Mountain Lake
Mountain Lake in Unknown County, Minnesota is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 19 ft visibility. Maximum depth: 130 ft. Surface area: 1344.33 acres.
Minnesota DNR surveys at Mountain Lake average 2.61 smallmouth bass per unit of sampling effort (CPUE) across 15 surveys. The most recent is a Targeted Survey dated August 11, 2025. Length samples in the latest survey that recorded them ran 7–18 inches. DNR survey details →
- Clarity
- 19 ft
- Max depth
- 130 ft
- Area
- 1,344 acres
- Trend
- ↓ Declining
- Public access
- Yes
- Ice-out
- May 5
- Open water
- 217 days
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Why Mountain 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 Mountain Lake, no EPA ecoregion for this lake, so the National Lakes Assessment condition thresholds cannot be applied. 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 MPCA and volunteer programs, and published through the EPA Water Quality Portal. Read the full grading methodology.
Mountain Lake is a 1,344-acre lake in Unknown County, Minnesota with a maximum depth of 130 feet (42-foot average). Overall water quality is declining over recent sampling years. The lake supports 7 documented fish species including burbot, cisco species, and green sunfish. Public access is available.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2024-07-31. Grade methodology: LakeQuality methodology.
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Swimming Safety
Not ratedLimited data
Not rated for swimming — there is not enough monitoring data to judge this lake
Water quality measurements for Mountain Lake
These readings exist but are not enough for an A–F grade — see why.
Crystal clear, you can see 19 ft down. Trophic State Index: 35.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 19 ft | Not graded |
| Phosphorus | No recent samples | |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 35 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Where to stay near Mountain Lake
Compare lakeside cabins, hotels, and vacation rentals near Mountain Lake in Unknown 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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Minnesota DNR surveys at Mountain Lake average 2.61 smallmouth bass per unit of sampling effort (CPUE) across 15 surveys. The most recent is a Targeted Survey dated August 11, 2025. Length samples in the latest survey that recorded them ran 7–18 inches.
A DNR fisheries survey reports fish abundance and catch rates, not a water-quality grade. LakeQuality does not assign an A–F grade on Mountain Lake — official chemistry is on file, not a letter, and the water-quality trend over 5 years of monitoring is declining. Catch rate is not a swim clearance and is not a weekly bite report.
DNR Fisheries Survey Summary
15 surveys on file from MN DNR Fisheries. Most recent: 2025-08-11 (Targeted Survey).
Top Species by Catch Rate
| Species | Avg catch rate (CPUE) | Avg Weight |
|---|---|---|
| Tullibee (Cisco) | 9.17 | 0.69 lb |
| Yellow Perch | 2.78 | 0.24 lb |
| Smallmouth Bass | 2.61 | 0.6 lb |
| Lake Trout | 2.54 | 1.74 lb |
| White Sucker | 1.62 | 2.05 lb |
CPUE = catch per unit effort, averaged across surveys (excludes juvenile shoreline seining). Higher CPUE means more abundant in standardized sampling. This is not a weekly bite report.
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.
Tullibee (Cisco)
Yellow Perch
Smallmouth Bass
Lake Trout
From the 2025-08-11 survey
Temperature and dissolved oxygen measurements were collected from the deepest basin in Clearwater Lake on August 11th, 2025. This was done to evaluate the quantity and quality of cold, oxygenated water (i.e., oxythermal habitat) available to Lake Trout, the most sensitive coldwater species present in this lake (Cisco,…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Mountain Lake. 3 reports on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fisheries Lake Survey — ClearwaterFisheries Lake Survey · MN DNR LakeFinder
- —Aquatic Plant Report — Clearwater (1828)Aquatic Plant Report · MN DNR LakeFinder
- —Public Water Access Sites — ClearwaterPublic Water Access Sites · MN DNR LakeFinder
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 130 ft |
| Average Depth | 42 ft |
| Surface Area | 1,344 acres |
| Shoreline Length | 18.5 mi |
| Littoral Zone | 19% |
| Public Access | Yes |
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Fish Species
Click a species to see every graded lake where it is documented.
Fishing Mountain Lake
Water Quality Trend: ↓ Declining
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.216 m/yr | 5 |
Ice Season
Historical ice data from Minnesota DNR (51 observations).
Estimated open water season: 217 days
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2012-04-03 (2012) | 2014-05-20 (2014) |
| Ice-In | 2018-11-20 (2018) | 2015-12-26 (2015) |
Most recent ice-out: 2024-04-26
EPA Impairment Status
Mountain Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit MN16-0093-00 · Official waterbody report
Graded lakes near Mountain Lake
Mountain Lake has no grade, but these lakes within a short drive do.
Nearby Lakes in Unknown County
Frequently asked questions about Mountain Lake
Where is Mountain Lake?
- Mountain Lake is a lake in Unknown County, Minnesota. LakeQuality has not rated this lake (UNRATED) — there is not enough recent sampling to assign a letter.
What is the water quality grade for Mountain Lake?
- Mountain Lake in Unknown County, Minnesota is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 19 ft visibility. Maximum depth: 130 ft. Surface area: 1344.33 acres.
Is Mountain Lake safe to swim in?
- Not rated for swimming — there is not enough monitoring data to judge this lake. Mountain Lake is not rated for water quality — its monitoring record does not meet the evidence bar. Always check local health department advisories before swimming.
How deep is Mountain Lake?
- Mountain Lake has a maximum depth of 130 feet and an average depth of 42 feet. The lake covers 1344.33 acres.
What fish are in Mountain Lake?
- Fish species found in Mountain Lake include burbot, cisco species, green sunfish, lake trout, smallmouth bass, white sucker, yellow perch. Mountain Lake is located in Unknown County, Minnesota.
What did the DNR fisheries survey find at Mountain Lake?
- Minnesota DNR surveys at Mountain Lake average 2.61 smallmouth bass per unit of sampling effort (CPUE) across 15 surveys. The most recent is a Targeted Survey dated August 11, 2025. Length samples in the latest survey that recorded them ran 7–18 inches. A DNR fisheries survey reports fish abundance and catch rates, not a water-quality grade. LakeQuality does not assign an A–F grade on Mountain Lake — official chemistry is on file, not a letter, and the water-quality trend over 5 years of monitoring is declining. Catch rate is not a swim clearance and is not a weekly bite report.
When does Mountain Lake thaw?
- Mountain Lake typically has ice-out around May 5. Ice typically forms around Dec 8. The open water season is approximately 217 days.
Explore Mountain Lake across LakeQuality
Depth, room, public access, and the state wake-sport rules
Fish species and top-graded lakes across Minnesota
Fish species and best lakes in Unknown County
Species activity outlook by water temperature and season
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Where this data comes from
1 measurement from 1 monitoring station, 2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA1 sample · 2024
Analytical methods
- Field measurement/observation, generic method
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