Nelson Lake is a 11-acre productive, nutrient-rich eutrophic lake in Crawford County, Iowa. Water quality is graded D based on 2 sampling years through 2024. Overall water quality is declining over recent sampling years. Compared to the 1 other monitored lakes in Crawford County, Nelson Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-09-03. Grade methodology: LakeGrade methodology.
Watch Nelson Lake water quality
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Swimming Safety
Swimming not recommended, poor water quality with high algae risk
Water Quality Grade: D, Poor
Murky, only visible to about 3.8 ft. Chlorophyll-a: 30.6 µg/L. Trophic State Index: 61.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 3.8 ft | D |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 30.6 µg/L | F |
| Trophic State Index (TSI) | 61 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 11 acres |
Where to stay near Nelson Lake
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Fishing Nelson Lake
Water Quality Trend: ↓ Declining
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.312 m/yr | 5 |
| Chlorophyll-a | ↓ Declining | +5.45 µg/L/yr | 5 |
Water Temperature
Nelson Lake averages about 78°F at the surface in summer, usually warmest in Jul (about 78°F) and coldest in May (about 63°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 29 field readings (2010–2023) via the EPA Water Quality Portal. See the full Nelson Lake water temperature breakdown →
Location & Depth Map
Free interactive depth-contour map of Nelson Lake, reaching a maximum surveyed depth of 22 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
County Ranking
Ranked #1 of 2 lakes in Crawford County
Cleaner Lakes Within 30 Miles
Nelson Lake holds Grade D. 3 nearby lakes hold higher grades.
- COldham LakeMonona County · 11.4 mi · Higher grade (D → C)
- CWillow LakeHarrison County · 15.5 mi · Higher grade (D → C)
- DMoorehead LakeIda County · 29.6 mi · Higher overall water quality score
Nearby Lakes in Crawford County
Explore Nelson Lake across LakeGrade
More Iowa Lake Rankings
Other Grade D Lakes in Iowa
Eutrophic Lakes in Iowa
More records, surveys & data sources
Reservoir Info (USACE NID)
Nelson Lake is a man-made reservoir impounded by the Iowa Noname8 (completed 1964), built primarily for recreation on the TR-BOYER RIVER; earth-type dam, 38 ft tall and 522 ft long.
- Surface area
- 11 ac
- Normal storage
- 111 ac-ft
- Max storage
- 220 ac-ft
- Drainage area
- 0.97 sq mi
- Hazard class
- Low
- Owner
- Crawford County Conservation Board
Source: USACE National Inventory of Dams, NID ID IA00233 · Operator website · Matched by proximity (0.08 km)
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Nelson Lake under Clean Water Act (NPDES) discharge permits. 1 facility has a reported Clean Water Act violation in the past three years.
- Dow City, City Of StpDow City · 4.9 mi away · NPDES IA0034371Recent 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)
Grading methodology based on Metropolitan Council standards
Most recent sample: 2024-09-03
Monitoring stations: 1