Chesterfield Reservoir is an important part of the Portneuf-Marsh Valley Canal Company’s (PMVCC) water supply. The reservoir stores up to 21,500 acre-feet of water and helps serve 5,103 acres through 28 miles of open canal in southeastern Idaho. Its stored water is especially valuable late in the irrigation season, when it helps farms and other water users make it through the drier months.

Heavy runoff in spring 2023 caused severe erosion and failure of the reservoir’s spillway. Franson Civil worked with PMVCC to secure financial assistance through NRCS EQIP and Idaho Aging Infrastructure grants while preparing the replacement design and coordinating approval with the Idaho Department of Water Resources Dam Safety Division (IDWR DSD).

Before construction could begin, IDWR DSD required an Interim Operations Plan for safely managing the reservoir until repairs were complete. Our team worked closely with the operator to digitize historical records, understand established practices, inspect the site, and evaluate reservoir, snowpack, and field data. Reaching the site was not always easy. When winter conditions closed normal access, two team members skied into the project area to check conditions firsthand. The completed plan gave PMVCC clear procedures for managing inflows, storage, and emergencies while preserving as much irrigation water as conditions safely allowed.

Construction began in spring 2024. Franson Civil managed construction, completed required reporting, and monitored the work for consistency with the approved design and IDWR standards. The project repaired the spillway foundation, improved energy dissipation and downstream erosion protection, and provided better control of reservoir inflows. Construction concluded in fall 2025 without disrupting irrigation deliveries. The completed work restored the spillway, improved reservoir operations, and strengthened the reliability of a water supply that local farms and water users depend on throughout the irrigation season.