Diamond Lake
Poweshiek County, Iowa93 acresView mapDirections
Iowa DNR publishes the water-quality measurements for Diamond Lake. Here is what has been measured.
- Hypereutrophic
- Impaired · Algal growth
- 29 ft max depth
Diamond Lake in Poweshiek County, Iowa has water-quality measurements published by Iowa DNR, shown here as reported. Water clarity: 1.6 ft visibility. Phosphorus: 82.5 µg/L. Chlorophyll-a (algae): 44.6 µg/L. Surface area: 93 acres.
Free interactive depth-contour map of Diamond Lake, reaching a maximum surveyed depth of 16 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
Diamond Lake is a 93-acre highly nutrient-loaded hypereutrophic lake in Poweshiek County, Iowa with an estimated average depth of about 29 feet. Overall water quality is declining over recent sampling years. Compared to the 1 other monitored lakes in Poweshiek County, Diamond Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-08-28. 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. Diamond Lake is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth. Check local advisories before you swim.
Whether Diamond 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 Diamond Lake
These are the measurements Iowa DNR publishes for Diamond Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Very murky, less than 1.6 ft of visibility. Phosphorus 82.5 µg/L. Chlorophyll-a 44.6 µg/L. TSI 69.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 1.6ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 82.5µg/L | Poor GoodFairPoor |
| Chlorophyll-a (algae) | 44.6µg/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. | 69 | Hypereutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Very high nutrients, dense algae, poor clarity
Ask about Diamond Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
Diamond Lake typically sits around 68°F at the surface in September. Summer average is about 77°F. Usually warmest in Aug (about 79°F) and coldest in Sep (about 68°F). These are typical values by month — not a live reading.
Column height runs from this lake’s coldest month (68°F) to its warmest (79°F).
Typical surface water temperature by month, from 29 field readings (2010–2023) via the EPA Water Quality Portal. See the full Diamond Lake water temperature breakdown →
Lake Details
Max depth
~29 ft est.
Surface area
93 acres
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Iowa has not published bathymetry for Diamond Lake.
Lake Size & Watershed
Watershed (drainage) area
6 sq mi
HydroLAKES / HydroSHEDS
Shoreline
2.6 mi
HydroLAKES / HydroSHEDS
Elevation
860 ft
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Stays via Stay22
Stays near Diamond Lake
Compare lakeside cabins, hotels, and vacation rentals near Diamond Lake in Poweshiek 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 Diamond Lake are listed below.
Top-rated places to stay near Diamond Lake
Prices for Oct 30–Nov 1, 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 Diamond Lake’s readings
Email me when new measurements post for this lake. Updates stay free.
Or email
Fishing Diamond Lake
Water Quality Trend: Worsening
Based on 5 years of monitoring (2020–2024).
Water clarity
Improving5 years
Phosphorus
Worsening+48.3 µg/L/yr4 years
Chlorophyll-a
Worsening+10.59 µg/L/yr5 years
Reservoir Info (USACE NID)
Diamond Lake is a man-made reservoir impounded by the Diamond Lake Dam (Poweshiek Co) (completed 1952), built primarily for water supply on the TR-MOON CREEK; earth-type dam, 40 ft tall and 900 ft long.
- Surface area
- 93 ac
- Normal storage
- 780 ac-ft
- Max storage
- 1,100 ac-ft
- Drainage area
- 4.42 sq mi
- Hazard class
- Significant
- Owner
- Montezuma Water Works
All listed purposes: Water Supply;Recreation.
Source: USACE National Inventory of Dams, NID ID IA01368 · Operator website · Matched by proximity (0.21 km)
EPA Impairment Status
Diamond Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Designated uses not supporting: Aquatic life.
Source: EPA ATTAINS assessment unit IA 03-NSK-861 · Official waterbody report
Regulated Pollution Sources Nearby
2 EPA-permitted facilities operate within about 5 miles of Diamond Lake under Clean Water Act (NPDES) discharge permits, including 1 classified by the EPA as major discharger. 2 facilities have a reported Clean Water Act violation in the past three years.
- Montezuma City Of StpMontezuma · 1.5 mi away · NPDES IA0036935MajorRecent violation
- Montezuma, City Of WtpMontezuma · 0.3 mi away · NPDES IA0084310Recent 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.
Nearby Lakes in Poweshiek County
Frequently asked questions about Diamond Lake
Where is Diamond Lake?
- Diamond Lake is a lake in Poweshiek County, Iowa. Its water-quality measurements are published by Iowa DNR and shown on this page as reported.
What is the water quality of Diamond Lake?
- Diamond Lake in Poweshiek County, Iowa has water-quality measurements published by Iowa DNR, shown here as reported. Water clarity: 1.6 ft visibility. Phosphorus: 82.5 µg/L. Chlorophyll-a (algae): 44.6 µg/L. Surface area: 93 acres.
Is Diamond Lake safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Diamond Lake is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth. Check local advisories before you swim.
What is the water temperature of Diamond Lake?
- Diamond Lake typically sits around 68°F at the surface in September, from 29 EPA Water Quality Portal field readings (2010–2023). That is a typical value for the month, not a live reading.
How deep is Diamond Lake?
- No published bathymetric survey covers Diamond Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 29 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate. The lake covers 93 acres.
Explore Diamond Lake across LakeQuality
More lakes in Iowa
Other Lakes in Iowa
Hypereutrophic Lakes in Iowa
Where this data comes from
62 measurements from 1 monitoring station, 2020–2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Iowa DNR Surface Water Monitoring Data
21IOWA_WQX62 samples · 2020–2024
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
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)