Forest Lake
Fond du Lac County, WisconsinMesotrophic
Forest Lake sits at the top of the Wisconsin DNR grading rubric with an A, a mark reached by the cleanest lakes in Wisconsin. With clarity and phosphorus scored, the sub-grades cluster within a letter of each other, pointing to stable conditions rather than one specific stressor.
The lake sits in the mesotrophic zone — the band where most well-managed Wisconsin lakes fall. The lake bottoms out at 32 ft — a moderate depth that supports a warm-water fishery without the year-round cold refuge a deeper basin provides. At 51 acres, Forest Lake sits below the Wisconsin median: a small water where shoreline activity has an outsized effect on water quality. Within Fond du Lac County's 10 graded waters, Forest Lake sits at rank 1, near the top of the local distribution.
Forest Lake carries an invasive watermilfoil record — boat-launch decontamination and shoreline care matter more here than at uninfested lakes. Walleye are documented at Forest Lake, one of 4 fish species on record for the lake. 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, Wisconsin DNR Surface Water, last sampled 2025-08-24. Grade methodology: LakeGrade methodology.
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Swimming Safety
Excellent for swimming, crystal clear water with minimal algae
Water Quality Grade: A, Excellent
Good clarity, visible to about 14 ft. Phosphorus level: 14.2 µg/L. Trophic State Index: 41.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 14 ft | B |
| Phosphorus | 14.2 µg/L | A |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 41 | Mesotrophic |
Moderate nutrients, good water quality
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 32 ft |
| Surface Area | 51 acres |
Fish Species
Click a species to see all Wisconsin and Minnesota lakes where it is found.
→ Best fishing times for Forest Lake (14-day solunar calendar)
→ Is it safe to eat fish from Forest Lake? (mercury & PFAS guide)
→ Forest Lake fishing regulations (limits, seasons, special rules)
Invasive & Introduced Species
Red chips are ecologically harmful invasive species. Gray chips are stocked or introduced fish that aren't a current ecological concern.
Water Quality Trend: ↑ Improving
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.893 m/yr | 6 |
| Phosphorus | ↑ Improving | -5.19 µg/L/yr | 6 |
Location
County Ranking
Ranked #1 of 10 lakes in Fond du Lac County
Nearby Lakes in Fond du Lac County
More Wisconsin Lake Rankings
State Parks Near Forest Lake
Other Grade A Lakes in Wisconsin
Washington County · Clarity: 11.8 ft
Washington County · Clarity: 12.6 ft
Walworth County · Clarity: 11 ft
Waukesha County · Clarity: 11.3 ft
Walworth County · Clarity: 20.2 ft
Kenosha County · Clarity: 10 ft
Mesotrophic Lakes in Wisconsin
WI DNR Lake Profile
Authoritative data from the Wisconsin DNR LakePages. Monitored by volunteers since 1986. 3 stations on file (most recent sample 2025).
Fish Species (DNR-rated)
EPA Impairment Status
Forest 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 WI10001196 · Official waterbody report · Matched by proximity (1.37 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
Most recent sample: 2025-08-24
Monitoring stations: 1