Three perched culverts (photo credit: USFWS).
A culvert is essentially a tunnel to pass flowing water, typically a small stream, under a man-made structure, usually a road. Depending on the size, placement, and design of a culvert, the impacts to a stream channel vary. These impacts, in turn, can affect fish habitat and fish migration.
Culverts are frequently barriers to fish movement. If culverts are perched, where the outlet is higher than the elevation of the downstream water, fish are required to leap up into the culvert (if they are able) to continue moving upstream. Often, culverts are not installed to be perched from the outset, but the situation is caused by erosion at the outlet of the culvert from high flows which scour the channel bed. This type of culvert creates a downstream pool, changes the flow velocity and habitat type, and, consequently, can alter the fish community as well.
Local, state, and federal managers often work to replace or retrofit culverts that have significant ramifications for fish passage. The U.S. Fish & Wildlife Service’s National Fish Passage Program is one example.