Rock Lake
Rock Lake in Cass County, Minnesota has a water quality grade of C (Fair). Water clarity: 5.9 ft visibility. Phosphorus: 29.5 µg/L. Chlorophyll-a (algae): 10.9 µg/L. Maximum depth: 17 ft. Surface area: 261.01 acres.
- Clarity
- 5.9 ft
- Typical in August
- 77°F
- Max depth
- 17 ft
- Area
- 261 acres
- Type
- Eutrophic
- Trend
- ↑ Improving
- Public access
- Yes
- Ice-out
- Apr 18
- Open water
- 221 days
Next lake in Cass County
Rock Lake is a 261-acre productive, nutrient-rich eutrophic lake in Cass County, Minnesota with a maximum depth of 17 feet. Water quality is graded C based on 2 sampling years through 2023. Overall water quality is improving over recent sampling years. The lake supports 11 documented fish species including black crappie, bluegill, and bowfin (dogfish). Public access is available with 2 public boat landings. Compared to the 103 other monitored lakes in Cass County, Rock Lake ranks #90 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2023-09-14. Grade methodology: LakeGrade methodology.
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Swimming Safety
Grade onlyModerate
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.
Generally safe for swimming, moderate water quality
Water Quality Grade: C (Fair)
Murky, only visible to about 5.9 ft. Phosphorus level: 29.5 µg/L. Chlorophyll-a: 10.9 µg/L. Trophic State Index: 53.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 5.9 ft | Not graded |
| Phosphorus | 29.5 µg/L | Fair |
| Chlorophyll-a (Algae) | 10.9 µg/L | Fair |
| Trophic State Index (TSI) | 53 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Where to stay near Rock Lake
Compare lakeside cabins, hotels, and vacation rentals near Rock Lake in Cass County, Minnesota — 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 whole trip? See where to stay at Minnesota’s top lakes →
Water Temperature
Rock Lake typically sits around 77°F at the surface in August. Summer average is about 75°F. Usually warmest in Aug (about 77°F) and coldest in Sep (about 63°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 35 field readings (2011–2023) 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 low (17,539 cells/mL) on May 16, 2026.
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 | 17 ft |
| Surface Area | 261 acres |
| Shoreline Length | 3.4 mi |
| Littoral Zone | 90% |
| Public Access | Yes |
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
- Green Bass LakePublic
- Rock LakePublic
Fish Species
Click a species to see every graded lake where it is documented.
Fishing Rock Lake
DNR Fisheries Survey Summary
3 surveys on file from MN DNR Fisheries. Most recent: 2025-08-12 (Standard Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| Black Bullhead | 52.17 | 0.11 lb |
| Brown Bullhead | 40.72 | 0.25 lb |
| Pumpkinseed | 30.07 | 0.1 lb |
| Yellow Bass | 29.00 | 0.52 lb |
| Yellow Perch | 11.38 | 0.18 lb |
| Northern Pike | 6.61 | 1.02 lb |
CPUE = catch per unit effort, averaged across surveys (excludes juvenile shoreline seining). Higher CPUE = more abundant in standardized sampling.
Length Distributions
Number of fish caught at each inch class in the most recent survey that recorded lengths. Red dashed line marks an approximate trophy threshold for that species.
Brown Bullhead
Pumpkinseed
Yellow Bass
Yellow Perch
From the 2025-08-12 survey
Rock Lake is a 261 acre lake located three miles east of Casino in southern Cass County. Rock Lake has a maximum depth of 17 feet and is susceptible to partial winterkill events during severe winters with at least two partial kills happening in the past decade. The management plan calls for stocking Walleye fry…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Rock Lake. 4 reports on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fisheries Lake Survey — RockFisheries Lake Survey · MN DNR LakeFinder
- —Aquatic Plant Report — Rock (1441)Aquatic Plant Report · MN DNR LakeFinder
- —Fish Stocking History — RockFish Stocking History · MN DNR LakeFinder
- —Public Water Access Sites — RockPublic Water Access Sites · MN DNR LakeFinder
Water Quality Trend: ↑ Improving
Based on 3 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.179 m/yr | 3 |
| Phosphorus | ↓ Declining | +9 µg/L/yr | 2 |
| Chlorophyll-a | ↑ Improving | -2.2 µg/L/yr | 2 |
Ice Season
Historical ice data from Minnesota DNR (6 observations).
Estimated open water season: 221 days
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2005-04-08 (2005) | 2008-04-24 (2008) |
| Ice-In | 2007-11-23 (2007) | 2006-11-30 (2006) |
Most recent ice-out: 2008-04-24
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Rock Lake under Clean Water Act (NPDES) discharge permits.
- PillagerPillager · 2.8 mi away · NPDES MN0048909
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 #90 of 104 lakes in Cass County
Nearby Lakes With Higher Grades
Rock Lake holds Grade C. One nearby lake has a higher nutrient-and-algae grade — useful for a swim day, not a reason to skip lodging here.
See full comparison →Nearby Lakes in Cass County
Frequently asked questions about Rock Lake
What is the water quality grade for Rock Lake?
- Rock Lake in Cass County, Minnesota has a water quality grade of C (Fair). Water clarity: 5.9 ft visibility. Phosphorus: 29.5 µg/L. Chlorophyll-a (algae): 10.9 µg/L. Maximum depth: 17 ft. Surface area: 261.01 acres.
Is Rock Lake safe to swim in?
- Generally safe for swimming, moderate water quality. Rock Lake has a water quality grade of C (Fair). Algae concentration (chlorophyll-a) is 10.9 µg/L. Always check local health department advisories before swimming.
What is the water temperature of Rock Lake?
- Rock Lake typically sits around 77°F at the surface in August, from 35 EPA Water Quality Portal field readings (2011–2023). That is a typical value for the month, not a live reading.
How deep is Rock Lake?
- Rock Lake has a maximum depth of 17 feet. The lake covers 261.01 acres.
What fish are in Rock Lake?
- Fish species found in Rock Lake include black crappie, bluegill, bowfin (dogfish), brown bullhead, hybrid sunfish, largemouth bass, northern pike, pumpkinseed, white sucker, yellow bullhead, yellow perch. Rock Lake is located in Cass County, Minnesota.
When does Rock Lake thaw?
- Rock Lake typically has ice-out around Apr 18. Ice typically forms around Nov 25. The open water season is approximately 221 days.
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 low (17,539 cells/mL). Blue-green algae can appear and clear within days, so check Minnesota's current advisory before swimming.
Are there boat ramps on Rock Lake?
- Rock Lake has 2 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Green Bass Lake, Rock Lake. Public access is listed as yes.
Explore Rock Lake across LakeGrade
Depth, room, public access, and the state wake-sport rules
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Fish species and best lakes in Cass County
Species activity outlook by water temperature and season
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Where to stay at Minnesota's top lakes, each with its grade
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High nutrients, frequent algae, reduced clarity
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More Minnesota Lake Rankings
State Parks Near Rock Lake
Other Grade C Lakes in Minnesota
Eutrophic Lakes in Minnesota
Where this data comes from
40 measurements from 1 monitoring station, 2020–2023. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA40 samples · 2020–2023
Analytical methods
- Field measurement/observation, generic method
- Phosphorus by Two Reagent Colorimetry
- Phosphorus by Colorimetry
- 10200 H 2 ~ Chlorophyll a-b-c Determination by spectrophotometer
- 10200 H ~ Chlorophyll a-b-c Determination
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)
Lake details from Minnesota DNR LakeFinder
Named boat-access points from the USGS National Boat Ramps inventory
Ice data from Minnesota DNR Climate