A culvert is a flowing body of water which passes underneath a road, railway, or embankment, or the part thereof that does so.
In the U.K., the culvert is the conduit through which the flow passes. (The U.S. reference below, seems consistent with this view.)
In the Pacific Northwest of the United States it has been found that many culverts impede upstream migration by salmon. Research is being carried out to determine how culverts can be adapted to make it easier for salmon and other fish to pass through them against the current.
References
