Union Reservoir
Weld County, Colorado717 acresView mapDirections
Nutrients and algae run above the regional average.
↑ up from F at the last regrade
- Eutrophic
- #2 of 5 in the county
- 26 ft max depth
- Limited data
Union Reservoir in Weld County, Colorado has a water quality grade of D (Nutrient-rich). Water clarity: 3.2 ft visibility. Phosphorus: 15.5 µg/L. Chlorophyll-a (algae): 14.5 µg/L. Surface area: 717 acres.
- Clarity
- 3.2 ft
- Max depth
- ~26 ft est.
- Area
- 717 acres
- Type
- Eutrophic
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Compare lakeside cabins, hotels, and vacation rentals near Union Reservoir 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 Union Reservoir are listed below.
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Watch Union Reservoir’s letter
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Union Reservoir is a 717-acre productive, nutrient-rich eutrophic lake in Weld County, Colorado with an estimated average depth of about 26 feet. Water quality is graded D based on 3 sampling years through 2022. Compared to the 4 other monitored lakes in Weld County, Union Reservoir ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, CDPHE, last sampled 2022-10-04. Grade methodology: LakeQuality methodology.
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 D is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Water Quality Grade: D (Nutrient-rich)
Very murky, less than 3.2 ft of visibility. Phosphorus 15.5 µg/L. Chlorophyll-a 14.5 µg/L. TSI 54. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 3.2 ft | Not graded |
| Phosphorus | 15.5 µg/L | Good |
| Chlorophyll-a (Algae) | 14.5 µg/L | Poor |
| Trophic State Index (TSI) | 54 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~26 ft est. |
| Surface Area | 717 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 Union 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 Union Reservoir
Reservoir Info (USACE NID)
Union Reservoir is a man-made reservoir impounded by the Union (completed 1902), built primarily for irrigation on the St Vrain Creek-Tr; earth-type dam, 33 ft tall and 1,200 ft long.
- Surface area
- 717 ac
- Normal storage
- 12,715 ac-ft
- Max storage
- 16,250 ac-ft
- Drainage area
- 6 sq mi
- Hazard class
- Significant
- Owner
- UNION DITCH COMPANY (KEN HUSON)
All listed purposes: Irrigation;Fish and Wildlife Pond;Recreation.
Source: USACE National Inventory of Dams, NID ID CO01219 · Operator website
County Ranking
Ranked #2 of 5 lakes in Weld County
Nearby Lakes in Weld County
Frequently asked questions about Union Reservoir
Where is Union Reservoir?
- Union Reservoir is a lake in Weld County, Colorado. LakeQuality grades it D for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Union Reservoir?
- Union Reservoir in Weld County, Colorado has a water quality grade of D (Nutrient-rich). Water clarity: 3.2 ft visibility. Phosphorus: 15.5 µg/L. Chlorophyll-a (algae): 14.5 µg/L. Surface area: 717 acres.
Is Union Reservoir safe to swim in?
- The D is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
How deep is Union Reservoir?
- No published bathymetric survey covers Union Reservoir, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 26 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 717 acres.
Explore Union Reservoir across LakeQuality
21.3 mi away · recorded at 79°F
Species activity outlook by water temperature and season
Where to stay at Colorado's top lakes, each with its grade
High nutrients, frequent algae, reduced clarity
Colorado's cleanest and most-polluted lakes, ranked
Planning a trip around Rocky Mountain?
Union Reservoir is about 37 miles from Rocky Mountain 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 Rocky Mountain →More Colorado Lake Rankings
Other Grade D Lakes in Colorado
Eutrophic Lakes in Colorado
Where this data comes from
23 measurements from 4 monitoring stations, 2021–2022. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Colorado Dept. of Public Health & Environment-WQCD
21COL001_WQX23 samples · 2021–2022
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 (4)
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)