Fish Passage through Culverts on Lolo National Forest

Kelsey Nielsen, Big Sky High School  and Traci Sylte,  Lolo National Forest

Road culverts often block fish migration, cutting fish off from critical habitat.  In the past, stream crossings were designed based on economic, road grade and alignment considerations with resource values like fish migration and stream condition considered secondary.  In the last ten years, knowledge of fish needs and swimming capabilities has increased, which has changed the focus of culvert design.

Culvert designers must consider not only efficient water passage, but also what water velocity and depth and culvert length and slope will accommodate the different swimming abilities of different fish species and age classes.  In most instances, culverts should span the active channel with a natural or naturalized bottom, especially where stream gradients exceed 2-3%.

A variety of structures, such as bottomless box culverts, bottomless arches, pipe arches, and corrugated metal pipes, are available. Baffles can be used to slow velocities within the culvert and to provide resting opportunities for fish; however, they are generally viewed as a last option because they reduce flow capacity and commonly require heavy maintenance.