Arrowrock Reservoir
Elmore County, Idaho3,256 acresView mapDirections
Not enough samples to grade — the only measurement available is water clarity, which is not a water-quality indicator on its own — EPA's National Lakes Assessment does not grade clarity. No letter yet. Here is what we do have.
- Limited data
Arrowrock Reservoir in Elmore County, Idaho is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 12.5 ft visibility. Phosphorus: 35 µg/L. Chlorophyll-a (algae): 1.5 µg/L. Surface area: 3256 acres.
- Clarity
- 12.5 ft
- Area
- 3,256 acres
- Phosphorus
- 35 µg/L
- Chlorophyll-a
- 1.5 µg/L
- Trend
- → Stable
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Watch Arrowrock Reservoir’s letter
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Why Arrowrock 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 Arrowrock Reservoir, the only measurement available is water clarity, which is not a water-quality indicator on its own — EPA's National Lakes Assessment does not grade clarity. Sampling here has recorded water clarity, phosphorus and algae (chlorophyll-a).
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.
Arrowrock Reservoir is a 3,256-acre lake in Elmore County, Idaho. Water clarity is improving over recent sampling years.
Source: EPA Water Quality Portal sampling records, IDEQ, last sampled 2015-02-11. Grade methodology: LakeQuality methodology.
What Arrowrock Reservoir is good for
From access and monitoring records, not editorial picks.
- Clear water12.5 ft of visibility
- Big water3,256 acres
Swimming Safety
Not ratedLimited data
The letter is not a swimming clearance. Check local advisories before you swim.
Water quality measurements for Arrowrock Reservoir
These readings exist but are not enough for an A–F grade — see why.
Good clarity, visible to about 12.5 ft. Phosphorus 35 µg/L. Chlorophyll-a 1.5 µg/L. TSI 44.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 12.5 ft | Not graded |
| Phosphorus | 35 µg/L | Not graded |
| Chlorophyll-a (Algae) | 1.5 µg/L | Not graded |
| Trophic State Index (TSI) | 44 | Mesotrophic |
Moderate nutrients, good water quality
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 3,256 acres |
No bathymetric survey has been published for Arrowrock Reservoir, so its depth is not documented here.
Fishing Arrowrock Reservoir
Water Quality Trend: → Stable
Based on 3 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.75 m/yr | 3 |
| Phosphorus | ↓ Declining | +11.5 µg/L/yr | 3 |
| Chlorophyll-a | → Stable | 0 µg/L/yr | 3 |
Reservoir Info (USACE NID)
Arrowrock Reservoir is a man-made reservoir impounded by the Arrowrock Dam (completed 1915), built primarily for flood risk reduction on the Boise River Snake River; concrete-type dam, 350 ft tall and 1,150 ft long.
- Surface area
- 3,256 ac
- Normal storage
- 272,224 ac-ft
- Max storage
- 272,224 ac-ft
- Drainage area
- 2,216.66 sq mi
- Hazard class
- High
- Owner
- BUREAU OF RECLAMATION
All listed purposes: Flood Risk Reduction;Irrigation;Fish and Wildlife Pond;Recreation;Water Supply.
Source: USACE National Inventory of Dams, NID ID ID00280 · Operator website
Nearby Lakes in Elmore County
Frequently asked questions about Arrowrock Reservoir
Where is Arrowrock Reservoir?
- Arrowrock Reservoir is a lake in Elmore County, Idaho. LakeQuality has not rated this lake (UNRATED) — the only measurement available is water clarity, which is not a water-quality indicator on its own — EPA's National Lakes Assessment does not grade clarity.
What is the water quality grade for Arrowrock Reservoir?
- Arrowrock Reservoir in Elmore County, Idaho is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 12.5 ft visibility. Phosphorus: 35 µg/L. Chlorophyll-a (algae): 1.5 µg/L. Surface area: 3256 acres.
Is Arrowrock 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
84 measurements from 1 monitoring station, 2012–2015. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Bureau of Reclamation
1119USBR_WQX84 samples · 2012–2015
Analytical methods
- Depth, Secchi disk depth
- Phosphorus by Single Reagent Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
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.