Culvert Havoc
Culverts may block the natural movement of both resident and migratory fish; even when adults are strong enough to swim through, their energy may be depleted, and juveniles may be completely obstructed.
Width
The diameter of fish-friendly culverts must synchronize with that of the stream. Channel width should be measured at the top of the streambank to reflect high-water conditions.
Thinking Ahead
Though fish migration related to spawning typically occurs during periods of high flow, culvert designers must ensure that culverts still permit passage when water levels are low---fish may also be traveling in such times, often seeking cooler parts of the stream.
The Bottom
Many successful fish-friendly culverts lack bottoms or are have bottoms sunken within the streambed, so that the current carves a channel within the tunnel.
Other Considerations
Matching the angle of the installed culvert to the gradient of the stream helps reduce erosion. In recommendations for fish-friendly culverts in California, the National Marine Fisheries Service (Southwest Region) notes that culverts exceeding 100 feet in length might need interior lighting.
