Onion Valley Reservoir @ Dam
Humboldt County, Nevada101 acresView mapDirections
Nutrients and algae sit mid-range for this region.
↓ down from B at the last regrade
- Eutrophic
- Impaired · Metals (other than mercury)
- #2 of 4 in the county
Onion Valley Reservoir @ Dam in Humboldt County, Nevada has a water quality grade of C (Fair). Phosphorus: 50 µg/L. Chlorophyll-a (algae): 4.6 µg/L. Surface area: 101 acres.
- Area
- 101 acres
- Type
- Eutrophic
- Phosphorus
- 50 µg/L
- Chlorophyll-a
- 4.6 µg/L
- Trend
- ↓ Declining
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Watch Onion Valley Reservoir @ Dam’s letter
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Onion Valley Reservoir @ Dam is a 101-acre productive, nutrient-rich eutrophic lake in Humboldt County, Nevada. Water quality is graded C based on 1 sampling years through 2021. Overall water quality is declining over recent sampling years. Compared to the 3 other monitored lakes in Humboldt County, Onion Valley Reservoir @ Dam ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, NDEP, last sampled 2021-10-05. Grade methodology: LakeQuality methodology.
Swimming Safety
Grade onlyModerate
Based on clarity and algae grades from past sampling — we do not hold live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
The C is nutrients and algae, not a swimming clearance. Onion Valley Reservoir @ Dam is listed impaired under Clean Water Act §303(d) (IR 5) for metals (other than mercury), nutrients (phosphorus, nitrogen), oxygen depletion (low DO), pH / acidity. Check local advisories before you swim.
Water Quality Grade: C (Fair)
No clarity data. Phosphorus 50 µg/L. Chlorophyll-a 4.6 µg/L. TSI 53. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 50 µg/L | Fair |
| Chlorophyll-a (Algae) | 4.6 µg/L | Not graded |
| Trophic State Index (TSI) | 53 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 101 acres |
No bathymetric survey has been published for Onion Valley Reservoir @ Dam, so its depth is not documented here.
Fishing Onion Valley Reservoir @ Dam
Water Quality Trend: ↓ Declining
Based on 7 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Phosphorus | ↓ Declining | +21.56 µg/L/yr | 7 |
| Chlorophyll-a | ↓ Declining | +0.46 µg/L/yr | 3 |
Reservoir Info (USACE NID)
Onion Valley Reservoir @ Dam is a man-made reservoir impounded by the Onion Dam, built primarily for irrigation on the East Fork Alder Creek; earth-type dam, 54 ft tall and 320 ft long.
- Surface area
- 101 ac
- Normal storage
- 1,500 ac-ft
- Max storage
- 1,500 ac-ft
- Drainage area
- 3.1 sq mi
- Hazard class
- Significant
- Owner
- ALDER CREEK RANCH LLC
All listed purposes: Irrigation;Recreation.
Source: USACE National Inventory of Dams, NID ID NV00005 · Operator website · Matched by proximity (0.16 km)
EPA Impairment Status
Onion Valley Reservoir @ Dam is officially listed as impaired under Clean Water Act §303(d) in the 2022 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit NV01-NW-06-B_00 · Official waterbody report · Matched by proximity (0 km)
County Ranking
Ranked #2 of 4 lakes in Humboldt County
Nearby Lakes in Humboldt County
Frequently asked questions about Onion Valley Reservoir @ Dam
Where is Onion Valley Reservoir @ Dam?
- Onion Valley Reservoir @ Dam is a lake in Humboldt County, Nevada. LakeQuality grades it C for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Onion Valley Reservoir @ Dam?
- Onion Valley Reservoir @ Dam in Humboldt County, Nevada has a water quality grade of C (Fair). Phosphorus: 50 µg/L. Chlorophyll-a (algae): 4.6 µg/L. Surface area: 101 acres.
Is Onion Valley Reservoir @ Dam safe to swim in?
- The C is nutrients and algae, not a swimming clearance. Onion Valley Reservoir @ Dam is listed impaired under Clean Water Act §303(d) (IR 5) for metals (other than mercury), nutrients (phosphorus, nitrogen), oxygen depletion (low DO), pH / acidity. Check local advisories before you swim.
Explore Onion Valley Reservoir @ Dam across LakeQuality
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Where this data comes from
21 measurements from 1 monitoring station, 2016–2021. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Nevada Division of Environmental Protection
21NEV1_WQX21 samples · 2016–2021
Analytical methods
- 4500 P E ~ Phosphorus in Water by Colorimetry- Ascorbic Acid Method
- 4500 P E ~ Phosphorus by Ascorbic Acid
- Phosphorus by Two Reagent Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
- 2130 B ~ Nephelometric Method
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.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database