Prospect Lake
Weld County, Colorado398 acresView mapDirections
Nutrients and algae run well above the regional average.
- Hypereutrophic
- #4 of 5 in the county
- 18 ft max depth
Prospect Lake in Weld County, Colorado has a water quality grade of F (Algae-prone). Phosphorus: 300 µg/L. Chlorophyll-a (algae): 42.5 µg/L. Surface area: 398 acres.
- Max depth
- ~18 ft est.
- Area
- 398 acres
- Type
- Hypereutrophic
- Trend
- ↓ Declining
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Stays near Prospect Lake
Compare lakeside cabins, hotels, and vacation rentals near Prospect Lake in Weld County, Colorado — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water. The water-quality grade measures nutrients and algae, not lodging — cabins and rentals around Prospect Lake are listed below.
Top-rated places to stay near Prospect Lake
Prices for Sep 4–6, 4 adults. Change dates on the map below.
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Watch Prospect Lake’s letter
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Prospect Lake is a 398-acre highly nutrient-loaded hypereutrophic lake in Weld County, Colorado with an estimated average depth of about 18 feet. Water quality is graded F based on 2 sampling years through 2024. Overall water quality is declining over recent sampling years. Compared to the 4 other monitored lakes in Weld County, Prospect Lake ranks #4 for water quality.
Source: EPA Water Quality Portal sampling records, CDPHE, last sampled 2024-09-25. Grade methodology: LakeQuality methodology.
Swimming Safety
Grade onlyAlgae risk
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 F is nutrients and algae, not a swimming clearance. Prospect Lake is listed impaired under Clean Water Act §303(d) (IR 5) for pH / acidity. Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
No clarity data. Phosphorus 300 µg/L. Chlorophyll-a 42.5 µg/L. TSI 77. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 300 µg/L | Poor |
| Chlorophyll-a (Algae) | 42.5 µg/L | Poor |
| Trophic State Index (TSI) | 77 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~18 ft est. |
| Average Depth | ~6 ft est. |
| Surface Area | 398 acres |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Colorado has not published bathymetry for Prospect Lake.
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 Prospect Lake
Water Quality Trend: ↓ Declining
Based on 3 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Phosphorus | ↓ Declining | +30.38 µg/L/yr | 3 |
| Chlorophyll-a | ↓ Declining | +16.29 µg/L/yr | 3 |
Reservoir Info (USACE NID)
Prospect Lake is a man-made reservoir impounded by the Prospect (completed 1914), built primarily for irrigation on the Lost Creek-Tr; earth-type dam, 45 ft tall and 5,301 ft long.
- Surface area
- 398 ac
- Normal storage
- 6,368 ac-ft
- Max storage
- 9,299 ac-ft
- Drainage area
- 9 sq mi
- Hazard class
- Significant
- Owner
- HENRYLYN IRRIGATION DISTRICT
Source: USACE National Inventory of Dams, NID ID CO01106 · Operator website
EPA Impairment Status
Prospect Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit COSPMS07_B · Official waterbody report
County Ranking
Ranked #4 of 5 lakes in Weld County
Nearby Lakes With Higher Grades
Prospect Lake holds Grade F. One nearby lake has a higher nutrient-and-algae grade — useful for a swim day, not a reason to skip lodging here.
See full comparison →Nearby Lakes in Weld County
Frequently asked questions about Prospect Lake
Where is Prospect Lake?
- Prospect Lake is a lake in Weld County, Colorado. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Prospect Lake?
- Prospect Lake in Weld County, Colorado has a water quality grade of F (Algae-prone). Phosphorus: 300 µg/L. Chlorophyll-a (algae): 42.5 µg/L. Surface area: 398 acres.
Is Prospect Lake safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Prospect Lake is listed impaired under Clean Water Act §303(d) (IR 5) for pH / acidity. Check local advisories before you swim.
How deep is Prospect Lake?
- No published bathymetric survey covers Prospect Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 18 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 398 acres.
Explore Prospect Lake across LakeQuality
Species activity outlook by water temperature and season
Where to stay at Colorado's top lakes, each with its grade
Very high nutrients, dense algae, poor clarity
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Where this data comes from
55 measurements from 1 monitoring station, 2020–2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Metro Water Recovery (formerly Metro Wastewater Reclamation District)
MWRD_WQX55 samples · 2020–2024
Analytical methods
- 4500 P H ~ Manual Digestion and Flow Injection Analysis for Total Phosphorus
- 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
Impairment status from EPA ATTAINS 303(d) database
Lake size & watershed from HydroLAKES (HydroSHEDS)