Ecological Processes

Depending on the degree to which road crossings constrict the river or stream channel, crossing structures can change the hydrology and hydraulics of the system by increasing the detention time of water upstream of the crossing. The more crossings on a particular river or stream the greater the potential impact on hydrology. If the changes in hydrology are significant enough, it can potentially result in changes in sediment transport and natural scouring of the channel during storm events or spring floods.

Large woody debris is an important component of stream ecosystems. Large logs in the stream can dam up water or create plunge pools on the downstream side of the log. Accumulations of woody debris can change the local hydraulics of the stream, scouring some areas and depositing the material in other places. Where woody debris forms dams across the stream they can create large and relatively deep pools. These habitat features (woody debris, scour holes, pools, deposited gravel) are important characteristics of habitat. However, they are not permanent features and woody debris will eventually break up or move downstream. When crossing structures restrict the ability of woody debris to pass downstream, road crossings can inhibit or prevent the formation of natural debris dams and deflectors that are important habitat features for fish and wildlife, and play an important role in shaping channel characteristics.

The movement of organisms within rivers and streams is an important ecological process that can be significantly affected by road crossings. If not properly designed, road crossings can block animal movements, delay migration (a process made worse where there are many crossings), and cause physiological stress as animals expend energy passing both natural and artificial obstacles. Delays in movement can also result in overlap of individuals that typically occupy different stream reaches. For example, culverts often concentrate migrating fish in large pools at their outlets. These pools often provide resident fish habitat, and these fish can experience increased predation or competition from upstream migrants when this overlap occurs. Increased susceptibility to fishing pressure and stress associated with over-crowding can also occur when fish movements are delayed at road crossings.