Oswego Lake
Kosciusko County, IndianaView mapDirections
Not Rated: one monitoring station are on record, but they measured water clarity only. LakeQuality’s A–F method grades total phosphorus, total nitrogen, chlorophyll-a, and turbidity; clarity is shown, not scored.
- Impaired · Impaired biota (cause unknown)
- Limited data
Oswego Lake in Kosciusko County, Indiana is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 9.9 ft visibility.
- Clarity
- 9.9 ft
- Trend
- ↓ Declining
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Stays near Oswego Lake
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Watch Oswego Lake’s letter
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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.
Why Oswego Lake has no grade
Oswego Lake has monitoring on record from one station, but only water clarity was measured. LakeQuality’s A–F letter uses total phosphorus, total nitrogen, chlorophyll-a, and turbidity against EPA National Lakes Assessment condition classes. With none of those grading measurements on record, Oswego Lake is Not Rated — not an F. Its 9.9-foot median from 11 readings collected 2021 is still shown as clarity, but clarity alone cannot earn a letter.
Not Rated is EPA Integrated Reporting Category 3 — insufficient information. Everything below that has been measured is still reported on this page.
Sampling is run by IDEM and volunteer programs, and published through the EPA Water Quality Portal. Read the full grading methodology.
Oswego Lake is a lake in Kosciusko County, Indiana. Overall water quality is declining over recent sampling years.
Source: EPA Water Quality Portal sampling records, IDEM, last sampled 2021-08-11. Grade methodology: LakeQuality methodology.
What Oswego Lake is good for
From access and monitoring records, not editorial picks.
- Clear water9.9 ft of visibility
Swimming Safety
Not ratedLimited data
The letter is not a swimming clearance. Oswego Lake is listed impaired under Clean Water Act §303(d) (IR 5) for impaired biota (cause unknown). Check local advisories before you swim.
Water quality measurements for Oswego Lake
These readings exist but are not enough for an A–F grade — see why.
Good clarity, visible to about 9.9 ft. TSI 44.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 9.9 ft | Not graded |
| Phosphorus | No recent samples | |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 44 | Mesotrophic |
Moderate nutrients, good water quality
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 |
| Phosphorus | ↓ Declining | +5.25 µg/L/yr | 4 |
| Chlorophyll-a | ↓ Declining | +0.2 µg/L/yr | 4 |
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
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.
Nearby Lakes in Kosciusko County
Frequently asked questions about Oswego Lake
Where is Oswego Lake?
- Oswego Lake is a lake in Kosciusko County, Indiana. LakeQuality has not rated this lake (UNRATED) — the only measurement available is water clarity, which is not a water-quality indicator on its own — EPA's National Lakes Assessment does not grade clarity.
What is the water quality grade for Oswego Lake?
- Oswego Lake in Kosciusko County, Indiana is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 9.9 ft visibility.
Is Oswego Lake safe to swim in?
- The letter is not a swimming clearance. Oswego Lake is listed impaired under Clean Water Act §303(d) (IR 5) for impaired biota (cause unknown). Check local advisories before you swim.
Explore Oswego Lake across LakeQuality
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Where this data comes from
11 measurements from 1 monitoring station, 2021. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- North American Lake Management Society
NALMS11 samples · 2021
Monitoring station identifiers (1)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated and data sources.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)