Prince Lake

Location

The Prince Lake No. 1 Dam Rehabilitation Project is in unincorporated Boulder County, immediately west of the Town of Erie. It is located just east of U.S. Highway 287 on North 111th Street between Erie Parkway and Arapahoe Road.

The project is located primarily on Boulder County’s Eddy open space property, with a portion extending into the Town of Erie’s right of way along North 111th Street and a private parcel of land east of the street.

Project Schedule

The project is currently out to bid for a general construction contractor to complete the work. The tentative schedule for construction is Aug. 1, 2025, to summer of 2026 and is dependent upon final contract award date and construction related permitting.

Parks & Open Space staff are coordinating construction related permitting with Boulder County Community Planning and Permitting and the Town of Erie.

Scope of Work

The dam rehabilitation project consists of:

1. Armoring using riprap and raising the dam embankment height one foot.
2. Improvements to the embankment will provide storage capacity to adequately store the decreed water storage rights.
3. Enlarging and armoring the emergency 90-foot-wide spillway with a concrete grade control wall.
4. Modernizing and enlarging the outlet works.
5. Installation of riprap lined drainage ditch to capture nuisance flow from spillway overtopping.
6. Installation of a toe drain along the downstream toe of the dam to filter and collect seepage through the embankment for its protection.
7. Installation of high-performance turf reinforcement mat downstream of N. 119^th Street to protect against substantial erosive velocity during flood event.

See the Construction Plans for complete details.

Improvements Needed

The county is implementing improvements to the Prince Lake No. 1 dam to modernize the facility, improve its operation, and ensure agricultural water can continue to be stored by bringing it into compliance with newly applicable Colorado dam safety regulations.

The hazard classification of the dam was recently increased by the Colorado Department of Natural Resources, Office of the State Engineer, Dam Safety Branch. In 2017, the county was notified that the dam inundation mapping had been updated, raising the reservoir’s hazard class from low to significant. The county was informed that modifications would be necessary to comply with dam safety regulations, including but not limited to raising the embankment height and increasing the size of the spillway.

This project will bring the dam into compliance with state dam safety regulations for significant hazard class dams, increase the resiliency of the reservoir, and restore its decreed capacity.

Prince Lake No. 1 provides agricultural water to the Eddy and Palizzi open space properties. These lands remain in agricultural production and are designated as having national and state importance. Updating the dam and outlet works to be resilient to future flooding will help ensure the continued viability of these lands, which are preserved for this purpose through the county’s open space program.

Project Funding

This project is substantially funded through a partnership with the Colorado Division of Homeland Security and Emergency Management through the Federal Emergency Management Agency’s Hazard Mitigation Grant Program.

History

The reservoir was originally constructed in 1879 to store irrigation water and has undergone several repairs and retrofits since then. It holds decreed irrigation water rights fed through the South Boulder Canon Ditch, which diverts water from South Boulder Creek, a tributary of Boulder Creek. The reservoir also collects surface and subsurface drainage from the immediate watershed.

Boulder County purchased the Prince Lake No. 1 dam in 2000 and is responsible for its maintenance and compliance with state dam safety regulations. In partnership with its agricultural tenant, the county maintains and operates the reservoir and associated infrastructure. The county and two private parties own the water rights to the water stored in the reservoir.