Long Lake
Long Lake in Fond du Lac County, Wisconsin has a water quality grade of B (Good). Water clarity: 9.1 ft visibility. Phosphorus: 19.3 µg/L. Maximum depth: 47 ft. Surface area: 423 acres.
- Clarity
- 9.1 ft
- Typical in August
- 79°F
- Max depth
- 47 ft
- Area
- 423 acres
- Type
- Mesotrophic
- Trend
- ↓ Declining
- Public access
- Yes
Next lake in Fond du Lac County
Long Lake is a 423-acre mesotrophic lake in Fond du Lac County, Wisconsin with a maximum depth of 47 feet. Water quality is graded B based on 1 sampling years through 2026. Overall water quality is declining over recent sampling years. The lake supports 4 documented fish species including Panfish, Largemouth Bass, and Northern Pike. Public access is available with 2 public boat landings and 2 public beaches. Compared to the 8 other monitored lakes in Fond du Lac County, Long Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Wisconsin DNR Surface Water, last sampled 2026-04-30. Grade methodology: LakeGrade methodology.
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Swimming Safety
Grade onlyBacteria B
Based on clarity and algae grades from past sampling — we do not hold live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Based on E. coli testing, Long Lake rarely exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 2.6% of 195 samples across 9 seasons.
This is a historical summary of past testing, not a current condition — bacteria levels change quickly, especially after rain. Always check Wisconsin’s current beach and health-department advisories before swimming.
Water Quality Grade: B (Good)
Moderate clarity, visible to about 9.1 ft. Phosphorus level: 19.3 µg/L. Trophic State Index: 46.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 9.1 ft | Not graded |
| Phosphorus | 19.3 µg/L | Good |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 46 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay near Long Lake
Compare lakeside cabins, hotels, and vacation rentals near Long Lake in Fond du Lac 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 Long Lake
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Water Temperature
Long Lake typically sits around 79°F at the surface in August. Summer average is about 79°F. Usually warmest in Jul (about 79°F) and coldest in Apr (about 51°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 10 field readings (2024–2026) via the EPA Water Quality Portal. See the full Long Lake water temperature breakdown →
Blue-Green Algae (Cyanobacteria)
There’s no recent EPA CyAN cyanobacteria estimate for Long Lake — clouds, ice, or an off-season gap in coverage. The most recent reading was moderate (40,179 cells/mL) on March 12, 2026.
Blue-green algae can appear and clear within days, so check your state’s current advisory before swimming. EPA CyAN ↗
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 47 ft |
| Surface Area | 423 acres |
| Public Access | Yes |
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
- Butler LakePublic
- Long LakePublic
Fish Species
Click a species to see every graded lake where it is documented.
Fishing Long Lake
Invasive & Introduced Species
Red chips are ecologically harmful invasive species. Gray chips are stocked or introduced fish that aren't a current ecological concern.
WI DNR Lake Profile
Authoritative data from the Wisconsin DNR LakePages. 2 stations on file (most recent sample 2025).
Fish Species (DNR-rated)
Water Quality Trend: ↓ Declining
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.76 m/yr | 6 |
| Phosphorus | ↓ Declining | +0.46 µg/L/yr | 6 |
Reservoir Info (USACE NID)
Long Lake is a man-made reservoir impounded by the Dundee (completed 1857), built primarily for recreation on the EAST BRANCH MILWAUKEE; earth-type dam, 8 ft tall and 280 ft long.
- Surface area
- 427 ac
- Normal storage
- 2,000 ac-ft
- Max storage
- 3,200 ac-ft
- Drainage area
- 24.5 sq mi
- Hazard class
- Significant
- Owner
- Town Of Osceola
Source: USACE National Inventory of Dams, NID ID WI00132 · Operator website
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Long Lake under Clean Water Act (NPDES) discharge permits.
- Kettle Moraine Correctional InstitutePlymouth, City Of · 3.9 mi away · NPDES WI0060721
Facilities permitted to discharge under the Clean Water Act within ~5 miles; proximity does not by itself mean a facility affects this lake. Source: EPA ECHO (ICIS-NPDES), as of 2026-07.
County Ranking
Ranked #1 of 9 lakes in Fond du Lac County
Nearby Lakes in Fond du Lac County
Frequently asked questions about Long Lake
What is the water quality grade for Long Lake?
- Long Lake in Fond du Lac County, Wisconsin has a water quality grade of B (Good). Water clarity: 9.1 ft visibility. Phosphorus: 19.3 µg/L. Maximum depth: 47 ft. Surface area: 423 acres.
Is Long Lake safe to swim in?
- Based on E. coli testing, Long Lake rarely exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 2.6% of 195 samples across 9 seasons. That works out to a bacteria swim-safety grade of B — a historical summary of past testing, not a current reading. Long Lake has a water quality grade of B (Good). Bacteria levels can spike quickly after heavy rain, so always check Wisconsin's current beach and health-department advisories before you swim.
What is the water temperature of Long Lake?
- Long Lake typically sits around 79°F at the surface in August, from 10 EPA Water Quality Portal field readings (2024–2026). That is a typical value for the month, not a live reading.
How deep is Long Lake?
- Long Lake has a maximum depth of 47 feet. The lake covers 423 acres.
What fish are in Long Lake?
- Fish species found in Long Lake include Panfish, Largemouth Bass, Northern Pike, Walleye. Long Lake is located in Fond du Lac County, Wisconsin.
Does Long Lake have blue-green algae?
- There is no recent EPA CyAN cyanobacteria estimate for Long Lake (clouds, ice, or an off-season gap). The most recent reading was moderate (40,179 cells/mL). Blue-green algae can appear and clear within days, so check Wisconsin's current advisory before swimming.
Are there boat ramps on Long Lake?
- Long Lake has 2 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Butler Lake, Long Lake. Public access is listed as yes.
Explore Long Lake across LakeGrade
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Where this data comes from
49 measurements from 1 monitoring station, 2020–2026. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Wisconsin Department of Natural Resources
WIDNR_WQX49 samples · 2020–2026
Analytical methods
- Phosphorus by Colorimetry
Monitoring station identifiers (1)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated and data sources.
Other records on this page
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Fish and recreation data from Wisconsin DNR Lakes
Named boat-access points from the USGS National Boat Ramps inventory