North Twin Lake is a highly nutrient-loaded hypereutrophic lake in Calhoun 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 2 public boat landings.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-09-04. Grade methodology: LakeGrade methodology.
Watch North Twin Lake water quality
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Swimming Safety
Based on E. coli testing, North Twin Lake rarely exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 4.8% of 252 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 1.1 ft of visibility. Chlorophyll-a: 31.4 µg/L. Trophic State Index: 70.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.1 ft | F |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 31.4 µg/L | F |
| Trophic State Index (TSI) | 70 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Where to stay near North Twin Lake
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Fishing North Twin Lake
Water Quality Trend: ↑ Improving
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.03 m/yr | 5 |
| Chlorophyll-a | ↑ Improving | -4.21 µg/L/yr | 5 |
Recreation & Access
Water Temperature
North Twin Lake averages about 77°F at the surface in summer, usually warmest in Jul (about 80°F) and coldest in May (about 66°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 178 field readings (2010–2023) via the EPA Water Quality Portal. See the full North Twin Lake water temperature breakdown →
Location & Depth Map
Free interactive depth-contour map of North Twin Lake, reaching a maximum surveyed depth of 11 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
County Ranking
Ranked #1 of 1 lakes in Calhoun County
Cleaner Lakes Within 30 Miles
North Twin Lake holds Grade F. 2 nearby lakes hold higher grades.
- CArrowhead LakeSac County · 24.5 mi · Two grade letters higher (F → C)
- CNewell Pit LakeBuena Vista County · 22.7 mi · Two grade letters higher (F → C)
Nearby Lakes in Calhoun County
Explore North Twin Lake across LakeGrade
More Iowa Lake Rankings
Other Grade F Lakes in Iowa
Hypereutrophic Lakes in Iowa
More records, surveys & data sources
EPA Impairment Status
North Twin Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 4A).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit IA 04-RAC-1167 · Official waterbody report
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Regulated Pollution Sources Nearby
2 EPA-permitted facilities operate within about 5 miles of North Twin 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.
- Rockwell City, City Of StpRockwell City · 4.9 mi away · NPDES IA0033138MajorRecent violation
- Twin Lakes Utilities StpRockwell City · 0.7 mi away · NPDES IA0070114Recent 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: 2024-09-04
Monitoring stations: 1