Inlet or Outlet Drop

Elevation drops at either the inlet, outlet or within a crossing structure can represent physical barriers to many animal species. Not all stream dwelling aquatic species have strong jumping capabilities, and many sub-adult life stages of strong jumpers are not developed well enough to navigate vertical drops associated with crossing structures. In addition, outlet pools are often insufficient in depth to allow fish to jump into structures, and the flat, hard surface of the structure bottom can cause significant harm to fish that are successful. Piping (water flowing through the fill material rather than the culvert) and scouring can result in culverts that are perched above the stream channel making passage impossible for most aquatic species.