Union Reservoir
Weld County, Colorado717 acresView mapDirections
Colorado WQCD publishes the water-quality measurements for Union Reservoir. Here is what has been measured.
- Eutrophic
- 26 ft max depth
- Limited data
Union Reservoir in Weld County, Colorado has water-quality measurements published by Colorado WQCD, shown here as reported. Water clarity: 3.2 ft visibility. Phosphorus: 15.5 µg/L. Chlorophyll-a (algae): 14.5 µg/L. Surface area: 717 acres.
Union Reservoir is a 717-acre productive, nutrient-rich eutrophic lake in Weld County, Colorado with an estimated average depth of about 26 feet. 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. Methodology: how these readings are compared.
Union Reservoir covers about 720 acres in Weld County. If you are planning to stay, you can search lake houses and cabins around Union Reservoir on Vrbo, or look for a hotel room near the lake on Booking.com or Expedia.
Swimming conditions
Readings onlyLimited data
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Whether Union Reservoir is safe to swim in today is a call for the Water Quality Control Division of the Colorado Department of Public Health and Environment and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Union Reservoir
These are the measurements Colorado WQCD publishes for Union Reservoir, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Very murky, less than 3.2 ft of visibility. Phosphorus 15.5 µg/L. Chlorophyll-a 14.5 µg/L. TSI 54.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 3.2ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 15.5µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 14.5µg/L | Poor GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 54 | Eutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
High nutrients, frequent algae, reduced clarity
Ask about Union Reservoir
Answers come straight from the public measurements and survey records on this page.
Lake Details
Max 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
Watershed (drainage) area
14 sq mi
HydroLAKES / HydroSHEDS
Shoreline
4.5 mi
HydroLAKES / HydroSHEDS
Elevation
4,957 ft
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.
Stays via Stay22
Stays near Union Reservoir
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 readings cover nutrients and algae, not lodging. Cabins and rentals around Union Reservoir are listed below.
Top-rated places to stay near Union Reservoir
Prices for Oct 23–25, 4 adults. Change dates on the map below.
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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
Nearby Lakes in Weld County
Frequently asked questions about Union Reservoir
Where is Union Reservoir?
- Union Reservoir is a lake in Weld County, Colorado. Its water-quality measurements are published by Colorado WQCD and shown on this page as reported.
What is the water quality of Union Reservoir?
- Union Reservoir in Weld County, Colorado has water-quality measurements published by Colorado WQCD, shown here as reported. 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?
- Nutrient and algae readings are 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
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 lakes in Colorado
Other 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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.
Other records on this page
Lake size & watershed from HydroLAKES (HydroSHEDS)