Holter Reservoir
Lewis and Clark County, Montana4,550 acresView mapDirections
Montana DEQ publishes the water-quality measurements for Holter Reservoir. Here is what has been measured.
- Impaired · Mercury
- Limited data
Holter Reservoir in Lewis and Clark County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Water clarity: 19 ft visibility. Surface area: 4550 acres.
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. Methodology: how these readings are compared.
Holter Reservoir covers about 4,600 acres in Lewis and Clark County. Vrbo is where to look for cabins and lake homes around Holter Reservoir, and Booking.com and Expedia search hotels near the lake for your dates.
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 conditions
No readingLimited data
Nutrient and algae readings are not a swimming clearance. Holter Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for mercury. Check local advisories before you swim.
Whether Holter Reservoir is safe to swim in today is a call for the Montana Department of Environmental Quality and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Holter Reservoir
These are the measurements Montana DEQ publishes for Holter Reservoir, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Crystal clear, you can see 19 ft down. TSI 35.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 19ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | No reading The nutrient that most limits algae growth in freshwater. More phosphorus, more algae potential. | |
| Chlorophyll-a (algae) | No reading The green pigment in algae — a direct measure of how much algae is in the water. | |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 35 | Oligotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Low nutrients, clear water. No reading is a swimming clearance.
Ask about Holter Reservoir
Answers come straight from the public measurements and survey records on this page.
Lake Details
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 Gulch Campground0.8 kmReserve
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
Stays via Stay22
Stays near Holter Reservoir
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Top-rated places to stay near Holter Reservoir
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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
EPA Impairment Status
Holter Reservoir is officially listed as impaired under Clean Water Act §303(d) in the 2021 EPA reporting cycle (IR category 5).
Causes of impairment
Designated uses not supporting: Aquatic life.
Source: EPA ATTAINS assessment unit MT41I007_020 · Official waterbody report
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Frequently asked questions about Holter Reservoir
Where is Holter Reservoir?
- Holter Reservoir is a lake in Lewis and Clark County, Montana. Its water-quality measurements are published by Montana DEQ and shown on this page as reported.
What is the water quality of Holter Reservoir?
- Holter Reservoir in Lewis and Clark County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Water clarity: 19 ft visibility. Surface area: 4550 acres.
Is Holter Reservoir safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Holter Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for mercury. 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 Campground. 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.
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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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database
Public campgrounds from USGS The National Map Structures, within about 3 miles