Black Hawk Lake is a 920-acre highly nutrient-loaded hypereutrophic lake in Sac County, Iowa. Water quality is graded F based on 2 sampling years through 2024. Overall water quality is improving over recent sampling years. Limited fish-species data available. Public access is available with 3 public boat landings. Compared to the 1 other monitored lakes in Sac County, Black Hawk Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-09-04. Grade methodology: LakeGrade methodology.
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Black Hawk Lake Water Quality Report Card (2026)
A printable one-page report card for Black Hawk Lake: grade, clarity, phosphorus, algae, multi-year trend, depth, and fish — with sources.
Swimming Safety
Based on E. coli testing, Black Hawk Lake frequently exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 13.4% of 202 samples across 7 seasons.
This is a historical summary of past testing, not a current condition — bacteria levels change quickly, especially after rain. Always check Iowa’s current beach and health-department advisories before swimming.
Water Quality Grade: F, Very Poor
Very murky, less than 0.9 ft of visibility. Chlorophyll-a: 54.1 µg/L. Trophic State Index: 74.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 0.9 ft | F |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 54.1 µg/L | F |
| Trophic State Index (TSI) | 74 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 920 acres |
| Public Access | Yes |
Where to stay near Black Hawk Lake
Compare lakeside cabins, hotels, and vacation rentals near Black Hawk Lake in Sac County, Iowa — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water.
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Fishing Black Hawk Lake
Water Quality Trend: ↑ Improving
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.016 m/yr | 5 |
| Chlorophyll-a | ↑ Improving | -4.99 µg/L/yr | 5 |
Recreation & Access
Current Conditions
Latest reading Jul 6, 2026, 5:15 AM from USGS gauge 05482315. See live data ↗ — readings update every 15–60 minutes; this page refreshes on each build.
Water Temperature
Black Hawk Lake averages about 78°F at the surface in summer, usually warmest in Jul (about 80°F) and coldest in May (about 65°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 60 field readings (2010–2023) via the EPA Water Quality Portal. See the full Black Hawk Lake water temperature breakdown →
Location & Depth Map
Free interactive depth-contour map of Black Hawk Lake, reaching a maximum surveyed depth of 10 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
County Ranking
Ranked #2 of 2 lakes in Sac County
Cleaner Lakes Within 30 Miles
Black Hawk Lake holds Grade F. 5 nearby lakes hold higher grades.
- CArrowhead LakeSac County · 1.2 mi · Two grade letters higher (F → C)
- CNewell Pit LakeBuena Vista County · 22.1 mi · Two grade letters higher (F → C)
- DMoorehead LakeIda County · 23.5 mi · Higher grade (F → D)
- DStorm Lake Max DepthBuena Vista County · 24.2 mi · Higher grade (F → D)
- DYellow Smoke LakeCrawford County · 24.3 mi · Higher grade (F → D)
Nearby Lakes in Sac County
Explore Black Hawk Lake across LakeGrade
More Iowa Lake Rankings
Other Grade F Lakes in Iowa
Hypereutrophic Lakes in Iowa
More records, surveys & data sources
Reservoir Info (USACE NID)
Black Hawk Lake is a man-made reservoir impounded by the Black Hawk Lake Dam (completed 1916), built primarily for flood risk reduction on the Wall Lake Outlet; earth-type dam, 12 ft tall.
- Surface area
- 920 ac
- Normal storage
- 5,520 ac-ft
- Max storage
- 10,500 ac-ft
- Drainage area
- 23.4 sq mi
- Hazard class
- Significant
- Owner
- Iowa Department of Natural Resources
All listed purposes: Flood Risk Reduction;Other;Recreation.
Source: USACE National Inventory of Dams, NID ID IA03524 · Operator website
EPA Impairment Status
Black Hawk Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit IA 04-RAC-1134 · Official waterbody report
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Data Sources
Water quality data from the EPA Water Quality Portal
Impairment status from EPA ATTAINS 303(d) database
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grading methodology based on Metropolitan Council standards
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2024-09-04
Monitoring stations: 1