Lake Koshkonong
Lake Koshkonong in Jefferson County, Wisconsin has a water quality grade of D (Nutrient-rich). 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 →
- Clarity
- 1.7 ft
- Typical in August
- 71°F
- Max depth
- 7 ft
- Area
- 10,595 acres
- Type
- Hypereutrophic
- Trend
- ↑ Improving
- Public access
- Yes
Grade D measures nutrients and algae, not whether this is a good place for a cabin trip. Cabins and rentals near Lake Koshkonong →
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Lake Koshkonong is a 10,595-acre highly nutrient-loaded hypereutrophic lake in Jefferson County, Wisconsin with a maximum depth of 7 feet. Water quality is graded D based on 1 sampling years through 2026. 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 2 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. Grade methodology: LakeGrade methodology.
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Lake Koshkonong Water Quality Report Card (2026)
A printable one-page report card for Lake Koshkonong: grade, clarity, phosphorus, algae, multi-year trend, depth, and fish — with sources.
Swimming Safety
Grade onlyBacteria C
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, 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.
Water Quality Grade: D (Nutrient-rich)
Very murky, less than 1.7 ft of visibility. Phosphorus level: 287 µg/L. Trophic State Index: 77.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.7 ft | Not graded |
| Phosphorus | 287 µg/L | Poor |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 77 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Where to stay 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 grade measures nutrients and algae, not lodging — cabins and rentals around Lake Koshkonong are listed below.
Top-rated places to stay near Lake Koshkonong
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Planning a trip? Cabins and lodging at Lake Koshkonong → · more Wisconsin lake trips
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.
Water Temperature
Lake Koshkonong typically sits around 71°F at the surface in August. 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.
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 a water-quality grade. LakeQuality grades Lake Koshkonong D from EPA Water Quality Portal chemistry, 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
| Characteristic | Value |
|---|---|
| Maximum Depth | 7 ft |
| Surface Area | 10,595 acres |
| Public Access | Yes |
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
- Royce-Dallman County ParkPublic
- Highway 59 BridgeSemi-private
Named access points from the USGS National Boat Ramps inventory, snapped within 5 km. More lakes with boat ramps
Fish Species
Click a species to see every graded 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 data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | +0.015 m/yr | 4 |
| Phosphorus | ↑ Improving | -25.2 µg/L/yr | 4 |
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
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit WI10001493 · Official waterbody report
County Ranking
Ranked #7 of 7 lakes in Jefferson County
Nearby Lakes With Higher Grades
Lake Koshkonong holds Grade D. 3 nearby lakes hold higher nutrient-and-algae grades — useful for a swim day, not a reason to skip lodging here.
See full comparison →Nearby Lakes in Jefferson County
Frequently asked questions about Lake Koshkonong
Where is Lake Koshkonong?
- Lake Koshkonong is a lake in Jefferson County, Wisconsin. LakeQuality grades it D for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Lake Koshkonong?
- Lake Koshkonong in Jefferson County, Wisconsin has a water quality grade of D (Nutrient-rich). 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 works out to a bacteria swim-safety grade of C — a historical summary of past testing, not a current reading. Lake Koshkonong has a water quality grade of D (Nutrient-rich). 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 Lake Koshkonong?
- Lake Koshkonong typically sits around 71°F at the surface in August, 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 a water-quality grade. LakeQuality grades Lake Koshkonong D from EPA Water Quality Portal chemistry, 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 2 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Royce-Dallman County Park, Highway 59 Bridge. Public access is listed as yes.
Explore Lake Koshkonong across LakeGrade
Depth, room, public access, and the state wake-sport rules
Fish species and top-graded 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 grade
Very high nutrients, dense algae, poor clarity
Wisconsin's cleanest and most-polluted lakes, ranked
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State Parks Near Lake Koshkonong
Other Grade D Lakes in Wisconsin
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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)
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
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