Fairfax County, Virginia · Water Quality Report Card
Lake Thoreau
Lake Thoreau in Fairfax County, Virginia is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 10 ft visibility.
- Clarity
- 10 ft
- Type
- Mesotrophic
- Trend
- ↑ Improving
Lake Thoreau is a mesotrophic lake in Fairfax County, Virginia. Overall water quality is improving over recent sampling years.
Source: EPA Water Quality Portal sampling records, Virginia DEQ, last sampled 2022-07-11. Grade methodology: LakeGrade methodology.
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Swimming Safety
Not rated for swimming — there is not enough monitoring data to judge this lake
Water Quality Grade: Not rated
Good clarity, visible to about 10 ft. Trophic State Index: 44.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 10 ft | |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 44 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay near Lake Thoreau
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Fishing Lake Thoreau
Water Quality Trend: ↑ Improving
Based on 10 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +2.578 m/yr | 10 |
Location
Nearby Lakes in Fairfax County
Explore Lake Thoreau across LakeGrade
6 mi away · 76 ft
Species activity outlook by water temperature and season
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Moderate nutrients, good water quality
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Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Where this data comes from
3 measurements from 1 monitoring station, 2019–2022. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- North American Lake Management Society
NALMS3 samples · 2019–2022
Monitoring station identifiers (1)
Grade thresholds come from the U.S. EPA National Lakes Assessment 2022 Technical Support Document (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated.
Data Sources
Water quality data from the EPA Water Quality Portal
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion
Most recent sample: 2022-07-11
Monitoring stations: 1