Blacktail Dam Lake
Williams County, North Dakota158 acresView mapDirections
Nutrients and algae sit mid-range for this region.
↓ down from B at the last regrade
- Eutrophic
- Impaired · Nutrients (phosphorus, nitrogen)
- #2 of 6 in the county
- 8 ft max depth
- Public access
Blacktail Dam Lake in Williams County, North Dakota has a water quality grade of C (Fair). Water clarity: 7.2 ft visibility. Phosphorus: 53 µg/L. Chlorophyll-a (algae): 8.6 µg/L. Surface area: 158 acres.
- Clarity
- 7.2 ft
- Typical in September
- 51°F
- Max depth
- ~8 ft est.
- Area
- 158 acres
- Type
- Eutrophic
- Phosphorus
- 53 µg/L
- Chlorophyll-a
- 8.6 µg/L
- Trend
- ↑ Improving
- Public access
- Yes
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Blacktail Dam Lake is a 158-acre productive, nutrient-rich eutrophic lake in Williams County, North Dakota with an estimated average depth of about 8 feet. Water quality is graded C based on 3 sampling years through 2025. Overall water quality is improving over recent sampling years. Limited fish-species data available. Public access is available with 1 public boat landing. Compared to the 5 other monitored lakes in Williams County, Blacktail Dam Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, NDDEQ, last sampled 2025-10-29. Grade methodology: LakeQuality methodology.
What Blacktail Dam Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch1 documented ramp
Swimming Safety
Grade onlyModerate
Based on clarity and algae grades from past sampling — we do not hold live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
The C is nutrients and algae, not a swimming clearance. Blacktail Dam Lake is listed impaired under Clean Water Act §303(d) (IR 4A) for nutrients (phosphorus, nitrogen), oxygen depletion (low DO). Check local advisories before you swim.
Water Quality Grade: C (Fair)
Moderate clarity, visible to about 7.2 ft. Phosphorus 53 µg/L. Chlorophyll-a 8.6 µg/L. TSI 54. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 7.2 ft | Not graded |
| Phosphorus | 53 µg/L | Good |
| Chlorophyll-a (Algae) | 8.6 µg/L | Good |
| Trophic State Index (TSI) | 54 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Water Temperature
Blacktail Dam Lake typically sits around 51°F at the surface in September. Summer average is about 62°F. Usually warmest in Jul (about 63°F) and coldest in Feb (about 38°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 167 field readings (2014–2025) via the EPA Water Quality Portal. See the full Blacktail Dam Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~8 ft est. |
| Surface Area | 158 acres |
| Public Access | Yes |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. North Dakota has not published bathymetry for Blacktail Dam Lake.
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Recreation & Access
- Blacktail DamPublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Blacktail Dam Lake
Water Quality Trend: ↑ Improving
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.375 m/yr | 3 |
| Phosphorus | ↑ Improving | -6.25 µg/L/yr | 3 |
| Chlorophyll-a | ↑ Improving | -2.02 µg/L/yr | 5 |
Reservoir Info (USACE NID)
Blacktail Dam Lake is a man-made reservoir impounded by the Blacktail Dam (completed 1959), built primarily for recreation on the Blacktail Creek; earth-type dam, 63 ft tall and 2,250 ft long.
- Surface area
- 158 ac
- Normal storage
- 2,668.4 ac-ft
- Max storage
- 5,317.8 ac-ft
- Drainage area
- 27.7 sq mi
- Hazard class
- Significant
- Owner
- ND Game & Fish
All listed purposes: Recreation;Fish and Wildlife Pond.
Source: USACE National Inventory of Dams, NID ID ND00033 · Operator website
EPA Impairment Status
Blacktail Dam Lake is officially listed as impaired under Clean Water Act §303(d) in the 2022 EPA reporting cycle (IR category 4A).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit ND-10110102-003-L_00 · Official waterbody report
County Ranking
Ranked #2 of 6 lakes in Williams County
Nearby Lakes in Williams County
Frequently asked questions about Blacktail Dam Lake
Where is Blacktail Dam Lake?
- Blacktail Dam Lake is a lake in Williams County, North Dakota. LakeQuality grades it C for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Blacktail Dam Lake?
- Blacktail Dam Lake in Williams County, North Dakota has a water quality grade of C (Fair). Water clarity: 7.2 ft visibility. Phosphorus: 53 µg/L. Chlorophyll-a (algae): 8.6 µg/L. Surface area: 158 acres.
Is Blacktail Dam Lake safe to swim in?
- The C is nutrients and algae, not a swimming clearance. Blacktail Dam Lake is listed impaired under Clean Water Act §303(d) (IR 4A) for nutrients (phosphorus, nitrogen), oxygen depletion (low DO). Check local advisories before you swim.
What is the water temperature of Blacktail Dam Lake?
- Blacktail Dam Lake typically sits around 51°F at the surface in September, from 167 EPA Water Quality Portal field readings (2014–2025). That is a typical value for the month, not a live reading.
How deep is Blacktail Dam Lake?
- No published bathymetric survey covers Blacktail Dam Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 8 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate. The lake covers 158 acres.
Are there boat ramps on Blacktail Dam Lake?
- Blacktail Dam Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include Blacktail Dam. Public access is listed as yes.
Explore Blacktail Dam Lake across LakeQuality
Species activity outlook by water temperature and season
Where to stay at North Dakota's top lakes, each with its grade
High nutrients, frequent algae, reduced clarity
North Dakota's cleanest and most-polluted lakes, ranked
More North Dakota Lake Rankings
Other Grade C Lakes in North Dakota
Eutrophic Lakes in North Dakota
Where this data comes from
165 measurements from 1 monitoring station, 2020–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- North Dakota Department Of Environmental Quality
21NDHDWQ_WQX165 samples · 2020–2025
Analytical methods
- Phosphorus by Colorimetry
- 447.0 ~ EPA; Chlorophyll a and b in phytoplankton by HPLC/UV
- 4500 NO3 E ~ Nitrate in Water- Cadmium Reduction
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
Impairment status from EPA ATTAINS 303(d) database
Lake size & watershed from HydroLAKES (HydroSHEDS)
Depth, fish, and access data from North Dakota Game & Fish
Named boat-access points from the USGS National Boat Ramps inventory