Qacmainstdup Lake is a 133,000-acre mesotrophic lake in Mercer County, North Dakota. Water quality is graded B based on 2 sampling years through 2020. The lake supports 1 documented fish species including Rainbow Trout. Public access is available with 2 public boat landings. Compared to the 4 other monitored lakes in Mercer County, Qacmainstdup Lake ranks #4 for water quality.
Source: EPA Water Quality Portal sampling records, North Dakota DNR, last sampled 2020-10-20. Grade methodology: LakeGrade methodology.
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Swimming Safety
Good for swimming, clear water with low algae levels
Water Quality Grade: B, Good
No clarity data. Phosphorus level: 10 µg/L. Chlorophyll-a: 11 µg/L. Trophic State Index: 46.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No data | |
| Phosphorus | 10 µg/L | A |
| Chlorophyll-a (Algae) | 11 µg/L | C |
| Trophic State Index (TSI) | 46 | Mesotrophic |
Moderate nutrients, good water quality
Lake Details
| Characteristic | Value |
|---|---|
| Average Depth | 6 ft |
| Surface Area | 133,000 acres |
| Shoreline Length | 0.5 mi |
| Public Access | Yes |
Where to stay near Qacmainstdup Lake
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Fish Species
Click a species to see every graded lake where it is documented.
Fishing Qacmainstdup Lake
Invasive & Introduced Species
Red chips are ecologically harmful invasive species. Gray chips are stocked or introduced fish that aren't a current ecological concern.
Source: USGS Nonindigenous Aquatic Species (NAS) Database. Hover a chip to see scientific name, ecological context, status, and first-observed year.
Recreation & Access
Location
County Ranking
Ranked #4 of 5 lakes in Mercer County
Cleaner Lakes Within 30 Miles
Qacmainstdup Lake holds Grade B. One nearby lake has a higher grade.
See full comparison →Nearby Lakes in Mercer County
Explore Qacmainstdup Lake across LakeGrade
Fish species and top-graded lakes across North Dakota
Fish species and best lakes in Mercer County
Species activity outlook by water temperature and season
Where to stay at North Dakota's top lakes, each with its grade
Moderate nutrients, good water quality
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Mesotrophic Lakes in North Dakota
Compare Qacmainstdup Lake With Nearby Lakes
More records, surveys & data sources
Reservoir Info (USACE NID)
Qacmainstdup Lake is a man-made reservoir impounded by the Garrison Dam (completed 1953), built primarily for hydroelectric on the MISSOURI RIVER; earth-type dam, 210 ft tall and 11,300 ft long.
- Surface area
- 133,000 ac
- Normal storage
- 18,500,000 ac-ft
- Max storage
- 26,000,000 ac-ft
- Drainage area
- 180,940 sq mi
- Hazard class
- High
- Owner
- USACE - Omaha District
All listed purposes: Hydroelectric;Irrigation;Navigation;Recreation;Flood Risk Reduction.
Source: USACE National Inventory of Dams, NID ID ND00145 · Operator website · Matched by proximity (0.09 km)
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Qacmainstdup Lake under Clean Water Act (NPDES) discharge permits. 1 facility has a reported Clean Water Act violation in the past three years.
- Garrison Dam NfhRiverdale · 0.5 mi away · NPDES ND0000388Recent violation
Facilities permitted to discharge under the Clean Water Act within ~5 miles; proximity does not by itself mean a facility affects this lake. Source: EPA ECHO (ICIS-NPDES), as of 2026-07.
Data Sources
Water quality data from the EPA Water Quality Portal
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grading methodology based on Metropolitan Council standards
Depth, fish, and access data from North Dakota Game & Fish
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2020-10-20
Monitoring stations: 1