Lake Koshkonong
Jefferson County, Wisconsin10,595 acresView mapDirections
Wisconsin DNR publishes the water-quality measurements for Lake Koshkonong. Here is what has been measured.
- Hypereutrophic
- Impaired · Nutrients (phosphorus, nitrogen)
- 3 aquatic invasives
- 7 ft max depth
- Public access
Lake Koshkonong in Jefferson County, Wisconsin has water-quality measurements published by Wisconsin DNR, shown here as reported. Water clarity: 1.7 ft visibility. Phosphorus: 287 µg/L. Maximum depth: 7 ft. Surface area: 10595 acres.
Wisconsin DNR lists Panfish as common on Lake Koshkonong. DNR survey details →
Lake Koshkonong is a 10,595-acre highly nutrient-loaded hypereutrophic lake in Jefferson County, Wisconsin with a maximum depth of 7 feet. Overall water quality is improving over recent sampling years. The lake supports 7 documented fish species including Musky, Panfish, and Largemouth Bass. Public access is available with 4 public boat landings and 2 public beaches. Compared to the 6 other monitored lakes in Jefferson County, Lake Koshkonong ranks #7 for water quality.
Source: EPA Water Quality Portal sampling records, Wisconsin DNR Surface Water, last sampled 2026-07-10. Methodology: how these readings are compared.
At about 11,000 acres, Lake Koshkonong is large enough that where you stay decides which shore you see most of. It has 4 public boat landings, so a lakefront rental is not the only way to get a boat on the water. For a place on the water, look through cabin and lake-house rentals near Lake Koshkonong on Vrbo. For a hotel or motel room, Booking.com and Expedia both search around the lake.
What Lake Koshkonong is good for
From access and monitoring records, not editorial picks.
- Boat launch4 documented ramps
- Beach2 documented beaches
- Fishing7 species recorded
- Big water10,595 acres
Lake Koshkonong Water Quality Summary (2026)
A printable one-page summary for Lake Koshkonong: clarity, phosphorus, algae, multi-year trend, depth, and fish — with sources.
Swimming conditions
Readings onlyRecreation use not supported
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, Lake Koshkonong occasionally exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 9.5% of 328 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 Lake Koshkonong 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 Lake Koshkonong
These are the measurements Wisconsin DNR publishes for Lake Koshkonong, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Very murky, less than 1.7 ft of visibility. Phosphorus 287 µg/L. TSI 77.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 1.7ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 287µg/L | Poor 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. | 77 | Hypereutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Very high nutrients, dense algae, poor clarity
Ask about Lake Koshkonong
Answers come straight from the public measurements and survey records on this page.
Current Conditions
USGS snapshot Jul 6, 2026, 5:00 AM from gauge 05427235 (Lake Koshkonong Near Newville, WI). Station page ↗ — a live reading replaces this snapshot when the gauge responds. Missing live data is not an all-clear.
→ Lake Koshkonong water level: the live USGS gauge reading
Water Temperature
Lake Koshkonong typically sits around 66°F at the surface in September. Summer average is about 77°F. Usually warmest in Jul (about 82°F) and coldest in Mar (about 39°F). These are typical values by month — not a live reading.
Column height runs from this lake’s coldest month (39°F) to its warmest (82°F).
Typical surface water temperature by month, from 21 field readings (2024–2026) via the EPA Water Quality Portal. See the full Lake Koshkonong water temperature breakdown →
Blue-Green Algae (Cyanobacteria)
There’s no recent EPA CyAN cyanobacteria estimate for Lake Koshkonong — clouds, ice, or an off-season gap in coverage. The most recent reading was very high (1,137,627 cells/mL) on July 18, 2026.
Blue-green algae can appear and clear within days, so check your state’s current advisory before swimming. EPA CyAN ↗
Wisconsin DNR lists Panfish as common on Lake Koshkonong.
A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Lake Koshkonong’s water-quality measurements separately, and the water-quality trend over 4 years of monitoring is improving. 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 2001. 5 stations on file (most recent sample 2025).
Fish Species (DNR-rated)
Lake Details
Max depth
7 ft
Surface area
10,595 acres
Public access
Yes
Listed by Wisconsin DNR
Lake Size & Watershed
Watershed (drainage) area
2,577 sq mi
HydroLAKES / HydroSHEDS
Shoreline
24.5 mi
HydroLAKES / HydroSHEDS
Elevation
771 ft
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
- Royce-Dallman County ParkPublic
- NewvillePublic
- Kuehn RoadPublic
- Highway 59 BridgeSemi-private
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Stays via Stay22
Stays near Lake Koshkonong
Compare lakeside cabins, hotels, and vacation rentals near Lake Koshkonong in Jefferson 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. The water-quality readings cover nutrients and algae, not lodging. Cabins and rentals around Lake Koshkonong are listed below.
Top-rated places to stay near Lake Koshkonong
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 trip? Cabins and lodging at Lake Koshkonong → · more Wisconsin lake trips
Follow Lake Koshkonong’s readings
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Fish Species
Click a species to see every lake where it is documented.
Fishing Lake Koshkonong
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 4 years of monitoring (2021–2025).
Water clarity
Stable4 years
Phosphorus
Improving-25.2 µg/L/yr4 years
EPA Impairment Status
Lake Koshkonong is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 4A).
Causes of impairment
Designated uses not supporting: Aquatic life, Recreation.
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit WI10001493 · Official waterbody report
Nearby Lakes in Jefferson County
Frequently asked questions about Lake Koshkonong
Where is Lake Koshkonong?
- Lake Koshkonong is a lake in Jefferson County, Wisconsin. Its water-quality measurements are published by Wisconsin DNR and shown on this page as reported.
What is the water quality of Lake Koshkonong?
- Lake Koshkonong in Jefferson County, Wisconsin has water-quality measurements published by Wisconsin DNR, shown here as reported. Water clarity: 1.7 ft visibility. Phosphorus: 287 µg/L. Maximum depth: 7 ft. Surface area: 10595 acres.
Is Lake Koshkonong safe to swim in?
- Based on E. coli testing, Lake Koshkonong occasionally exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 9.5% of 328 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 Lake Koshkonong 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 Lake Koshkonong?
- Lake Koshkonong typically sits around 66°F at the surface in September, from 21 EPA Water Quality Portal field readings (2024–2026). That is a typical value for the month, not a live reading.
How deep is Lake Koshkonong?
- Lake Koshkonong has a maximum depth of 7 feet. The lake covers 10595 acres.
What fish are in Lake Koshkonong?
- Fish species found in Lake Koshkonong include Musky, Panfish, Largemouth Bass, Smallmouth Bass, Northern Pike, Walleye, Catfish. Lake Koshkonong is located in Jefferson County, Wisconsin.
What did the DNR fisheries survey find at Lake Koshkonong?
- Wisconsin DNR lists Panfish as common on Lake Koshkonong. A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Lake Koshkonong’s water-quality measurements separately, and the water-quality trend over 4 years of monitoring is improving. Catch rate is not a swim clearance and is not a weekly bite report.
What is the water level at Lake Koshkonong?
- The USGS snapshot on this page for Lake Koshkonong is 776.62 ft of water-surface elevation, recorded Jul 6, 2026, 5:00 AM at gauge 05427235. Current Conditions hydrates a live NWIS reading from that same station when the gauge responds. A missed live fetch is not an all-clear.
Does Lake Koshkonong have blue-green algae?
- There is no recent EPA CyAN cyanobacteria estimate for Lake Koshkonong (clouds, ice, or an off-season gap). The most recent reading was very high (1,137,627 cells/mL). Blue-green algae can appear and clear within days, so check Wisconsin's current advisory before swimming.
Are there boat ramps on Lake Koshkonong?
- Lake Koshkonong has 4 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Royce-Dallman County Park, Newville, Kuehn Road, Highway 59 Bridge. Public access is listed as yes.
Can I fish at Lake Koshkonong?
- Lake Koshkonong has 4 documented boat landings 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 Lake Koshkonong across LakeQuality
Depth, room, public access, and the state wake-sport rules
Fish species and the cleanest lakes across Wisconsin
Fish species and best lakes in Jefferson County
Species activity outlook by water temperature and season
Where to stay at Wisconsin's top lakes, each with its readings
Wisconsin's cleanest lakes, ranked on agency measurements
More lakes in Wisconsin
State Parks Near Lake Koshkonong
Other Lakes in Wisconsin
Hypereutrophic Lakes in Wisconsin
Compare Lake Koshkonong With Nearby Lakes
Where this data comes from
109 measurements from 4 monitoring stations, 2020–2026. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Wisconsin Department of Natural Resources
WIDNR_WQX109 samples · 2020–2026
Analytical methods
- Phosphorus by Colorimetry
Monitoring station identifiers (4)
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
Impairment status from EPA ATTAINS 303(d) database
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