Lake Koocanusa
Lincoln County, Montana46,500 acresView mapDirections
Montana DEQ publishes the water-quality measurements for Lake Koocanusa. Here is what has been measured.
- Impaired · Hydrologic alteration
- Public access
- Limited data
Lake Koocanusa in Lincoln County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Water clarity: 14.8 ft visibility. Surface area: 46500 acres.
Lake Koocanusa is a 46,500-acre lake in Lincoln County, Montana. Overall water quality is improving over recent sampling years. Limited fish-species data available. Public access is available with 3 public boat landings.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2018-10-23. Methodology: how these readings are compared.
What Lake Koocanusa is good for
From access and monitoring records, not editorial picks.
- Clear water14.8 ft of visibility
- Boat launch3 documented ramps
- Big water46,500 acres
Swimming conditions
No readingLimited data
Nutrient and algae readings are not a swimming clearance. Lake Koocanusa is listed impaired under Clean Water Act §303(d) (IR 5) for hydrologic alteration, metals (other than mercury). Check local advisories before you swim.
Whether Lake Koocanusa 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 Lake Koocanusa
These are the measurements Montana DEQ publishes for Lake Koocanusa, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Good clarity, visible to about 14.8 ft. TSI 38.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 14.8ft | 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. | 38 | Oligotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Low nutrients, clear water. No reading is a swimming clearance.
Ask about Lake Koocanusa
Answers come straight from the public measurements and survey records on this page.
Lake Details
Surface area
46,500 acres
Public access
Yes
Listed by Montana DEQ
No bathymetric survey has been published for Lake Koocanusa, so its depth is not documented here.
Recreation & Access
- Souse Gulch Recreation AreaPublic
- Dunn CreekPublic
- RipleyPublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Stays via Stay22
Stays near Lake Koocanusa
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Fishing Lake Koocanusa
Water Quality Trend: Improving
Based on 9 years of monitoring (2013–2023).
Water clarity
Improving5 years
Phosphorus
Improving-0.3 µg/L/yr4 years
Chlorophyll-a
Improving-0.23 µg/L/yr4 years
Reservoir Info (USACE NID)
Lake Koocanusa is a man-made reservoir impounded by the Libby Dam (completed 1973), built primarily for hydroelectric on the Kootenai River; gravity-type dam, 422 ft tall and 2,890 ft long.
- Surface area
- 46,500 ac
- Normal storage
- 5,809,000 ac-ft
- Max storage
- 6,027,000 ac-ft
- Drainage area
- 8,985 sq mi
- Hazard class
- High
- Owner
- USACE - Seattle District
All listed purposes: Hydroelectric;Recreation;Flood Risk Reduction.
Source: USACE National Inventory of Dams, NID ID MT00652 · Operator website
EPA Impairment Status
Lake Koocanusa 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 MT76D003_010 · Official waterbody report
Nearby Lakes in Lincoln County
Frequently asked questions about Lake Koocanusa
Where is Lake Koocanusa?
- Lake Koocanusa is a lake in Lincoln County, Montana. Its water-quality measurements are published by Montana DEQ and shown on this page as reported.
What is the water quality of Lake Koocanusa?
- Lake Koocanusa in Lincoln County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Water clarity: 14.8 ft visibility. Surface area: 46500 acres.
Is Lake Koocanusa safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Lake Koocanusa is listed impaired under Clean Water Act §303(d) (IR 5) for hydrologic alteration, metals (other than mercury). Check local advisories before you swim.
Are there boat ramps on Lake Koocanusa?
- Lake Koocanusa has 3 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Souse Gulch Recreation Area, Dunn Creek, Ripley. Public access is listed as yes.
Can I fish at Lake Koocanusa?
- Lake Koocanusa has 3 documented boat landings in the USGS National Boat Ramps inventory. Seasons, size and bag limits, and whether you need a license are set by Montana's fish and wildlife agency, so check its current regulations before you go.
Explore Lake Koocanusa across LakeQuality
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Where this data comes from
33 measurements from 1 monitoring station, 2013–2018. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Montana DEQ WQPB
MDEQ_WQ_WQX33 samples · 2013–2018
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
Named boat-access points from the USGS National Boat Ramps inventory