Deep Mill Lake
Yellowstone County, MontanaView mapDirections
Clarity, phosphorus, and algae sit in the clean band for this region.
- Mesotrophic
- #1 of 1 in the county
- Limited data
Deep Mill Lake in Yellowstone County, Montana has a water quality grade of B (Good). Phosphorus: 20 µg/L.
- Type
- Mesotrophic
- Phosphorus
- 20 µg/L
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Stays near Deep Mill Lake
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Watch Deep Mill Lake’s letter
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Deep Mill Lake is a mesotrophic lake in Yellowstone County, Montana. Water quality is graded B based on 2 sampling years through 2025.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2025-09-17. Grade methodology: LakeQuality methodology.
What Deep Mill Lake is good for
From access and monitoring records, not editorial picks.
- Camping1 public campground nearby
Swimming Safety
Grade onlyLimited data
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.
The B is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Water Quality Grade: B (Good)
No clarity data. Phosphorus 20 µg/L. TSI 47. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 20 µg/L | Good |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 47 | Mesotrophic |
Moderate nutrients, good water quality
Recreation & Access
Public campgrounds near Deep Mill Lake
- Billings KOA Holiday4.7 km
Nearest public campgrounds in the USGS National Map inventory, within about 3 miles. An empty list means USGS records no campground that close, which is not the same as camping being unavailable. Where the source gives an official reservation link, it is used; the cabin and hotel search does not cover campsites. More camping lakes
Fishing Deep Mill Lake
County Ranking
Ranked #1 of 1 lakes in Yellowstone County
Nearby Lakes in Yellowstone County
Frequently asked questions about Deep Mill Lake
Where is Deep Mill Lake?
- Deep Mill Lake is a lake in Yellowstone County, Montana. LakeQuality grades it B for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Deep Mill Lake?
- Deep Mill Lake in Yellowstone County, Montana has a water quality grade of B (Good). Phosphorus: 20 µg/L.
Is Deep Mill Lake safe to swim in?
- The B is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Are there campgrounds on Deep Mill Lake?
- Deep Mill Lake has 1 public campground from USGS The National Map within 5 km, including Billings KOA Holiday. This is a location join, not a booking. Cabins and hotels are listed separately; campsite reservations use the official link when the source provides one.
Explore Deep Mill Lake across LakeQuality
More Montana Lake Rankings
Other Grade B Lakes in Montana
Mesotrophic Lakes in Montana
Where this data comes from
6 measurements from 1 monitoring station, 2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Montana Volunteer Water Quality Monitoring
MTVOLWQM_WQX6 samples · 2025
Analytical methods
- Phosphorus by Colorimetry
- 4500 N C ~ Persufate Method for Total Nitrogen
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
Public campgrounds from USGS The National Map Structures, within about 3 miles