Replacement of natural streambed and banks with an artificial crossing structure will usually result in direct loss of some habitat value. Culvert crossings provide very little habitat within the culvert. Some habitat can be provided if the culvert is sufficiently embedded that substrate in the culvert resembles that in the natural streambed. Open-bottom or arch culverts and bridge crossings often maintain natural streambeds, although some habitat may be lost to footings, piers, and abutments. Fords may or may not significantly affect habitat at the crossing, depending on how much the streambed and banks were altered to create the ford.
Erosion and sedimentation are two significant impacts of road crossings. This often occurs during construction if “best management practices” (BMPs) are not used, but can also occur even when BMPs are in place. On-going erosion of embankments, the road surface, and drainage ways are of more long-term concern. Excess sedimentation degrades river and stream habitats by increasing suspended solids in the water and altering downstream substrate and channel characteristics. Increased turbidity in the water can adversely affect visual predators and increase the amount of inorganic particles (relative to organic particles) available to filter feeders downstream. Drainage from the road surface may contain contaminants that are toxic to aquatic organisms.
Crossing structures often have hydraulic effects on stream systems. Where crossings constricts the stream or river, water typically ponds upstream and may result in the accumulation of sediment above the crossings. Below crossings, increased velocities caused by the constriction can scour streambeds, creating scour pools and removing all but the coarsest substrate from channels. Scouring at the downstream end of these crossings may undermine the culvert or necessitate rip-rap or other armoring techniques to prevent scouring. Such scouring can also result in drops at culvert outlets that function as barriers to animal movement. Ultimately, these conditions result in habitat degradation both upstream and downstream of the crossing and can increase the risk of road failure with potentially catastrophic effects on stream, floodplain, and riparian habitats.