Access by fish is not only enhanced, but water velocity in the culvert is decreased. The downstream dams should not create outfall barriers. In some streams, the range of flows is so great that it is impossible not to have the culvert outlet above the tailwater at some time. Also where severe fluctuations in flow require large culverts, fish passage may be impeded during low flows because of shallow flow over the broad culvert bottom. Placing the stacked culverts at different elevations ensures adequate discharge capacity as well as fish passage over a wider range of flows.
Trash racks are detrimental to fish passage.
The storms that often bring debris downstream are those in which many fish can move up to spawning areas. Although the protected culvert may not be a velocity or outfall barrier, a debris-laden trash rack can be impassable to fish. Therefore, debris-catching structures should be avoided on streams used by migrating fish, and crossings should be large enough to transmit debris downstream. However, increasing the culvert diameter may be impractical because of the increased cost. To compensate for the absence of culvert protection from avoiding debris-catching structures, the culvert should be large enough to allow debris to pass through it.