Arrowrock Reservoir
Elmore County, Idaho3,256 acresView mapDirections
Idaho DEQ publishes the water-quality measurements for Arrowrock Reservoir. Here is what has been measured.
- Limited data
Arrowrock Reservoir in Elmore County, Idaho has water-quality measurements published by Idaho DEQ, shown here as reported. Water clarity: 12.5 ft visibility. Phosphorus: 35 µg/L. Chlorophyll-a (algae): 1.5 µg/L. Surface area: 3256 acres.
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. Methodology: how these readings are compared.
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 conditions
No readingLimited data
Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Whether Arrowrock Reservoir is safe to swim in today is a call for the Idaho Department of Environmental Quality and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Arrowrock Reservoir
These are the measurements Idaho DEQ publishes for Arrowrock Reservoir, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Good clarity, visible to about 12.5 ft. Phosphorus 35 µg/L. Chlorophyll-a 1.5 µg/L. TSI 44.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 12.5ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 35µg/L | Measured, not scored The nutrient that most limits algae growth in freshwater. More phosphorus, more algae potential. |
| Chlorophyll-a (algae) | 1.5µg/L | Measured, not scored The green pigment in algae — a direct measure of how much algae is in the water. |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 44 | Mesotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Moderate nutrients, good water quality
Ask about Arrowrock Reservoir
Answers come straight from the public measurements and survey records on this page.
Lake Details
Surface area
3,256 acres
No bathymetric survey has been published for Arrowrock Reservoir, so its depth is not documented here.
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Fishing Arrowrock Reservoir
Water quality readings over time
3 seasons of samples on record (2012–2014). Calling a direction takes at least 4 years of the same measurement.
Water clarity
Improving3 years
Phosphorus
Worsening+11.5 µg/L/yr3 years
Chlorophyll-a
Stable0 µg/L/yr3 years
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. Its water-quality measurements are published by Idaho DEQ and shown on this page as reported.
What is the water quality of Arrowrock Reservoir?
- Arrowrock Reservoir in Elmore County, Idaho has water-quality measurements published by Idaho DEQ, shown here as reported. 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?
- Nutrient and algae readings are 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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.