Long Lake
Fond du Lac County, Wisconsin423 acresView mapDirections
Wisconsin DNR publishes the water-quality measurements for Long Lake. Here is what has been measured.
- Mesotrophic
- 3 aquatic invasives
- 47 ft max depth
- Public access
Long Lake in Fond du Lac County, Wisconsin has water-quality measurements published by Wisconsin DNR, shown here as reported. Water clarity: 9.1 ft visibility. Phosphorus: 19.3 µg/L. Maximum depth: 47 ft. Surface area: 423 acres.
Wisconsin DNR rates Panfish as abundant on Long Lake. DNR survey details →
Long Lake is a 423-acre mesotrophic lake in Fond du Lac County, Wisconsin with a maximum depth of 47 feet. 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 1 public boat landing 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. Methodology: how these readings are compared.
Long Lake covers about 420 acres in Fond du Lac County. It has one public boat landing. For a place on the water, look through cabin and lake-house rentals near Long Lake on Vrbo. For a hotel or motel room, Booking.com and Expedia both search around the lake.
What Long Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch1 documented ramp
- Beach2 documented beaches
- Fishing4 species recorded
- Deep water47 ft surveyed max depth
Swimming conditions
Readings onlyBacteria tested: low
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.
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.
Whether Long 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 Long Lake
These are the measurements Wisconsin DNR publishes for Long Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Moderate clarity, visible to about 9.1 ft. Phosphorus 19.3 µg/L. TSI 46.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 9.1ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 19.3µg/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. | 46 | Mesotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Moderate nutrients, good water quality
Ask about Long Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
Long Lake typically sits around 71°F at the surface in September. 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.
Column height runs from this lake’s coldest month (51°F) to its warmest (79°F).
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 ↗
Wisconsin DNR rates Panfish as abundant on Long Lake.
A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Long Lake’s water-quality measurements separately, and the water-quality trend over 6 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. 2 stations on file (most recent sample 2025).
Fish Species (DNR-rated)
Lake Details
Max depth
47 ft
Surface area
423 acres
Public access
Yes
Listed by Wisconsin DNR
Lake Size & Watershed
Watershed (drainage) area
21 sq mi
HydroLAKES / HydroSHEDS
Shoreline
5.9 mi
HydroLAKES / HydroSHEDS
Elevation
1,001 ft
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
- Long LakePublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Stays via Stay22
Stays 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
Prices for Oct 9–11, 4 adults. Change dates on the map below.
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Planning a whole trip? See where to stay at Wisconsin’s top lakes →
Follow Long Lake’s readings
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Fish Species
Click a species to see every 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.
Water Quality Trend: Worsening
Based on 6 years of monitoring (2020–2025).
Water clarity
Worsening6 years
Phosphorus
Worsening+0.46 µg/L/yr6 years
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.
Nearby Lakes in Fond du Lac County
Frequently asked questions about Long Lake
Where is Long Lake?
- Long Lake is a lake in Fond du Lac County, Wisconsin. Its water-quality measurements are published by Wisconsin DNR and shown on this page as reported.
What is the water quality of Long Lake?
- Long Lake in Fond du Lac County, Wisconsin has water-quality measurements published by Wisconsin DNR, shown here as reported. 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 is a historical summary of past bacteria testing, not a current reading and not a swimming clearance. Bacteria levels can spike quickly after heavy rain, so whether Long Lake is safe to swim in today is a call for Wisconsin's water agency and the county health department; check their current beach advisories before you swim.
What is the water temperature of Long Lake?
- Long Lake typically sits around 71°F at the surface in September, 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.
What did the DNR fisheries survey find at Long Lake?
- Wisconsin DNR rates Panfish as abundant on Long Lake. A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Long Lake’s water-quality measurements separately, and the water-quality trend over 6 years of monitoring is declining. Catch rate is not a swim clearance and is not a weekly bite report.
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 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include Long Lake. Public access is listed as yes.
Can I fish at Long Lake?
- Long Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Seasons, size and bag limits, and whether you need a license are set by Wisconsin's fish and wildlife agency, so check its current regulations before you go.
Explore Long Lake across LakeQuality
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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)
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
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