Lake Helena Outlet
Lewis and Clark County, MontanaView mapDirections
Nutrients and algae run above the regional average.
↑ up from F at the last regrade
- Hypereutrophic
- Impaired · Metals (other than mercury)
- #3 of 3 in the county
Lake Helena Outlet in Lewis and Clark County, Montana has a water quality grade of D (Nutrient-rich). Phosphorus: 115.5 µg/L.
- Type
- Hypereutrophic
- Phosphorus
- 115.5 µg/L
- Trend
- ↓ Declining
Stays via Stay22
Stays near Lake Helena Outlet
Compare lakeside cabins, hotels, and vacation rentals near Lake Helena Outlet in Lewis and Clark County, Montana — 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. The water-quality grade measures nutrients and algae, not lodging — cabins and rentals around Lake Helena Outlet are listed below.
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Lake Helena Outlet is a highly nutrient-loaded hypereutrophic lake in Lewis and Clark County, Montana. Water quality is graded D based on 2 sampling years through 2023. Overall water quality is declining over recent sampling years. Compared to the 2 other monitored lakes in Lewis and Clark County, Lake Helena Outlet ranks #3 for water quality.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2023-10-17. Grade methodology: LakeQuality methodology.
Swimming Safety
Grade onlyElevated nutrients
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 D is nutrients and algae, not a swimming clearance. Lake Helena Outlet is listed impaired under Clean Water Act §303(d) (IR 4A) for metals (other than mercury), nutrients (phosphorus, nitrogen). Check local advisories before you swim.
Water Quality Grade: D (Nutrient-rich)
No clarity data. Phosphorus 115.5 µg/L. TSI 73. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 115.5 µg/L | Poor |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 73 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Fishing Lake Helena Outlet
Water Quality Trend: ↓ Declining
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Phosphorus | ↓ Declining | +17.15 µg/L/yr | 5 |
EPA Impairment Status
Lake Helena Outlet is officially listed as impaired under Clean Water Act §303(d) in the 2021 EPA reporting cycle (IR category 4A).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit MT41I007_010 · Official waterbody report · Matched by proximity (2.64 km)
County Ranking
Ranked #3 of 3 lakes in Lewis and Clark County
Nearby Lakes in Lewis and Clark County
Frequently asked questions about Lake Helena Outlet
Where is Lake Helena Outlet?
- Lake Helena Outlet is a lake in Lewis and Clark County, Montana. LakeQuality grades it D for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Lake Helena Outlet?
- Lake Helena Outlet in Lewis and Clark County, Montana has a water quality grade of D (Nutrient-rich). Phosphorus: 115.5 µg/L.
Is Lake Helena Outlet safe to swim in?
- The D is nutrients and algae, not a swimming clearance. Lake Helena Outlet is listed impaired under Clean Water Act §303(d) (IR 4A) for metals (other than mercury), nutrients (phosphorus, nitrogen). Check local advisories before you swim.
Explore Lake Helena Outlet across LakeQuality
2 thermal springs within 18 mi, nearest Broadwater (Helena) Hot Springs
Species activity outlook by water temperature and season
Where to stay at Montana's top lakes, each with its grade
Very high nutrients, dense algae, poor clarity
Montana's cleanest and most-polluted lakes, ranked
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Other Grade D Lakes in Montana
Hypereutrophic Lakes in Montana
Where this data comes from
30 measurements from 1 monitoring station, 2019–2023. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Montana DEQ WQPB
MDEQ_WQ_WQX30 samples · 2019–2023
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
Impairment status from EPA ATTAINS 303(d) database