Half Moon Lake
Polk County, WisconsinOligotrophic
Half Moon Lake sits at the top of the Wisconsin DNR grading rubric with an A, a mark reached by the cleanest lakes in Wisconsin. Sub-grades cluster within a single letter of each other, which usually means the lake is in stable trophic balance rather than fighting one specific stressor.
A TSI value of 40 puts Half Moon Lake in the oligotrophic range, where biological productivity stays low and the water column remains clear. The lake's 60 ft maximum depth is in the upper tier for Wisconsin — deep enough for cold-water fish to find refuge in summer. Half Moon Lake covers 550 acres alongside partial shoreline records — a mid-sized water that supports a working fishery without being so large that conditions diverge between basins. Within Polk County's 74 graded waters, Half Moon Lake sits at rank 6, near the top of the local distribution.
Eurasian watermilfoil is established at Half Moon Lake, which can dampen recreational access and pressure the native plant community. Walleye are documented at Half Moon Lake, one of 4 fish species on record for the lake. A documented public access point at Half Moon Lake makes the lake usable for shore fishing, paddle craft, and trailered boats. 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 2025-08-31. 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 15.3 ft down. Phosphorus level: 13.9 µg/L. Trophic State Index: 40.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 15.3 ft | A |
| Phosphorus | 13.9 µg/L | A |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 40 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 60 ft |
| Surface Area | 550 acres |
| Public Access | Yes |
Fish Species
Click a species to see every graded lake where it is documented.
→ Best fishing times for Half Moon Lake (14-day solunar calendar)
→ Is it safe to eat fish from Half Moon Lake? (mercury & PFAS guide)
→ Half Moon 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: ↓ Declining
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | +0.093 m/yr | 6 |
| Phosphorus | ↓ Declining | +5.8 µg/L/yr | 3 |
Recreation & Access
Location
County Ranking
Ranked #6 of 74 lakes in Polk County
Nearby Lakes in Polk County
More Wisconsin Lake Rankings
State Parks Near Half Moon 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
Oligotrophic Lakes in Wisconsin
Compare Half Moon Lake With Nearby Lakes
WI DNR Lake Profile
Authoritative data from the Wisconsin DNR LakePages. Monitored by volunteers since 1993. 13 stations on file (most recent sample 2025).
Fish Species (DNR-rated)
Data Sources
Water quality data from the EPA Water Quality Portal
Grading methodology based on Metropolitan Council standards
Fish and recreation data from Wisconsin DNR Lakes
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2025-08-31
Monitoring stations: 2
Where to stay near Half Moon Lake
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