Rock Lake
Jefferson County, Wisconsin1,365 acresView mapDirections
Wisconsin DNR publishes the water-quality measurements for Rock Lake. Here is what has been measured.
- Mesotrophic
- 2 aquatic invasives
- 60 ft max depth
- Public access
Rock Lake in Jefferson County, Wisconsin has water-quality measurements published by Wisconsin DNR, shown here as reported. Water clarity: 12 ft visibility. Phosphorus: 18.1 µg/L. Maximum depth: 60 ft. Surface area: 1365 acres.
Wisconsin DNR lists Panfish as common on Rock Lake. DNR survey details →
Rock Lake is a 1,365-acre mesotrophic lake in Jefferson County, Wisconsin with a maximum depth of 60 feet. Overall water quality is improving over recent sampling years. The lake supports 5 documented fish species including Panfish, Largemouth Bass, and Smallmouth Bass. Public access is available with 4 public boat landings and 2 public beaches. Compared to the 6 other monitored lakes in Jefferson County, Rock Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Wisconsin DNR Surface Water, last sampled 2026-06-22. Methodology: how these readings are compared.
Rock Lake covers about 1,400 acres in Jefferson County. It has 4 public boat landings, so a lakefront rental is not the only way to get a boat on the water. If you are planning to stay, you can search lake houses and cabins around Rock Lake on Vrbo, or look for a hotel room near the lake on Booking.com or Expedia.
What Rock Lake is good for
From access and monitoring records, not editorial picks.
- Clear water12 ft of visibility
- Boat launch4 documented ramps
- Beach2 documented beaches
- Fishing5 species recorded
- Deep water60 ft surveyed max depth
- Big water1,365 acres
Swimming conditions
Readings onlyBacteria tested: exceedances
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, Rock Lake occasionally exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 7.7% of 561 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 Rock 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 Rock Lake
These are the measurements Wisconsin DNR publishes for Rock 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 12 ft. Phosphorus 18.1 µg/L. TSI 44.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 12ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 18.1µg/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 Rock Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
Rock Lake typically sits around 74°F at the surface in August. Summer average is about 77°F. Usually warmest in Jul (about 80°F) and coldest in Mar (about 40°F). These are typical values by month — not a live reading.
Column height runs from this lake’s coldest month (40°F) to its warmest (80°F).
Typical surface water temperature by month, from 14 field readings (2024–2026) via the EPA Water Quality Portal. See the full Rock Lake water temperature breakdown →
Blue-Green Algae (Cyanobacteria)
There’s no recent EPA CyAN cyanobacteria estimate for Rock Lake — clouds, ice, or an off-season gap in coverage. The most recent reading was moderate (30,479 cells/mL) on October 18, 2025.
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 Rock Lake.
A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Rock Lake’s water-quality measurements separately, and the water-quality trend over 7 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 1988. 4 stations on file (most recent sample 2026).
Fish Species (DNR-rated)
DNR Reports & Resources
Wisconsin DNR Fisheries scientists publish multi-year survey reports for Rock Lake covering fish populations, stocking, water quality, and management. 1 report on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- 2022Rock Lake comprehensive survey, 2022Comprehensive Survey · WI DNR · 2022 (PDF)
Lake Details
Max depth
60 ft
Surface area
1,365 acres
Public access
Yes
Listed by Wisconsin DNR
Lake Size & Watershed
Watershed (drainage) area
19 sq mi
HydroLAKES / HydroSHEDS
Shoreline
8.9 mi
HydroLAKES / HydroSHEDS
Elevation
820 ft
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
- Elm PointPublic
- Rock Lake County ParkPublic
- Lake LanePublic
- Sandy BeachPublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Stays via Stay22
Stays near Rock Lake
Compare lakeside cabins, hotels, and vacation rentals near Rock Lake 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.
Top-rated places to stay near Rock 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 trip? Cabins and lodging at Rock Lake → · more Wisconsin lake trips
Follow Rock Lake’s readings
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Fish Species
Click a species to see every lake where it is documented.
Fishing Rock 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: Improving
Based on 7 years of monitoring (2020–2026).
Water clarity
Stable7 years
Phosphorus
Improving-1.87 µg/L/yr7 years
Reservoir Info (USACE NID)
Rock Lake is a man-made reservoir impounded by the Lake Mills (completed 1865), built primarily for recreation on the ROCK CREEK; gravity-type dam, 17 ft tall and 400 ft long.
- Surface area
- 1,371 ac
- Normal storage
- 25,590 ac-ft
- Max storage
- 28,330 ac-ft
- Drainage area
- 11 sq mi
- Hazard class
- High
- Owner
- City of Lake Mills
Source: USACE National Inventory of Dams, NID ID WI10222 · Operator website
Regulated Pollution Sources Nearby
2 EPA-permitted facilities operate within about 5 miles of Rock Lake under Clean Water Act (NPDES) discharge permits, including 1 classified by the EPA as major discharger. 1 facility has a reported Clean Water Act violation in the past three years.
- Lake Mills Wastewater Treatment FacilityLake Mills · 1.8 mi away · NPDES WI0031194MajorRecent violation
- Daybreak Foods Inc Lake Mills ComplexMarshall · 4.2 mi away · NPDES WI0057550
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 Jefferson County
Frequently asked questions about Rock Lake
Where is Rock Lake?
- Rock Lake 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 Rock Lake?
- Rock Lake in Jefferson County, Wisconsin has water-quality measurements published by Wisconsin DNR, shown here as reported. Water clarity: 12 ft visibility. Phosphorus: 18.1 µg/L. Maximum depth: 60 ft. Surface area: 1365 acres.
Is Rock Lake safe to swim in?
- Based on E. coli testing, Rock Lake occasionally exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 7.7% of 561 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 Rock 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 Rock Lake?
- Rock Lake typically sits around 74°F at the surface in August, from 14 EPA Water Quality Portal field readings (2024–2026). That is a typical value for the month, not a live reading.
How deep is Rock Lake?
- Rock Lake has a maximum depth of 60 feet. The lake covers 1365 acres.
What fish are in Rock Lake?
- Fish species found in Rock Lake include Panfish, Largemouth Bass, Smallmouth Bass, Northern Pike, Walleye. Rock Lake is located in Jefferson County, Wisconsin.
What did the DNR fisheries survey find at Rock Lake?
- Wisconsin DNR lists Panfish as common on Rock Lake. A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Rock Lake’s water-quality measurements separately, and the water-quality trend over 7 years of monitoring is improving. Catch rate is not a swim clearance and is not a weekly bite report.
Does Rock Lake have blue-green algae?
- There is no recent EPA CyAN cyanobacteria estimate for Rock Lake (clouds, ice, or an off-season gap). The most recent reading was moderate (30,479 cells/mL). Blue-green algae can appear and clear within days, so check Wisconsin's current advisory before swimming.
Are there boat ramps on Rock Lake?
- Rock Lake has 4 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Elm Point, Rock Lake County Park, Lake Lane, Sandy Beach. Public access is listed as yes.
Can I fish at Rock Lake?
- Rock Lake 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 Rock Lake 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 Rock Lake
Other Lakes in Wisconsin
Mesotrophic Lakes in Wisconsin
Compare Rock Lake With Nearby Lakes
Where this data comes from
104 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_WQX104 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