Lake Keomah is a 82-acre highly nutrient-loaded hypereutrophic lake in Mahaska County, Iowa. Water quality is graded F based on 2 sampling years through 2024. Overall water quality is improving over recent sampling years. Limited fish-species data available. Public access is available with 1 public boat landing. Compared to the 2 other monitored lakes in Mahaska County, Lake Keomah ranks #3 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-05-22. Grade methodology: LakeGrade methodology.
Watch Lake Keomah water quality
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Swimming Safety
Based on E. coli testing, Lake Keomah frequently exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 22.3% of 112 samples across 7 seasons.
This is a historical summary of past testing, not a current condition — bacteria levels change quickly, especially after rain. Always check Iowa’s current beach and health-department advisories before swimming.
Water Quality Grade: F, Very Poor
Very murky, less than 1.5 ft of visibility. Chlorophyll-a: 63 µg/L. Trophic State Index: 71.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.5 ft | F |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 63 µg/L | F |
| Trophic State Index (TSI) | 71 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 82 acres |
| Public Access | Yes |
Where to stay near Lake Keomah
Compare lakeside cabins, hotels, and vacation rentals near Lake Keomah in Mahaska 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.
Top-rated places to stay near Lake Keomah
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Fishing Lake Keomah
Water Quality Trend: ↑ Improving
Based on 4 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.006 m/yr | 4 |
| Chlorophyll-a | ↑ Improving | -3.56 µg/L/yr | 4 |
Recreation & Access
Water Temperature
Lake Keomah averages about 82°F at the surface in summer, usually warmest in Jul (about 84°F) and coldest in Sep (about 69°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 109 field readings (2010–2023) via the EPA Water Quality Portal. See the full Lake Keomah water temperature breakdown →
Location & Depth Map
Free interactive depth-contour map of Lake Keomah, reaching a maximum surveyed depth of 18 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
County Ranking
Ranked #3 of 3 lakes in Mahaska County
Cleaner Lakes Within 30 Miles
Lake Keomah holds Grade F. 2 nearby lakes hold higher grades.
- BYenruogis PondKeokuk County · 18.4 mi · Two grade letters higher (F → B)
- DHawthorn LakeMahaska County · 13.2 mi · Higher grade (F → D)
Nearby Lakes in Mahaska County
Explore Lake Keomah across LakeGrade
More Iowa Lake Rankings
Other Grade F Lakes in Iowa
Hypereutrophic Lakes in Iowa
More records, surveys & data sources
Reservoir Info (USACE NID)
Lake Keomah is a man-made reservoir impounded by the Lake Keomah Dam (completed 1935), built primarily for recreation on the TR-SOUTH SKUNK RIVER; earth-type dam, 42 ft tall and 1,100 ft long.
- Surface area
- 82 ac
- Normal storage
- 802 ac-ft
- Max storage
- 1,710 ac-ft
- Drainage area
- 3.1 sq mi
- Hazard class
- Significant
- Owner
- Iowa Department of Natural Resources
Source: USACE National Inventory of Dams, NID ID IA00642 · Operator website
EPA Impairment Status
Lake Keomah is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit IA 03-SSK-930 · Official waterbody report
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Regulated Pollution Sources Nearby
3 EPA-permitted facilities operate within about 5 miles of Lake Keomah under Clean Water Act (NPDES) discharge permits, including 1 classified by the EPA as major discharger. 3 facilities have a reported Clean Water Act violation in the past three years.
- Oskaloosa City Of Stp (Northeast)Oskaloosa · 4.7 mi away · NPDES IA0038539MajorRecent violation
- Keomah Village City Of StpOskaloosa · 0.2 mi away · NPDES IA0062073Recent violation
- University Park City Of StpUniversity Park · 3.9 mi away · NPDES IA0047741Recent 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
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grading methodology based on Metropolitan Council standards
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2024-05-22
Monitoring stations: 1