Franklin Lake
Forest County, WisconsinOligotrophic
Franklin Lake sits at the top of the Wisconsin DNR grading rubric with an A, a mark reached by the cleanest lakes in Wisconsin. clarity and phosphorus rate similarly, so there is no obvious single lever to pull on watershed management.
A TSI value of 38 puts Franklin Lake in the oligotrophic range, where biological productivity stays low and the water column remains clear. A maximum depth of 46 ft puts Franklin Lake in the middle of Wisconsin's depth distribution. Franklin Lake covers 839 acres alongside partial shoreline records — a mid-sized water that supports a working fishery without being so large that conditions diverge between basins. Among the 34 graded lakes in Forest County, Franklin Lake ranks 4 — in the top quartile locally.
Franklin Lake has at least one documented invasive species (Banded Mystery Snail), so boaters and anglers should observe clean-drain-dry steps when leaving the lake. Walleye are documented at Franklin Lake, one of 5 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 2025-08-21. Grade methodology: LakeGrade methodology.
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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 16 ft down. Phosphorus level: 10.8 µg/L. Trophic State Index: 38.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 16 ft | A |
| Phosphorus | 10.8 µg/L | A |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 38 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 46 ft |
| Surface Area | 839 acres |
Fish Species
Click a species to see all Wisconsin and Minnesota lakes where it is found.
→ Best fishing times for Franklin Lake (14-day solunar calendar)
→ Is it safe to eat fish from Franklin Lake? (mercury & PFAS guide)
→ Franklin 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 | → Stable | +0.086 m/yr | 6 |
| Phosphorus | ↑ Improving | -0.35 µg/L/yr | 5 |
Location
County Ranking
Ranked #4 of 34 lakes in Forest County
Nearby Lakes in Forest County
More Wisconsin Lake Rankings
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
Oligotrophic Lakes in Wisconsin
Compare Franklin Lake With Nearby Lakes
WI DNR Lake Profile
Authoritative data from the Wisconsin DNR LakePages. Monitored by volunteers since 1986. 2 stations on file (most recent sample 2025).
Fish Species (DNR-rated)
DNR Reports & Resources
Wisconsin DNR Fisheries scientists publish multi-year survey reports for Franklin Lake covering fish populations, stocking, water quality, and management. 1 report on file.
- 2025Franklin Lake comprehensive survey, 2025Comprehensive Survey · WI DNR · 2025 (PDF)
Data Sources
Water quality data from the EPA Water Quality Portal
Grading methodology based on Metropolitan Council standards
Most recent sample: 2025-08-21
Monitoring stations: 1