Holter Reservoir is a cold, clear oligotrophic lake in Lewis and Clark County, Montana. Water quality is graded A based on 1 sampling years through 2026. Compared to the 11 other monitored lakes in Lewis and Clark County, Holter Reservoir ranks #4 for water quality.
Source: EPA Water Quality Portal sampling records, Montana DNR, last sampled 2026-06-17. Grade methodology: LakeGrade methodology.
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Swimming Safety
Excellent for swimming, crystal clear water with minimal algae
Water Quality Grade: A, Excellent
Crystal clear, you can see 19 ft down. Trophic State Index: 35.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 19 ft | A |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 35 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Where to stay near Holter Reservoir
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Fishing Holter Reservoir
Location
County Ranking
Ranked #4 of 12 lakes in Lewis and Clark County
Nearby Lakes in Lewis and Clark County
Explore Holter Reservoir across LakeGrade
More Montana Lake Rankings
Other Grade A Lakes in Montana
Oligotrophic Lakes in Montana
More records, surveys & data sources
Data Sources
Water quality data from the EPA Water Quality Portal
Grading methodology based on Metropolitan Council standards
Most recent sample: 2026-06-17
Monitoring stations: 1