Lake Mary Northwest Basin; Mastodon Township is a cold, clear oligotrophic lake in Iron County, Michigan. Water quality is graded A based on 2 sampling years through 2023. Overall water quality is improving over recent sampling years. Limited fish-species data available. Public access is available with 1 public boat landing. Compared to the 38 other monitored lakes in Iron County, Lake Mary Northwest Basin; Mastodon Township ranks #14 for water quality.
Source: EPA Water Quality Portal sampling records, Michigan DNR, last sampled 2023-09-16. Grade methodology: LakeGrade methodology.
Watch Lake Mary Northwest Basin; Mastodon Township water quality
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Swimming Safety
Excellent for swimming, crystal clear water with minimal algae
Water Quality Grade: A, Excellent
Crystal clear, you can see 18 ft down. Chlorophyll-a: 3 µg/L. Trophic State Index: 38.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 18 ft | A |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 3 µg/L | A |
| Trophic State Index (TSI) | 38 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Where to stay near Lake Mary Northwest Basin; Mastodon Township
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Fishing Lake Mary Northwest Basin; Mastodon Township
Water Quality Trend: ↑ Improving
Based on 3 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +2.67 m/yr | 3 |
| Chlorophyll-a | ↑ Improving | -0.75 µg/L/yr | 3 |
Recreation & Access
Location
County Ranking
Ranked #14 of 39 lakes in Iron County
Nearby Lakes in Iron County
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Low nutrients, clear water, excellent for swimming
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More records, surveys & data sources
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Regulated Pollution Sources Nearby
2 EPA-permitted facilities operate within about 5 miles of Lake Mary Northwest Basin; Mastodon Township under Clean Water Act (NPDES) discharge permits.
- Great Lakes Gas TransmissionCrystal Falls · 4.4 mi away · NPDES MIG670311
- City Of Crystal FallsCrystal Falls · 4.7 mi away · NPDES MI0048879
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
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grading methodology based on Metropolitan Council standards
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2023-09-16
Monitoring stations: 1