Rainy Lake
Rainy Lake in Cass County, Minnesota has a water quality grade of A (Excellent). Water clarity: 13.9 ft visibility. Phosphorus: 11.5 µg/L. Chlorophyll-a (algae): 1.8 µg/L. Maximum depth: 29 ft. Surface area: 129.39 acres.
- Clarity
- 13.9 ft
- Typical in August
- 69°F
- Max depth
- 29 ft
- Area
- 129 acres
- Type
- Oligotrophic
- Trend
- ↑ Improving
- Public access
- No
- Ice-out
- Apr 15
- Open water
- 220 days
Next lake in Cass County
Rainy Lake is a 129-acre cold, clear oligotrophic lake in Cass County, Minnesota with a maximum depth of 29 feet. Water quality is graded A based on 2 sampling years through 2024. Overall water quality is improving over recent sampling years. Limited fish-species data available. No public access — the lake is private or restricted. Compared to the 103 other monitored lakes in Cass County, Rainy Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2024-09-12. Grade methodology: LakeGrade methodology.
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Swimming Safety
Grade onlyLow algae
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.
Excellent for swimming, crystal clear water with minimal algae
Water Quality Grade: A (Excellent)
Good clarity, visible to about 13.9 ft. Phosphorus level: 11.5 µg/L. Chlorophyll-a: 1.8 µg/L. Trophic State Index: 38.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 13.9 ft | Not graded |
| Phosphorus | 11.5 µg/L | Good |
| Chlorophyll-a (Algae) | 1.8 µg/L | Good |
| Trophic State Index (TSI) | 38 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Where to stay near Rainy Lake
Compare lakeside cabins, hotels, and vacation rentals near Rainy 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.
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Water Temperature
Rainy Lake typically sits around 69°F at the surface in August. Summer average is about 66°F. Usually warmest in Aug (about 69°F) and coldest in May (about 52°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 90 field readings (2023–2024) via the EPA Water Quality Portal. See the full Rainy Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 29 ft |
| Surface Area | 129 acres |
| Shoreline Length | 2.8 mi |
| Littoral Zone | 72% |
| Public Access | No |
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Fishing Rainy Lake
DNR Fisheries Survey Summary
3 surveys on file from MN DNR Fisheries. Most recent: 1987-07-07 (Standard Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| Bluegill | 37.75 | 0.13 lb |
| Yellow Perch | 6.09 | 0.11 lb |
| Pumpkinseed | 5.84 | 0.22 lb |
| Yellow Bass | 4.46 | 0.49 lb |
| Black Crappie | 3.33 | 0.3 lb |
| Northern Pike | 2.56 | 3.3 lb |
CPUE = catch per unit effort, averaged across surveys (excludes juvenile shoreline seining). Higher CPUE = more abundant in standardized sampling.
From the 1987-07-07 survey
The fish population does not seem to have changed appreciably since the lake survey done in 1962. The bulk of the fish biomass is still predominantly centrarchids, with bluegills, black crappies, and largemouth bass being the most sought-after fish by sportsmen. Looking at the age and growth data, it can be seen that…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Rainy Lake. 2 reports on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fisheries Lake Survey — RainyFisheries Lake Survey · MN DNR LakeFinder
- —Aquatic Plant Report — Rainy (1579)Aquatic Plant Report · MN DNR LakeFinder
Water Quality Trend: ↑ Improving
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.274 m/yr | 6 |
| Phosphorus | ↑ Improving | -1.79 µg/L/yr | 6 |
| Chlorophyll-a | ↑ Improving | -0.95 µg/L/yr | 6 |
Ice Season
Historical ice data from Minnesota DNR (51 observations).
Estimated open water season: 220 days
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2016-03-30 (2016) | 1996-05-03 (1996) |
| Ice-In | 1993-11-06 (1993) | 1998-12-08 (1998) |
Most recent ice-out: 2023-05-01
EPA Impairment Status
Rainy Lake 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 MN11-0308-01 · Official waterbody report · Matched by proximity (1.6 km)
County Ranking
Ranked #1 of 104 lakes in Cass County
Nearby Lakes in Cass County
Frequently asked questions about Rainy Lake
What is the water quality grade for Rainy Lake?
- Rainy Lake in Cass County, Minnesota has a water quality grade of A (Excellent). Water clarity: 13.9 ft visibility. Phosphorus: 11.5 µg/L. Chlorophyll-a (algae): 1.8 µg/L. Maximum depth: 29 ft. Surface area: 129.39 acres.
Is Rainy Lake safe to swim in?
- Excellent for swimming, crystal clear water with minimal algae. Rainy Lake has a water quality grade of A (Excellent). Algae concentration (chlorophyll-a) is 1.8 µg/L. Always check local health department advisories before swimming.
What is the water temperature of Rainy Lake?
- Rainy Lake typically sits around 69°F at the surface in August, from 90 EPA Water Quality Portal field readings (2023–2024). That is a typical value for the month, not a live reading.
How deep is Rainy Lake?
- Rainy Lake has a maximum depth of 29 feet. The lake covers 129.39 acres.
When does Rainy Lake thaw?
- Rainy Lake typically has ice-out around Apr 15. Ice typically forms around Nov 21. The open water season is approximately 220 days.
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Where this data comes from
37 measurements from 1 monitoring station, 2020–2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA37 samples · 2020–2024
Analytical methods
- Field measurement/observation, generic method
- Phosphorus by Colorimetry
- 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
Impairment status from EPA ATTAINS 303(d) database
Lake size & watershed from HydroLAKES (HydroSHEDS)
Lake details from Minnesota DNR LakeFinder
Ice data from Minnesota DNR Climate