Prewitt Reservoir
Washington County, Colorado2,329 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.
- 82 ft max depth
- Public access
- Limited data
Prewitt Reservoir in Washington County, Colorado is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 1.8 ft visibility. Phosphorus: 87 µg/L. Chlorophyll-a (algae): 31.5 µg/L. Surface area: 2329 acres.
- Clarity
- 1.8 ft
- Max depth
- ~82 ft est.
- Area
- 2,329 acres
- Public access
- Yes
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Why Prewitt 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 Prewitt Reservoir, no nutrient or algae measurement meets the evidence bar (at least 3 samples within 10 years). 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 CDPHE and volunteer programs, and published through the EPA Water Quality Portal. Read the full grading methodology.
Prewitt Reservoir is a 2,329-acre lake in Washington County, Colorado with an estimated average depth of about 82 feet. Limited fish-species data available. Public access is available.
Source: EPA Water Quality Portal sampling records, CDPHE, last sampled 2023-11-06. Grade methodology: LakeQuality methodology.
What Prewitt Reservoir is good for
From access and monitoring records, not editorial picks.
- Big water2,329 acres
Swimming Safety
Not ratedLimited data
The letter is not a swimming clearance. Check local advisories before you swim.
Water quality measurements for Prewitt Reservoir
These readings exist but are not enough for an A–F grade — see why.
Very murky, less than 1.8 ft of visibility. Phosphorus 87 µg/L. Chlorophyll-a 31.5 µg/L. TSI 67.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.8 ft | Not graded |
| Phosphorus | 87 µg/L | Not graded |
| Chlorophyll-a (Algae) | 31.5 µg/L | Not graded |
| Trophic State Index (TSI) | 67 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~82 ft est. |
| Average Depth | ~25 ft est. |
| Surface Area | 2,329 acres |
| Public Access | Yes |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Colorado has not published bathymetry for Prewitt Reservoir.
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Fishing Prewitt Reservoir
Reservoir Info (USACE NID)
Prewitt Reservoir is a man-made reservoir impounded by the Prewitt (completed 1912), built primarily for irrigation on the South Platte River-Os; earth-type dam, 43 ft tall and 15,400 ft long.
- Surface area
- 2,329 ac
- Normal storage
- 28,840 ac-ft
- Max storage
- 51,387 ac-ft
- Drainage area
- 105 sq mi
- Hazard class
- High
- Owner
- LOGAN IRRIGATION DISTRICT (YAHN, JIM)
Source: USACE National Inventory of Dams, NID ID CO00385 · Operator website
Graded lakes near Prewitt Reservoir
Prewitt Reservoir has no grade, but these lakes within a short drive do.
Nearby Lakes in Washington County
Frequently asked questions about Prewitt Reservoir
Where is Prewitt Reservoir?
- Prewitt Reservoir is a lake in Washington County, Colorado. 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 Prewitt Reservoir?
- Prewitt Reservoir in Washington County, Colorado is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 1.8 ft visibility. Phosphorus: 87 µg/L. Chlorophyll-a (algae): 31.5 µg/L. Surface area: 2329 acres.
Is Prewitt Reservoir safe to swim in?
- The letter is not a swimming clearance. Check local advisories before you swim.
How deep is Prewitt Reservoir?
- No published bathymetric survey covers Prewitt Reservoir, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 82 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate. The lake covers 2329 acres.
Explore Prewitt Reservoir across LakeQuality
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Where this data comes from
7 measurements from 2 monitoring stations, 2021–2023. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Colorado Dept. of Public Health & Environment-WQCD
21COL001_WQX7 samples · 2021–2023
Analytical methods
- Secchi disk depth measurement
- Phosphorus by Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
- Nitrogen in Water by Pyrolysis Detection
Monitoring station identifiers (2)
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
Lake size & watershed from HydroLAKES (HydroSHEDS)