Mann Creek Reservoir
Washington County, Idaho283 acresView mapDirections
Not enough samples to grade — no nutrient or algae measurement meets the evidence bar (at least 3 samples within 10 years). No letter yet. Here is what we do have.
- Limited data
Mann Creek Reservoir in Washington County, Idaho is not rated for water quality — its monitoring record does not meet the evidence bar. Phosphorus: 49 µg/L. Chlorophyll-a (algae): 179 µg/L. Surface area: 283 acres.
- Area
- 283 acres
- Phosphorus
- 49 µg/L
- Chlorophyll-a
- 179 µg/L
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Watch Mann Creek Reservoir’s letter
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Why Mann Creek Reservoir has no grade
We grade a lake only when the monitoring record supports a letter: at least three phosphorus or chlorophyll-a samples in the last ten years, scored against EPA National Lakes Assessment thresholds for this lake’s ecoregion. For Mann Creek Reservoir, no nutrient or algae measurement meets the evidence bar (at least 3 samples within 10 years). Sampling here has recorded phosphorus and algae (chlorophyll-a), but not water clarity.
Not Rated is not an F. It is EPA Integrated Reporting Category 3 — insufficient information. The water here may be excellent or poor; nobody has measured enough of it to say. Everything below that has been measured is still reported on this page.
Sampling is run by IDEQ and volunteer programs, and published through the EPA Water Quality Portal. Read the full grading methodology.
Mann Creek Reservoir is a 283-acre lake in Washington County, Idaho.
Source: EPA Water Quality Portal sampling records, IDEQ, last sampled 2021-07-21. Grade methodology: LakeQuality methodology.
Swimming Safety
Not ratedLimited data
The letter is not a swimming clearance. Check local advisories before you swim.
Water quality measurements for Mann Creek Reservoir
These readings exist but are not enough for an A–F grade — see why.
No clarity data. Phosphorus 49 µg/L. Chlorophyll-a 179 µg/L. TSI 71.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 49 µg/L | Not graded |
| Chlorophyll-a (Algae) | 179 µg/L | Not graded |
| Trophic State Index (TSI) | 71 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 283 acres |
No bathymetric survey has been published for Mann Creek Reservoir, so its depth is not documented here.
Fishing Mann Creek Reservoir
Reservoir Info (USACE NID)
Mann Creek Reservoir is a man-made reservoir impounded by the Mann Creek Dam (completed 1967), built primarily for irrigation on the Mann Creek; earth-type dam, 148 ft tall and 1,176 ft long.
- Surface area
- 283 ac
- Max storage
- 12,536 ac-ft
- Drainage area
- 55.98 sq mi
- Hazard class
- High
- Owner
- BUREAU OF RECLAMATION
Source: USACE National Inventory of Dams, NID ID ID00285 · Operator website
Graded lakes near Mann Creek Reservoir
Mann Creek Reservoir has no grade, but these lakes within a short drive do.
Nearby Lakes in Washington County
Frequently asked questions about Mann Creek Reservoir
Where is Mann Creek Reservoir?
- Mann Creek Reservoir is a lake in Washington County, Idaho. LakeQuality has not rated this lake (UNRATED) — no nutrient or algae measurement meets the evidence bar (at least 3 samples within 10 years).
What is the water quality grade for Mann Creek Reservoir?
- Mann Creek Reservoir in Washington County, Idaho is not rated for water quality — its monitoring record does not meet the evidence bar. Phosphorus: 49 µg/L. Chlorophyll-a (algae): 179 µg/L. Surface area: 283 acres.
Is Mann Creek Reservoir safe to swim in?
- The letter is not a swimming clearance. Check local advisories before you swim.
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Where this data comes from
3 measurements from 1 monitoring station, 2021. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Bureau of Reclamation
1119USBR_WQX3 samples · 2021
Analytical methods
- Phosphorus by Single Reagent Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
- Turbidity by Nephelometry
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.