Rock Lake
Rock Lake in Jefferson County, Wisconsin has a water quality grade of B (Good). Water clarity: 12 ft visibility. Phosphorus: 18.1 µg/L. Maximum depth: 60 ft. Surface area: 1365 acres.
- Clarity
- 12 ft
- Typical in August
- 74°F
- Max depth
- 60 ft
- Area
- 1,365 acres
- Type
- Mesotrophic
- Trend
- ↑ Improving
- Public access
- Yes
Next lake in Jefferson County
Rock Lake is a 1,365-acre mesotrophic lake in Jefferson County, Wisconsin with a maximum depth of 60 feet. Water quality is graded B based on 1 sampling years through 2026. 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 5 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. Grade methodology: LakeGrade methodology.
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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, 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.
Water Quality Grade: B (Good)
Good clarity, visible to about 12 ft. Phosphorus level: 18.1 µg/L. Trophic State Index: 44.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 12 ft | Not graded |
| Phosphorus | 18.1 µg/L | Good |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 44 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay 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
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Planning a trip? Cabins and lodging at Rock Lake → · more Wisconsin lake trips
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.
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 ↗
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 60 ft |
| Surface Area | 1,365 acres |
| Public Access | Yes |
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
- Elm PointPublic
- Rock Lake Mill PondPublic
- Rock Lake County ParkPublic
- Lake LanePublic
- Sandy BeachPublic
Fish Species
Click a species to see every graded 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.
Source: USGS Nonindigenous Aquatic Species (NAS) Database. Hover a chip to see scientific name, ecological context, status, and first-observed year.
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)
Water Quality Trend: ↑ Improving
Based on 7 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.161 m/yr | 7 |
| Phosphorus | ↑ Improving | -1.87 µg/L/yr | 7 |
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.
County Ranking
Ranked #1 of 7 lakes in Jefferson County
Nearby Lakes in Jefferson County
Frequently asked questions about Rock Lake
What is the water quality grade for Rock Lake?
- Rock Lake in Jefferson County, Wisconsin has a water quality grade of B (Good). 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 works out to a bacteria swim-safety grade of C — a historical summary of past testing, not a current reading. Rock 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 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.
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 5 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Elm Point, Rock Lake Mill Pond, Rock Lake County Park, Lake Lane, among others. Public access is listed as yes.
Explore Rock Lake 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
Moderate nutrients, good water quality
Wisconsin's cleanest and most-polluted lakes, ranked
More Wisconsin Lake Rankings
State Parks Near Rock Lake
Other Grade B 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)
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