Thayer Lake is a 14-acre productive, nutrient-rich eutrophic lake in Union 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 5 other monitored lakes in Union County, Thayer Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-09-15. Grade methodology: LakeGrade methodology.
Watch Thayer 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 3.5 ft. Chlorophyll-a: 34.3 µg/L. Trophic State Index: 62.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 3.5 ft | D |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 34.3 µg/L | F |
| Trophic State Index (TSI) | 62 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 14 acres |
Where to stay near Thayer Lake
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Fishing Thayer Lake
Water Quality Trend: ↓ Declining
Based on 4 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.02 m/yr | 4 |
| Chlorophyll-a | ↓ Declining | +34.4 µg/L/yr | 4 |
Water Temperature
Thayer Lake averages about 79°F at the surface in summer, usually warmest in Jul (about 82°F) and coldest in May (about 70°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 32 field readings (2010–2023) via the EPA Water Quality Portal. See the full Thayer Lake water temperature breakdown →
Location & Depth Map
Free interactive depth-contour map of Thayer Lake, reaching a maximum surveyed depth of 12 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
County Ranking
Ranked #2 of 6 lakes in Union County
Nearby Lakes in Union County
Explore Thayer 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)
Thayer Lake is a man-made reservoir impounded by the Thayer Lake Dam (completed 1930), built primarily for recreation on the TR-FOUR MILE CREEK; earth-type dam, 30 ft tall and 590 ft long.
- Surface area
- 14.1 ac
- Normal storage
- 91 ac-ft
- Max storage
- 222 ac-ft
- Drainage area
- 0.79 sq mi
- Hazard class
- Low
- Owner
- Iowa Department of Natural Resources
Source: USACE National Inventory of Dams, NID ID IA01377 · Operator website
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Thayer Lake under Clean Water Act (NPDES) discharge permits.
- Thayer City Of StpThayer · 1.3 mi away · NPDES IA0079936
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: 2024-09-15
Monitoring stations: 1