Any structure over or in a stream which results in limitations on the movement of fish or other ecological elements can be an issue.
Often expressed in cubic feet per second (cfs), conveyance capacity is the amount of water a stream crossing structure will pass.
This may result in substantial erosion as flood water cuts a gully on the side of road.
[1] What happens when a heavy truck passes over, or flood water plugs an opening with debris or overtops the structure.
Passage of wildlife, particularly fish such as salmon which spawn in streams, is a design consideration as is habitat continuity.