Blue Lake
Monona County, IowaView mapDirections
Nutrients and algae run well above the regional average.
- Hypereutrophic
- Impaired · Turbidity
- #2 of 2 in the county
- 38 ft max depth
- Public access
Blue Lake in Monona County, Iowa has a water quality grade of F (Algae-prone). Water clarity: 0.9 ft visibility. Phosphorus: 194.5 µg/L. Chlorophyll-a (algae): 37 µg/L.
- Clarity
- 0.9 ft
- Typical in August
- 80°F
- Max depth
- ~38 ft est.
- Type
- Hypereutrophic
- Trend
- ↓ Declining
- Public access
- Yes
Stays via Stay22
Stays near Blue Lake
Compare lakeside cabins, hotels, and vacation rentals near Blue Lake in Monona 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 Blue Lake are listed below.
Top-rated places to stay near Blue Lake
Prices for Sep 11–13, 4 adults. Change dates on the map below.
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Free interactive depth-contour map of Blue Lake, reaching a maximum surveyed depth of 10 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Iowa DNR.
Blue Lake is a highly nutrient-loaded hypereutrophic lake in Monona County, Iowa with an estimated average depth of about 38 feet. Water quality is graded F based on 1 sampling years through 2024. Overall water quality is declining over recent sampling years. Limited fish-species data available. Public access is available. Compared to the 1 other monitored lakes in Monona County, Blue Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Iowa DNR, last sampled 2024-09-19. Grade methodology: LakeQuality methodology.
Swimming Safety
Grade onlyBacteria A
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.
Based on E. coli testing, Blue Lake never exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 0% of 5 samples across 2 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 (Algae-prone)
Very murky, less than 0.9 ft of visibility. Phosphorus 194.5 µg/L. Chlorophyll-a 37 µg/L. TSI 75. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 0.9 ft | Not graded |
| Phosphorus | 194.5 µg/L | Not graded |
| Chlorophyll-a (Algae) | 37 µg/L | Poor |
| Trophic State Index (TSI) | 75 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Water Temperature
Blue Lake typically sits around 80°F at the surface in August. Summer average is about 80°F. Usually warmest in Jul (about 83°F) and coldest in May (about 68°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 96 field readings (2010–2023) via the EPA Water Quality Portal. See the full Blue Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~38 ft est. |
| Average Depth | ~8 ft est. |
| Public Access | Yes |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Iowa has not published bathymetry for Blue Lake.
Lake Size & Watershed
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.
Fishing Blue Lake
Water Quality Trend: ↓ Declining
Based on 4 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.122 m/yr | 4 |
| Phosphorus | ↓ Declining | +79.65 µg/L/yr | 4 |
| Chlorophyll-a | ↓ Declining | +48.51 µg/L/yr | 4 |
EPA Impairment Status
Blue 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 06-WEM-1728 · Official waterbody report · Matched by proximity (1.31 km)
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Blue Lake under Clean Water Act (NPDES) discharge permits. 1 facility has a reported Clean Water Act violation in the past three years.
- Onawa City Of StpOnawa · 4.1 mi away · NPDES IA0036145Recent 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.
County Ranking
Ranked #2 of 2 lakes in Monona County
Nearby Lakes With Higher Grades
Blue 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 Monona County
Frequently asked questions about Blue Lake
Where is Blue Lake?
- Blue Lake is a lake in Monona County, Iowa. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Blue Lake?
- Blue Lake in Monona County, Iowa has a water quality grade of F (Algae-prone). Water clarity: 0.9 ft visibility. Phosphorus: 194.5 µg/L. Chlorophyll-a (algae): 37 µg/L.
Is Blue Lake safe to swim in?
- Based on E. coli testing, Blue Lake never exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 0% of 5 samples across 2 seasons. That works out to a bacteria swim-safety grade of A — a historical summary of past testing, not a current reading. Blue Lake has a water quality grade of F (Algae-prone). Bacteria levels can spike quickly after heavy rain, so always check Iowa's current beach and health-department advisories before you swim.
What is the water temperature of Blue Lake?
- Blue Lake typically sits around 80°F at the surface in August, from 96 EPA Water Quality Portal field readings (2010–2023). That is a typical value for the month, not a live reading.
How deep is Blue Lake?
- No published bathymetric survey covers Blue Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 38 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.
Explore Blue 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
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)