Deep Portage Lake
Deep Portage Lake in Cass County, Minnesota has a water quality grade of A (Excellent). Water clarity: 15.4 ft visibility. Phosphorus: 9 µg/L. Chlorophyll-a (algae): 1.6 µg/L. Maximum depth: 105 ft. Surface area: 129.02 acres.
- Clarity
- 15.4 ft
- Typical in August
- 53°F
- Max depth
- 105 ft
- Area
- 129 acres
- Type
- Oligotrophic
- Trend
- ↑ Improving
- Public access
- No
Next lake in Cass County
Deep Portage Lake is a 129-acre cold, clear oligotrophic lake in Cass County, Minnesota with a maximum depth of 105 feet (40-foot average). Water quality is graded A based on 2 sampling years through 2025. Overall water quality is improving over recent sampling years. The lake supports 9 documented fish species including black crappie, bluegill, and largemouth bass. No public access — the lake is private or restricted. Compared to the 103 other monitored lakes in Cass County, Deep Portage Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-09-02. 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)
Crystal clear, you can see 15.4 ft down. Phosphorus level: 9 µg/L. Chlorophyll-a: 1.6 µg/L. Trophic State Index: 36.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 15.4 ft | Not graded |
| Phosphorus | 9 µg/L | Good |
| Chlorophyll-a (Algae) | 1.6 µg/L | Good |
| Trophic State Index (TSI) | 36 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Where to stay near Deep Portage Lake
Compare lakeside cabins, hotels, and vacation rentals near Deep Portage 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
Deep Portage Lake typically sits around 53°F at the surface in August. Summer average is about 66°F. Usually warmest in Jul (about 73°F) and coldest in Aug (about 53°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 84 field readings (2011–2025) via the EPA Water Quality Portal. See the full Deep Portage Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 105 ft |
| Average Depth | 40 ft |
| Surface Area | 129 acres |
| Shoreline Length | 1.8 mi |
| Littoral Zone | 24% |
| Public Access | No |
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 Deep Portage Lake
DNR Fisheries Survey Summary
5 surveys on file from MN DNR Fisheries. Most recent: 2024-08-01 (Targeted Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| Northern Pike | 7.70 | 1.65 lb |
| Yellow Perch | 3.24 | 0.13 lb |
| Walleye | 1.84 | 3.1 lb |
| Bluegill | 1.74 | 0.13 lb |
| Largemouth Bass | 1.55 | 0.65 lb |
| Tullibee (Cisco) | 1.53 | 0.32 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.
Northern Pike
Yellow Perch
Walleye
Bluegill
From the 2024-08-01 survey
Temperature and dissolved oxygen profiles were collected in the deepest basin in Deep Portage Lake on August 1, 2024, to evaluate the quantity and quality of cold, oxygenated water (i.e., oxythermal habitat) available to Cisco, also known as Tullibee, 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 Deep Portage Lake. 1 report on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fisheries Lake Survey — Deep PortageFisheries Lake Survey · MN DNR LakeFinder
Water Quality Trend: ↑ Improving
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.049 m/yr | 6 |
| Phosphorus | ↑ Improving | -0.81 µg/L/yr | 6 |
| Chlorophyll-a | → Stable | -0.04 µg/L/yr | 6 |
EPA Impairment Status
Deep Portage 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 MN11-0308-02 · Official waterbody report · Matched by proximity (1.36 km)
County Ranking
Ranked #2 of 104 lakes in Cass County
Nearby Lakes in Cass County
Frequently asked questions about Deep Portage Lake
What is the water quality grade for Deep Portage Lake?
- Deep Portage Lake in Cass County, Minnesota has a water quality grade of A (Excellent). Water clarity: 15.4 ft visibility. Phosphorus: 9 µg/L. Chlorophyll-a (algae): 1.6 µg/L. Maximum depth: 105 ft. Surface area: 129.02 acres.
Is Deep Portage Lake safe to swim in?
- Excellent for swimming, crystal clear water with minimal algae. Deep Portage Lake has a water quality grade of A (Excellent). Algae concentration (chlorophyll-a) is 1.6 µg/L. Always check local health department advisories before swimming.
What is the water temperature of Deep Portage Lake?
- Deep Portage Lake typically sits around 53°F at the surface in August, from 84 EPA Water Quality Portal field readings (2011–2025). That is a typical value for the month, not a live reading.
How deep is Deep Portage Lake?
- Deep Portage Lake has a maximum depth of 105 feet and an average depth of 40 feet. The lake covers 129.02 acres.
What fish are in Deep Portage Lake?
- Fish species found in Deep Portage Lake include black crappie, bluegill, largemouth bass, northern pike, pumpkinseed, rock bass, tullibee (cisco), walleye, yellow bullhead. Deep Portage Lake is located in Cass County, Minnesota.
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Where this data comes from
42 measurements from 1 monitoring station, 2020–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA42 samples · 2020–2025
Analytical methods
- Field measurement/observation, generic method
- Phosphorus by Two Reagent Colorimetry
- 10200 H 2 ~ Chlorophyll a-b-c Determination by spectrophotometer
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