Fourth Lake
St. Louis County, MinnesotaLimited DataHypereutrophic
On the LakeGrade rubric Fourth Lake fails the cutoffs — a TSI of 72 and persistent algal pressure put it in the bottom tier. The three sub-grades — clarity, phosphorus, and chlorophyll-a — track close together, so no single parameter is dragging the average.
The hypereutrophic status — TSI 72 — reflects sustained heavy nutrient loading from the surrounding watershed and frequent algal pressure. A maximum depth of 20 ft puts Fourth Lake in the middle of Minnesota's depth distribution. At 205 acres, Fourth Lake fits the Minnesota median for monitored lakes, with 7.8 miles of shoreline. Within the 215 graded lakes of St. Louis County, Fourth Lake sits at rank 210, near the bottom of the county list.
No invasive species are currently listed at Fourth Lake — the lake remains off the Minnesota infested-waters roster. Walleye are documented at Fourth Lake, one of 13 fish species on record for the lake. No formal public access is documented at Fourth Lake — most use is by shoreline residents and their guests. Monitoring depth is thin here: the LakeGrade rubric is applied to a small number of sample years, and the grade will be revised as more data accumulates.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2024-10-09. Grade methodology: LakeGrade methodology.
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Swimming Safety
Avoid swimming, very poor water quality, potential algae toxins
Water Quality Grade: F, Very Poor
Very murky, less than 1.5 ft of visibility. Trophic State Index: 72.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.5 ft | F |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 72 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 20 ft |
| Surface Area | 204.9 acres |
| Shoreline Length | 7.8 mi |
| Littoral Zone | 86% |
| Public Access | No |
Fish Species
Click a species to see every graded lake where it is documented.
→ Best fishing times for Fourth Lake (14-day solunar calendar)
→ Is it safe to eat fish from Fourth Lake? (mercury & PFAS guide)
→ Fourth Lake fishing regulations (limits, seasons, special rules)
Water Quality Trend: ↓ Declining
Based on 4 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.327 m/yr | 4 |
Location
County Ranking
Ranked #210 of 215 lakes in St. Louis County
Cleaner Lakes Within 30 Miles
Fourth Lake holds Grade F. 4 nearby lakes hold higher grades.
Nearby Lakes in St. Louis County
More Minnesota Lake Rankings
State Parks Near Fourth Lake
Other Grade F Lakes in Minnesota
Hypereutrophic Lakes in Minnesota
EPA Impairment Status
Fourth 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 MN69-0565-00 · Official waterbody report · Matched by proximity (1.22 km)
Data Sources
Water quality data from the EPA Water Quality Portal
Impairment status from EPA ATTAINS 303(d) database
Grading methodology based on Metropolitan Council standards
Lake details from Minnesota DNR LakeFinder
Most recent sample: 2024-10-09
Monitoring stations: 1