Rock River Lake
Rock River Lake in Rock County, Wisconsin is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 21 ft visibility. Surface area: 548 acres.
- Clarity
- 21 ft
- Typical in August
- 75°F
- Area
- 548 acres
- Public access
- Yes
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Why Rock River Lake has no grade
We grade a lake only when the monitoring record supports a letter: at least three phosphorus or chlorophyll-a samples in the last ten years, scored against EPA National Lakes Assessment thresholds for this lake’s ecoregion. For Rock River Lake, no nutrient or algae measurement meets the evidence bar (at least 3 samples within 10 years). Sampling here has recorded water clarity, but not phosphorus and algae (chlorophyll-a).
Not Rated is not an F. It is EPA Integrated Reporting Category 3 — insufficient information. The water here may be excellent or poor; nobody has measured enough of it to say. Everything below that has been measured is still reported on this page.
Sampling is run by WI DNR and volunteer programs, and published through the EPA Water Quality Portal. Read the full grading methodology.
Rock River Lake is a 548-acre lake in Rock County, Wisconsin. Limited fish-species data available. Public access is available with 6 public boat landings.
Source: EPA Water Quality Portal sampling records, Wisconsin DNR Surface Water, last sampled 2021-07-06. Grade methodology: LakeQuality methodology.
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Swimming Safety
Not ratedBacteria C
Based on E. coli testing, Rock River Lake occasionally exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 7.9% of 164 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 measurements for Rock River Lake
These readings exist but are not enough for an A–F grade — see why.
Crystal clear, you can see 21 ft down. Trophic State Index: 33.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 21 ft | Not graded |
| Phosphorus | No recent samples | |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 33 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Where to stay near Rock River Lake
Compare lakeside cabins, hotels, and vacation rentals near Rock River Lake in Rock 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 River Lake
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Planning a trip? Cabins and lodging at Rock River Lake → · more Wisconsin lake trips
Water Temperature
Rock River Lake typically sits around 75°F at the surface in August. Summer average is about 75°F. Usually warmest in Jul (about 77°F) and coldest in May (about 66°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 30 field readings (2024–2026) via the EPA Water Quality Portal. See the full Rock River Lake water temperature breakdown →
DNR Reports & Resources
Wisconsin DNR Fisheries scientists publish multi-year survey reports for Rock River 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).
- 2025Rock River fall shocking survey, 2025Fisheries Survey · WI DNR · 2025 (PDF)
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 548 acres |
| Public Access | Yes |
No bathymetric survey has been published for Rock River Lake, so its depth is not documented here.
Recreation & Access
- Riverside ParkPublic
- Rockport RoadPublic
- Lions Park PondPublic
- WIllard Avenue BridgePublic
- Kiwanis PondPublic
- Traxler ParkPublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Rock River Lake
Reservoir Info (USACE NID)
Rock River Lake is a man-made reservoir impounded by the Central (completed 1927), built primarily for hydroelectric on the Rock River; timber crib-type dam, 8 ft tall and 325 ft long.
- Surface area
- 548 ac
- Normal storage
- 3,675 ac-ft
- Drainage area
- 3,205 sq mi
- Hazard class
- Low
All listed purposes: Hydroelectric;Recreation.
Source: USACE National Inventory of Dams, NID ID WI00790 · Operator website · Matched by proximity (0.7 km)
Regulated Pollution Sources Nearby
3 EPA-permitted facilities operate within about 5 miles of Rock River 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.
- Janesville Wastewater UtilityJanesville · 4 mi away · NPDES WI0030350MajorRecent violation
- General Motors Corp/Truck AndJanesville · 1.8 mi away · NPDES WI0001945
- Seneca Foods Corp Janesville La Prairie PlantLa Prairie · 3 mi away · NPDES WI0050687
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.
Graded lakes near Rock River Lake
Rock River Lake has no grade, but these lakes within a short drive do.
Nearby Lakes in Rock County
Frequently asked questions about Rock River Lake
Where is Rock River Lake?
- Rock River Lake is a lake in Rock County, Wisconsin. LakeQuality has not rated this lake (UNRATED) — there is not enough recent sampling to assign a letter.
What is the water quality grade for Rock River Lake?
- Rock River Lake in Rock County, Wisconsin is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 21 ft visibility. Surface area: 548 acres.
Is Rock River Lake safe to swim in?
- Based on E. coli testing, Rock River Lake occasionally exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 7.9% of 164 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 River Lake is not rated for water quality — its monitoring record does not meet the evidence bar. 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 River Lake?
- Rock River Lake typically sits around 75°F at the surface in August, from 30 EPA Water Quality Portal field readings (2024–2026). That is a typical value for the month, not a live reading.
Are there boat ramps on Rock River Lake?
- Rock River Lake has 6 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Riverside Park, Rockport Road, Lions Park Pond, WIllard Avenue Bridge, among others. Public access is listed as yes.
Explore Rock River Lake across LakeQuality
Depth, room, public access, and the state wake-sport rules
Fish species and top-graded lakes across Wisconsin
Fish species and best lakes in Rock County
Species activity outlook by water temperature and season
Where to stay at Wisconsin's top lakes, each with its grade
Wisconsin's cleanest and most-polluted lakes, ranked
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State Parks Near Rock River Lake
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Oligotrophic Lakes in Wisconsin
Where this data comes from
1 measurement from 1 monitoring station, 2021. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Wisconsin Department of Natural Resources
WIDNR_WQX1 sample · 2021
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)
Fish and recreation data from Wisconsin DNR Lakes
Named boat-access points from the USGS National Boat Ramps inventory