Saint Mary'S River Offshore Lake is a mesotrophic lake in Chippewa County, Michigan. Water quality is graded A based on 2 sampling years through 2025. Water clarity is improving over recent sampling years. Compared to the 17 other monitored lakes in Chippewa County, Saint Mary'S River Offshore Lake ranks #5 for water quality.
Source: EPA Water Quality Portal sampling records, Michigan DNR, last sampled 2025-10-15. Grade methodology: LakeGrade methodology.
Watch Saint Mary'S River Offshore Lake water quality
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Swimming Safety
Excellent for swimming, crystal clear water with minimal algae
Water Quality Grade: A, Excellent
No clarity data. Phosphorus level: 17.5 µg/L. Chlorophyll-a: 1.8 µg/L. Trophic State Index: 41.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No data | |
| Phosphorus | 17.5 µg/L | A |
| Chlorophyll-a (Algae) | 1.8 µg/L | A |
| Trophic State Index (TSI) | 41 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay near Saint Mary'S River Offshore Lake
Compare lakeside cabins, hotels, and vacation rentals near Saint Mary'S River Offshore Lake in Chippewa County, Michigan — 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.
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Fishing Saint Mary'S River Offshore Lake
Water Quality Trend: → Stable
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.234 m/yr | 4 |
| Phosphorus | → Stable | +0.24 µg/L/yr | 5 |
| Chlorophyll-a | ↓ Declining | +20.7 µg/L/yr | 3 |
Water Temperature
Saint Mary'S River Offshore Lake averages about 59°F at the surface in summer, usually warmest in Aug (about 67°F) and coldest in May (about 44°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 212 field readings (2010–2025) via the EPA Water Quality Portal. See the full Saint Mary'S River Offshore Lake water temperature breakdown →
Location
County Ranking
Ranked #5 of 18 lakes in Chippewa County
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Moderate nutrients, good water quality
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Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Data Sources
Water quality data from the EPA Water Quality Portal
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grading methodology based on Metropolitan Council standards
Most recent sample: 2025-10-15
Monitoring stations: 1