Thayer Lake
Union County, Iowa14 acresView mapDirections
Nutrients and algae run well above the regional average.
↓ down from D at the last regrade
- Hypereutrophic
- #6 of 6 in the county
Thayer Lake in Union County, Iowa has a water quality grade of F (Algae-prone). Water clarity: 3.5 ft visibility. Phosphorus: 68 µg/L. Chlorophyll-a (algae): 34.3 µg/L. Surface area: 14.1 acres.
- Clarity
- 3.5 ft
- Typical in September
- 73°F
- Area
- 14 acres
- Type
- Hypereutrophic
- Phosphorus
- 68 µg/L
- Chlorophyll-a
- 34.3 µg/L
- Trend
- → Stable
Stays via Stay22
Stays near Thayer Lake
Compare lakeside cabins, hotels, and vacation rentals near Thayer Lake in Union 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. The water-quality grade measures nutrients and algae, not lodging — cabins and rentals around Thayer Lake are listed below.
Top-rated places to stay near Thayer Lake
Prices for Sep 11–13, 4 adults. Change dates on the map below.
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Watch Thayer Lake’s letter
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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.
Thayer Lake is a 14-acre highly nutrient-loaded hypereutrophic lake in Union County, Iowa. Water quality is graded F based on 2 sampling years through 2024. Phosphorus level is improving over recent sampling years. Compared to the 5 other monitored lakes in Union County, Thayer Lake ranks #6 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-09-15. Grade methodology: LakeQuality methodology.
Swimming Safety
Grade onlyAlgae risk
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 F is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
Murky, only visible to about 3.5 ft. Phosphorus 68 µg/L. Chlorophyll-a 34.3 µg/L. TSI 63. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 3.5 ft | Not graded |
| Phosphorus | 68 µg/L | Poor |
| Chlorophyll-a (Algae) | 34.3 µg/L | Poor |
| Trophic State Index (TSI) | 63 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Water Temperature
Thayer Lake typically sits around 73°F at the surface in September. Summer average is about 79°F. 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 →
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 14 acres |
No bathymetric survey has been published for Thayer Lake, so its depth is not documented here.
Fishing Thayer Lake
Water Quality Trend: → Stable
Based on 4 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.02 m/yr | 4 |
| Phosphorus | ↑ Improving | -7.65 µg/L/yr | 4 |
| Chlorophyll-a | ↓ Declining | +34.4 µg/L/yr | 4 |
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.
County Ranking
Ranked #6 of 6 lakes in Union County
Nearby Lakes With Higher Grades
Thayer Lake holds Grade F. 2 nearby lakes hold higher nutrient-and-algae grades — useful for a swim day, not a reason to skip lodging here.
See full comparison →Nearby Lakes in Union County
Frequently asked questions about Thayer Lake
Where is Thayer Lake?
- Thayer Lake is a lake in Union County, Iowa. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Thayer Lake?
- Thayer Lake in Union County, Iowa has a water quality grade of F (Algae-prone). Water clarity: 3.5 ft visibility. Phosphorus: 68 µg/L. Chlorophyll-a (algae): 34.3 µg/L. Surface area: 14.1 acres.
Is Thayer Lake safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
What is the water temperature of Thayer Lake?
- Thayer Lake typically sits around 73°F at the surface in September, from 32 EPA Water Quality Portal field readings (2010–2023). That is a typical value for the month, not a live reading.
Explore Thayer Lake across LakeQuality
More Iowa Lake Rankings
Other Grade F Lakes in Iowa
Hypereutrophic Lakes in Iowa
Where this data comes from
48 measurements from 1 monitoring station, 2021–2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Iowa DNR Surface Water Monitoring Data
21IOWA_WQX48 samples · 2021–2024
Analytical methods
- Unknown
- Phosphorus by Colorimetry
- 445.0 ~ EPA; Chlorophyll and Pheophytin in Algae by Fluorescence
- In-Vitro Determination of Chlorophyll
- ISU modification of APHA 2130-B
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
Regulated discharger data from EPA ECHO (ICIS-NPDES)