Õisu Järv
Õisu Järv in Viljandi, Estonia has official water-quality measurements, not a LakeQuality A–F grade. Water clarity: 1.5 m visibility. Phosphorus: 39.5 µg/L. Chlorophyll-a (algae): 17 µg/L. Maximum depth: 14.1 ft. Surface area: 471.7 acres.
- Clarity
- 1.5 m
- Max depth
- 4.3 m
- Area
- 191 ha
Why Õisu Järv has no A–F grade
Õisu Järv in Estonia is published with the authority's own measurements, not a LakeQuality letter. official authority class; not a LakeQuality A–F letter Sampling here has recorded water clarity, phosphorus and algae (chlorophyll-a).
A letter on the LakeQuality scale would be an invention, not an official class. Everything below that has been measured is still reported on this page.
Monitoring: the Estonian Environment Agency (Keskkonnaagentuur).
Õisu Järv is a 472-acre lake in Viljandi County, Estonia with a maximum depth of 14 feet.
Source: EPA Water Quality Portal sampling records, your state environmental agency, last sampled 2025-09-15. Grade methodology: LakeGrade methodology.
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Swimming Safety
Not ratedLimited data
Not rated for swimming — there is not enough monitoring data to judge this lake
Water quality measurements for Õisu Järv
These readings exist but do not meet the evidence bar for an A–F grade — see why.
Murky, only visible to about 4.8 ft. Phosphorus level: 39.5 µg/L. Chlorophyll-a: 17 µg/L. Trophic State Index: 57.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.45 m | Not graded |
| Phosphorus | 39.5 µg/L | Not graded |
| Chlorophyll-a (Algae) | 17 µg/L | Not graded |
| Trophic State Index (TSI) | 57 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 4.3 m |
| Surface Area | 191 ha |
Where to stay near Õisu Järv
Compare lakeside cabins, hotels, and vacation rentals near Õisu Järv in Viljandi, Estonia — 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.
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Fishing Õisu Järv
Frequently asked questions about Õisu Järv
What is the water quality of Õisu Järv?
- Õisu Järv in Viljandi, Estonia has official water-quality measurements, not a LakeQuality A–F grade. Water clarity: 1.5 m visibility. Phosphorus: 39.5 µg/L. Chlorophyll-a (algae): 17 µg/L. Maximum depth: 14.1 ft. Surface area: 471.7 acres.
Is Õisu Järv safe to swim in?
- Õisu Järv has official nutrient and clarity measurements, not a bacteria swim class and not a safety clearance. Algal blooms, currents, and depth are not covered. Always check local notices before swimming.
How deep is Õisu Järv?
- Õisu Järv has a maximum depth of 14.1 feet. The lake covers 471.7 acres.
Explore Õisu Järv across LakeGrade
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Where this data comes from
36 measurements from 2 monitoring stations, 2019–2025. Retrieved from KESE / Estonian Environment Agency (not the EPA Water Quality Portal) on 2026-08-16.
Collected and reported by
- Estonian Environment Agency (Keskkonnaagentuur)
kese-ee36 samples · 2019–2025
Analytical methods
- KESE I-level monitoring
Monitoring station identifiers (2)
SJA1938000SJA1938003
Authority classifications are stored separately from LakeQuality A–F letters and are never blended. See methodology and data sources.
Data Sources
Water quality data from the EPA Water Quality Portal
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion
Most recent sample: 2025-09-15
Monitoring stations: 2