Rimrock Reservoir
Yakima County, Washington2,525 acresView mapDirections
Nutrients and algae sit mid-range for this region.
↓ down from A at the last regrade
- Mesotrophic
- #4 of 4 in the county
- Public access
- Limited data
Rimrock Reservoir in Yakima County, Washington has a water quality grade of C (Fair). Phosphorus: 15 µg/L. Chlorophyll-a (algae): 2.1 µg/L. Surface area: 2525 acres.
- Area
- 2,525 acres
- Type
- Mesotrophic
- Phosphorus
- 15 µg/L
- Chlorophyll-a
- 2.1 µg/L
- Trend
- ↑ Improving
- Public access
- Yes
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Watch Rimrock Reservoir’s letter
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Rimrock Reservoir is a 2,525-acre mesotrophic lake in Yakima County, Washington. Water quality is graded C based on 2 sampling years through 2023. Overall water quality is improving over recent sampling years. Limited fish-species data available. Public access is available with 3 public boat landings. Compared to the 3 other monitored lakes in Yakima County, Rimrock Reservoir ranks #4 for water quality.
Source: EPA Water Quality Portal sampling records, WA Ecology, last sampled 2023-07-27. Grade methodology: LakeQuality methodology.
What Rimrock Reservoir is good for
From access and monitoring records, not editorial picks.
- Boat launch3 documented ramps
- Big water2,525 acres
Swimming Safety
Grade onlyLimited data
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. Check local advisories before you swim.
Water Quality Grade: C (Fair)
No clarity data. Phosphorus 15 µg/L. Chlorophyll-a 2.1 µg/L. TSI 41. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 15 µg/L | Good |
| Chlorophyll-a (Algae) | 2.1 µg/L | Fair |
| Trophic State Index (TSI) | 41 | Mesotrophic |
Moderate nutrients, good water quality
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 2,525 acres |
| Public Access | Yes |
No bathymetric survey has been published for Rimrock Reservoir, so its depth is not documented here.
Recreation & Access
- Horseshoe CovePublic
- Silver Beach ResortSemi-private
- Rimrock Lake ResortSemi-private
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Rimrock Reservoir
Water Quality Trend: ↑ Improving
Based on 3 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Phosphorus | ↑ Improving | -3 µg/L/yr | 3 |
| Chlorophyll-a | ↑ Improving | -1.9 µg/L/yr | 3 |
Reservoir Info (USACE NID)
Rimrock Reservoir is a man-made reservoir impounded by the Tieton Dam (completed 1925), built primarily for flood risk reduction on the Tieton River; rockfill-type dam, 319 ft tall and 920 ft long.
- Surface area
- 2,525 ac
- Normal storage
- 198,000 ac-ft
- Max storage
- 198,000 ac-ft
- Drainage area
- 186.37 sq mi
- Hazard class
- High
- Owner
- BUREAU OF RECLAMATION
All listed purposes: Flood Risk Reduction;Irrigation;Recreation.
Source: USACE National Inventory of Dams, NID ID WA00273 · Operator website
County Ranking
Ranked #4 of 4 lakes in Yakima County
Nearby Lakes in Yakima County
Frequently asked questions about Rimrock Reservoir
Where is Rimrock Reservoir?
- Rimrock Reservoir is a lake in Yakima County, Washington. LakeQuality grades it C for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Rimrock Reservoir?
- Rimrock Reservoir in Yakima County, Washington has a water quality grade of C (Fair). Phosphorus: 15 µg/L. Chlorophyll-a (algae): 2.1 µg/L. Surface area: 2525 acres.
Is Rimrock Reservoir safe to swim in?
- The C is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Are there boat ramps on Rimrock Reservoir?
- Rimrock Reservoir has 3 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Horseshoe Cove, Silver Beach Resort, Rimrock Lake Resort. Public access is listed as yes.
Explore Rimrock Reservoir across LakeQuality
19.5 mi away · 60 ft
2 thermal springs within 19.3 mi, nearest Simcoe Soda Springs
Species activity outlook by water temperature and season
Where to stay at Washington's top lakes, each with its grade
Moderate nutrients, good water quality
Washington's cleanest and most-polluted lakes, ranked
Planning a trip around Mount Rainier?
Rimrock Reservoir is about 29 miles from Mount Rainier National Park. Crowds there swing hard by season, so the month you pick changes both how busy the park is and what lodging costs.
Best time to visit Mount Rainier →More Washington Lake Rankings
Other Grade C Lakes in Washington
Mesotrophic Lakes in Washington
Where this data comes from
8 measurements from 1 monitoring station, 2019–2023. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Bureau of Reclamation
1119USBR_WQX8 samples · 2019–2023
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.
Other records on this page
Named boat-access points from the USGS National Boat Ramps inventory