St. Johns Lake
Forest County, Wisconsin107 acresView mapDirections
Wisconsin DNR publishes the water-quality measurements for St. Johns Lake. Here is what has been measured.
- Mesotrophic
- 21 ft max depth
St. Johns Lake in Forest County, Wisconsin has water-quality measurements published by Wisconsin DNR, shown here as reported. Water clarity: 10 ft visibility. Phosphorus: 14.9 µg/L. Maximum depth: 21 ft. Surface area: 107 acres.
Wisconsin DNR lists Panfish as common on St. Johns Lake. DNR survey details →
St. Johns Lake is a 107-acre mesotrophic lake in Forest County, Wisconsin with a maximum depth of 21 feet. Overall water quality is declining over recent sampling years. The lake supports 3 documented fish species including Panfish, Largemouth Bass, and Walleye. Compared to the 22 other monitored lakes in Forest County, St. Johns Lake ranks #17 for water quality.
Source: EPA Water Quality Portal sampling records, Wisconsin DNR Surface Water, last sampled 2026-07-06. Methodology: how these readings are compared.
What St. Johns Lake is good for
From access and monitoring records, not editorial picks.
- Clear water10 ft of visibility
- Fishing3 species recorded
Swimming conditions
Readings onlyLow algae
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Whether St. Johns Lake is safe to swim in today is a call for the Wisconsin Department of Natural Resources and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for St. Johns Lake
These are the measurements Wisconsin DNR publishes for St. Johns Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Good clarity, visible to about 10 ft. Phosphorus 14.9 µg/L. TSI 44.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 10ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 14.9µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | No reading The green pigment in algae — a direct measure of how much algae is in the water. | |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 44 | Mesotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Moderate nutrients, good water quality
Ask about St. Johns Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
St. Johns Lake typically sits around 71°F at the surface in September. Summer average is about 76°F. Usually warmest in Jul (about 78°F) and coldest in Mar (about 41°F). These are typical values by month — not a live reading.
Column height runs from this lake’s coldest month (41°F) to its warmest (78°F).
Typical surface water temperature by month, from 25 field readings (2024–2026) via the EPA Water Quality Portal. See the full St. Johns Lake water temperature breakdown →
Wisconsin DNR lists Panfish as common on St. Johns Lake.
A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows St. Johns Lake’s water-quality measurements separately, and the water-quality trend over 7 years of monitoring is declining. Catch rate is not a swim clearance and is not a weekly bite report.
WI DNR Lake Profile
Authoritative data from the Wisconsin DNR LakePages. Monitored by volunteers since 2022. 1 station on file (most recent sample 2026).
Fish Species (DNR-rated)
Lake Details
Max depth
21 ft
Surface area
107 acres
Lake Size & Watershed
Watershed (drainage) area
2 sq mi
HydroLAKES / HydroSHEDS
Shoreline
2.2 mi
HydroLAKES / HydroSHEDS
Elevation
1,585 ft
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Stays via Stay22
Stays near St. Johns Lake
Compare lakeside cabins, hotels, and vacation rentals near St. Johns Lake in Forest County, Wisconsin — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water.
Top-rated places to stay near St. Johns Lake
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Follow St. Johns Lake’s readings
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Fish Species
Click a species to see every lake where it is documented.
Fishing St. Johns Lake
Water Quality Trend: Worsening
Based on 7 years of monitoring (2020–2026).
Water clarity
Worsening7 years
Phosphorus
Worsening+1.95 µg/L/yr3 years
Nearby Lakes in Forest County
Frequently asked questions about St. Johns Lake
Where is St. Johns Lake?
- St. Johns Lake is a lake in Forest County, Wisconsin. Its water-quality measurements are published by Wisconsin DNR and shown on this page as reported.
What is the water quality of St. Johns Lake?
- St. Johns Lake in Forest County, Wisconsin has water-quality measurements published by Wisconsin DNR, shown here as reported. Water clarity: 10 ft visibility. Phosphorus: 14.9 µg/L. Maximum depth: 21 ft. Surface area: 107 acres.
Is St. Johns Lake safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
What is the water temperature of St. Johns Lake?
- St. Johns Lake typically sits around 71°F at the surface in September, from 25 EPA Water Quality Portal field readings (2024–2026). That is a typical value for the month, not a live reading.
How deep is St. Johns Lake?
- St. Johns Lake has a maximum depth of 21 feet. The lake covers 107 acres.
What fish are in St. Johns Lake?
- Fish species found in St. Johns Lake include Panfish, Largemouth Bass, Walleye. St. Johns Lake is located in Forest County, Wisconsin.
What did the DNR fisheries survey find at St. Johns Lake?
- Wisconsin DNR lists Panfish as common on St. Johns Lake. A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows St. Johns Lake’s water-quality measurements separately, and the water-quality trend over 7 years of monitoring is declining. Catch rate is not a swim clearance and is not a weekly bite report.
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Where this data comes from
64 measurements from 1 monitoring station, 2022–2026. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Wisconsin Department of Natural Resources
WIDNR_WQX64 samples · 2022–2026
Analytical methods
- Phosphorus by Colorimetry
Monitoring station identifiers (1)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.
Other records on this page
Lake size & watershed from HydroLAKES (HydroSHEDS)
Fish and recreation data from Wisconsin DNR Lakes