Monona County Lake Quality
Iowa, 4 lakes, average grade D (Nutrient-rich)
Monona County has 4 monitored lakes — a thin enough sample that one or two stressed lakes can move the county average noticeably. Use the per-lake pages below for what actually matters. 2 of those lakes carry an A–F grade. The county-level summary describes that subset, not every water body listed. On average, Monona County lakes grade D, with phosphorus loading and reduced clarity showing up across most of the dataset.
Most of the county's monitored lakes are shallow — under 50 feet — which means wind keeps the water column mixed all summer and sediment-bound phosphorus releases back into the surface layer whenever the bottom is disturbed. Oldham Lake (C) is the cleanest in the county.
Where to stay near Monona County
Top-rated places to stay near Monona County
Prices for Sep 11–13, 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 trip to Monona County?
Get lake water-quality report cards plus the best cabin & lodging deals near Monona County — straight to your inbox. No spam, unsubscribe anytime.
Quick Answers for Monona County
Planning a trip? Check special fishing regulations for Oldham Lake — or check the official Iowa fishing regulations.
All Lakes in Monona County
4 lakes · 2 graded, 2 not rated2 graded lakes · 2 not rated, hidden
| # | Lake | Grade | Clarity | Phosphorus | Algae | Trophic | Access |
|---|---|---|---|---|---|---|---|
| 1 | COldham LakeMonona | C | 5.1 ft | 46 µg/L | 8.5 µg/L | No recent samples | Not recorded |
| 2 | FBlue LakeMonona | F | 0.9 ft | 194.5 µg/L | 37 µg/L | No recent samples | ✓ Public |
| NRBlue Lake BeachMonona | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded | |
| NRBadger LakeMonona | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded |
NR = not rated: there is not enough recent sampling to support a letter. That is EPA Integrated Reporting Category 3 (insufficient information), not a failing grade. How grading works.
Lake Match
Which lake suits how you actually use one?
Six questions — swimming, fishing, quiet, cabins, drive time — and a shortlist of graded lakes that fit, with the water quality behind each one.
Other Counties in Iowa
See how Monona County stacks up in the national county water-quality rankings, or browse the full directory of graded lakes across all United States (49 states), Canada (Ontario, Québec, Nova Scotia, British Columbia and Newfoundland and Labrador), and 38 other country hubs..
Source: EPA National Aquatic Resource Surveys, 2026.