Boulder County worked with the Niwot Ditch Company and associated unincorporated ditches to rebuild the Niwot Ditch in-stream diversion dam on St. Vrain Creek between 75th Street and Airport Road.
Niwot Ditch Diversion Repair & Fish Passage Installation Project
The primary goal of the project was to protect and restore native fish populations while ensuring the efficient delivery of decreed water rights. This project replaced the existing diversion dam with two engineered diversion structures, 532 CY of soil riprap, 36” constructed boulder riffle drops, 48” stacked boulder walls, 560 LF of rootwad revetments, 1120 LF coir wrapped soil lifts, 2000 live willow stakes, and 3800 landscape plantings along 1200 LF of stream length.
This was a collaborative project which also included the US Fish and Wildlife Service, Colorado Parks and Wildlife, and Trout Unlimited.
Wildlife Habitat Enhancements
This diversion dam is located on a reach of St. Vrain Creek that is one of that last strongholds for native transition zone fish in Colorado (e.g. plains top minnow) and is one of Colorado Parks and Wildlife’s highest priorities for native fish conservation in the state. One of the management goals for conservation of these sensitive native fish populations is to restore upstream-downstream connectivity and allow for natural movements of both native and non-native sport fish by removing instream barriers to movement.
The project included ecological restoration elements designed to support native fish species by improving in-stream habitat conditions and reducing the stream gradient and velocity. The project restored some geomorphic and hydrologic processes, improved the sediment transport capacity and provided attenuation of flood flows within the stream reach. The project benefits the Preble’s meadow jumping mouse by providing habitat structure, improving habitat connectivity, and enhancing production of insect prey and other food resources.
New Diversion Dam Structure
The project replaced the existing diversion dam with an engineered structure approximately 400 feet upstream of the existing dam location that includes two headgates. Using design concepts from natural channel design, the new diversion dam is a multi-objective structure built from natural materials that is more resilient to high flows, reduces flood risk, allows passage of native and non-native fish, and facilitates natural stream processes and function.
The design and construction of the project were funded by US Fish and Wildlife Service National Fish Passage Program, Colorado Parks and Wildlife Wetland Grant, CWCB Species Conservation Trust Fund, Trout Unlimited, Community Development Block Grant – Disaster Recovery (CDBGDR), and Boulder County. Boulder County also contributed native fill material in-kind from a flood recovery project and cut logs from a forestry project. The design was been developed by GEI Consultants Inc.