Holter Reservoir
Lewis and Clark County, Montana4,550 acresView mapDirections
Not Rated: one monitoring station are on record, but they measured water clarity only. LakeQuality’s A–F method grades total phosphorus, total nitrogen, chlorophyll-a, and turbidity; clarity is shown, not scored.
- Limited data
Holter Reservoir in Lewis and Clark County, Montana is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 19 ft visibility. Surface area: 4550 acres.
- Clarity
- 19 ft
- Area
- 4,550 acres
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Watch Holter Reservoir’s letter
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Why Holter Reservoir has no grade
Holter Reservoir has monitoring on record from one station, but only water clarity was measured. LakeQuality’s A–F letter uses total phosphorus, total nitrogen, chlorophyll-a, and turbidity against EPA National Lakes Assessment condition classes. With none of those grading measurements on record, Holter Reservoir is Not Rated — not an F. Its 19-foot median from 26 readings collected 2026 is still shown as clarity, but clarity alone cannot earn a letter.
Not Rated is EPA Integrated Reporting Category 3 — insufficient information. Everything below that has been measured is still reported on this page.
Sampling is run by Montana DEQ and volunteer programs, and published through the EPA Water Quality Portal. Read the full grading methodology.
Holter Reservoir is a 4,550-acre lake in Lewis and Clark County, Montana.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2026-06-17. Grade methodology: LakeQuality methodology.
What Holter Reservoir is good for
From access and monitoring records, not editorial picks.
- Clear water19 ft of visibility
- Camping2 public campgrounds nearby
- Big water4,550 acres
Swimming Safety
Not ratedLimited data
The letter is not a swimming clearance. Check local advisories before you swim.
Water quality measurements for Holter Reservoir
These readings exist but are not enough for an A–F grade — see why.
Crystal clear, you can see 19 ft down. TSI 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. The letter is not a swimming clearance.
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 4,550 acres |
No bathymetric survey has been published for Holter Reservoir, so its depth is not documented here.
Recreation & Access
Public campgrounds near Holter Reservoir
- Departure Point0.6 km
- Log Gulch0.8 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 Holter Reservoir
Reservoir Info (USACE NID)
Holter Reservoir is a man-made reservoir impounded by the Holter Dam (completed 1918), built primarily for hydroelectric on the Missouri River; gravity-type dam, 124 ft tall and 1,364 ft long.
- Surface area
- 4,550 ac
- Normal storage
- 245,000 ac-ft
- Max storage
- 306,000 ac-ft
- Drainage area
- 17,150 sq mi
- Hazard class
- High
- Owner
- NorthWestern Energy
All listed purposes: Hydroelectric;Flood Risk Reduction.
Source: USACE National Inventory of Dams, NID ID MT00559 · Operator website
Graded lakes near Holter Reservoir
Holter Reservoir has no grade, but these lakes within a short drive do.
Nearby Lakes in Lewis and Clark County
Frequently asked questions about Holter Reservoir
Where is Holter Reservoir?
- Holter Reservoir is a lake in Lewis and Clark County, Montana. LakeQuality has not rated this lake (UNRATED) — no nutrient or algae measurement meets the evidence bar (at least 3 samples within 10 years).
What is the water quality grade for Holter Reservoir?
- Holter Reservoir in Lewis and Clark County, Montana is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 19 ft visibility. Surface area: 4550 acres.
Is Holter Reservoir safe to swim in?
- The letter is not a swimming clearance. Check local advisories before you swim.
Are there campgrounds on Holter Reservoir?
- Holter Reservoir has 2 public campgrounds from USGS The National Map within 5 km, including Departure Point, Log Gulch. 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 Holter Reservoir across LakeQuality
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Where this data comes from
26 measurements from 1 monitoring station, 2026. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Montana DEQ WQPB
MDEQ_WQ_WQX26 samples · 2026
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