Thayer Lake
Union County, Iowa14 acresView mapDirections
Iowa DNR publishes the water-quality measurements for Thayer Lake. Here is what has been measured.
- Hypereutrophic
Thayer Lake in Union County, Iowa has water-quality measurements published by Iowa DNR, shown here as reported. Water clarity: 3.5 ft visibility. Phosphorus: 68 µg/L. Chlorophyll-a (algae): 34.3 µg/L. Surface area: 14.1 acres.
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. 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. Methodology: how these readings are compared.
Swimming conditions
Readings onlyAlgae risk
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Whether Thayer Lake is safe to swim in today is a call for the Iowa Department of Natural Resources and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Thayer Lake
These are the measurements Iowa DNR publishes for Thayer Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Murky, only visible to about 3.5 ft. Phosphorus 68 µg/L. Chlorophyll-a 34.3 µg/L. TSI 63.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 3.5ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 68µg/L | Poor GoodFairPoor |
| Chlorophyll-a (algae) | 34.3µg/L | Poor GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 63 | Hypereutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Very high nutrients, dense algae, poor clarity
Ask about Thayer Lake
Answers come straight from the public measurements and survey records on this page.
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.
Column height runs from this lake’s coldest month (70°F) to its warmest (82°F).
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
Surface area
14 acres
No bathymetric survey has been published for Thayer Lake, so its depth is not documented here.
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 readings cover nutrients and algae, not lodging. Cabins and rentals around Thayer Lake are listed below.
Top-rated places to stay near Thayer Lake
Prices for Oct 23–25, 4 adults. Change dates on the map below.
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Planning a whole trip? See where to stay at Iowa’s top lakes →
Follow Thayer Lake’s readings
Email me when new measurements post for this lake. Updates stay free.
Or email
Fishing Thayer Lake
Water Quality Trend: Stable
Based on 4 years of monitoring (2021–2024).
Water clarity
Stable4 years
Phosphorus
Improving-7.65 µg/L/yr4 years
Chlorophyll-a
Worsening+34.4 µg/L/yr4 years
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.
Nearby Lakes in Union County
Frequently asked questions about Thayer Lake
Where is Thayer Lake?
- Thayer Lake is a lake in Union County, Iowa. Its water-quality measurements are published by Iowa DNR and shown on this page as reported.
What is the water quality of Thayer Lake?
- Thayer Lake in Union County, Iowa has water-quality measurements published by Iowa DNR, shown here as reported. 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?
- Nutrient and algae readings are 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 lakes in Iowa
Other 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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.
Other records on this page
Regulated discharger data from EPA ECHO (ICIS-NPDES)