Washington County Lake Quality
Missouri, 2 lakes, average grade A (Excellent)
Washington County has 2 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. 1 of those lake carry an A–F grade. The county-level summary describes that subset, not every water body listed. The county's average grade is A — among the cleanest in Missouri. 1 lakes pull an A on their own, and almost none fall into the D or F brackets.
The county's lakes lean shallow, which makes the grades sensitive to wind, sediment-resuspension, and any nutrient inputs from the watershed. The cleanest lake on the books is Lake Shayne-Terre du Lac Lakes (A); the most-stressed is Lake Shayne-Terre du Lac Lakes (A).
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Quick Answers for Washington County
Planning a trip? Check special fishing regulations for Lake Shayne-Terre du Lac Lakes — or check the official Missouri fishing regulations.
All Lakes in Washington County
2 lakes · 1 graded, 1 not rated1 graded lake · 1 not rated, hidden
| # | Lake | Grade | Clarity | Phosphorus | Algae | Trophic | Access |
|---|---|---|---|---|---|---|---|
| 1 | ALake Shayne-Terre du Lac LakesWashington | A | 11.1 ft | 5.2 µg/L | 1.2 µg/L | No recent samples | Not recorded |
| NRSunnen LakeWashington | NR | 1 ft | 61 µg/L | 14.3 µg/L | 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 Missouri
See how Washington County stacks up in the national county water-quality rankings, or browse the full directory of graded lakes across all 49 states and 2 Canadian provinces.
Source: EPA National Aquatic Resource Surveys, 2026.