Crossing Problems

There are a number of ways for a road to cross a river or stream. These include bridges, fords, open-bottom or arch culverts, box culverts, and pipe culverts. Depending on the type of crossing, its size, method of installation, and maintenance, a road crossing may have many or relatively few adverse impacts on a river or stream ecosystem.

The movement of organisms within rivers and streams is an important ecological process that can be significantly affected by road crossings. There are several ways that crossing structures can impede or prevent the movement of animals:

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.

Physical Barriers

Animal movement can be blocked by clogged or collapsed culverts. Also, weirs or baffles, which are typically designed to increase depth or decrease velocities within a crossing structure, can create barriers for some weak swimming or crawling species.

As rigid structures that typically constrict the natural stream channel, crossing structures can trap debris is such a way that blocks or inhibits aquatic organism passage. Beavers often find these undersized structures to be convenient places to dam up streams. Larger structures are not only more likely to pass woody debris but are also less likely to be blocked by beavers.

Excessive Velocities

Water velocities can be too high to pass fish or other organisms during some or all of the year. This is a particularly relevant factor with many culverts. As stream discharge increases, velocities within culverts increase correspondingly. These velocities can easily exceed 10 feet per second, which is nearly twice that of even the strongest swimming salmonid's capabilities. In addition, flows within crossing structures are generally uniform, allowing no resting areas for aquatic species attempting to pass through them. The result is that the species must be able to swim the entire length of the structure in one burst effort.

Insufficient Water Depth

Absence of a low-flow channel can result in water depths too shallow to allow passage for fish or other organisms. In systems with highly variable flows, it is challenging to construct a structure capable of passing high flows but still maintaining a defined low-flow channel similar to the natural streambed. In these systems, often the most successful structures are those that provide bank-edges and a floodplain within the structure. When designing these types of crossings, pay particular attention to the size, location, and spacing of substrate within the structure to emulate the natural streambed as closely as possible.

Turbulence

Flow contraction at the inlet or outlet can create turbulence that inhibits or prevents animal passage. Oxygen forced into the water column at culvert outlets can disorient aquatic species and not allow them to navigate the structure opening. This is especially prevalent with perched structures. Baffles or other artificially roughened channels or structures can create turbulence that blocks movement.

Absence of Bank Edge

Passage by weak-swimming organisms can be inhibited or prevented by the absence of bank edge areas within crossing structures. It is often challenging to construct a crossing structure that allows for bank edge areas because of the increased cost associated with the larger structure. However, when long-term costs to species are considered, the additional cost of constructing a structure that provides bank edge areas may be justified.

Discontinuity of Channel Substrate

Crossing structures that lack any natural substrate or contain substrates (including rip-rap, baffles, or other armoring) that contrast with the natural stream channel create discontinuities in streambed habitats. Many benthic (streambed-dwelling) organisms are confined to the streambed and can move only through appropriate substrates. Streambed discontinuities caused by crossing structures disrupt and fragment populations of these benthic organisms.