthington Agriculture PartsLeesburg · 3.5 mi away · NPDES INRM01600
- Leesburg Dollar GeneralLeesburg · 3.5 mi away · NPDES INR10K096
- Wwc Parking Expansion 2013Warsaw · 4.2 mi away · NPDES INR10H135
Facilities permitted to discharge under the Clean Water Act within ~5 miles; proximity does not by itself mean a facility affects this lake. Source: EPA ECHO (ICIS-NPDES), as of 2026-07.
Data Sources
Water quality data from the EPA Water Quality Portal
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grading methodology based on Metropolitan Council standards
Most recent sample: 2021-08-11
Monitoring stations: 1