Green Lake
Green Lake County, WisconsinLimited DataMesotrophic
On the LakeGrade scale Green Lake grades a B, with clarity at 13.0 ft and phosphorus readings still being added placing it above the Wisconsin median.
The lake sits in the mesotrophic zone — the band where most well-managed Wisconsin lakes fall. At 236 ft of maximum depth, Green Lake stratifies reliably through the summer — the hypolimnion stays cooler and clearer than the surface layer suggests. At 7,920 acres, Green Lake is one of the larger lakes in Green Lake County, with partial shoreline records supporting multiple distinct use patterns. Among the 13 graded lakes in Green Lake County, Green Lake ranks 2 — in the top quartile locally.
Green Lake is on the Wisconsin infested-waters list for zebra mussels — boaters should follow clean-drain-dry protocols when moving gear to or from the lake. Walleye are documented at Green Lake, one of 7 fish species on record for the lake. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.
Source: EPA Water Quality Portal sampling records, Wisconsin DNR Surface Water, last sampled 2024-10-21. Grade methodology: LakeGrade methodology.
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Swimming Safety
Good for swimming, clear water with low algae levels
Water Quality Grade: B, Good
Good clarity, visible to about 13 ft. Trophic State Index: 40.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 13 ft | B |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 40 | Mesotrophic |
Moderate nutrients, good water quality
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 236 ft |
| Surface Area | 7.9K acres |
Fish Species
Click a species to see all Wisconsin and Minnesota lakes where it is found.
→ Best fishing times for Green Lake (14-day solunar calendar)
→ Is it safe to eat fish from Green Lake? (mercury & PFAS guide)
→ Green 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 3 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.61 m/yr | 3 |
Location
County Ranking
Ranked #2 of 13 lakes in Green Lake County
Nearby Lakes in Green Lake County
More Wisconsin Lake Rankings
Other Grade B Lakes in Wisconsin
Mesotrophic Lakes in Wisconsin
Compare Green Lake With Nearby Lakes
WI DNR Lake Profile
Authoritative data from the Wisconsin DNR LakePages. 85 stations on file (most recent sample 2026).
Fish Species (DNR-rated)
EPA Impairment Status
Green Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 4A).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit WI10000969 · Official waterbody report
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: 2024-10-21
Monitoring stations: 1