To maintain or restore the long-term viability of river and stream ecosystems, we must ensure that transportation systems are designed to protect habitat quality and ecosystem processes that maintain habitats and populations over time. When designing a road system, the first BMP is to carefully design the road system to minimize the number of river and stream crossings. Where crossings are necessary, a careful analysis of the area should be made so that they can be located away from sensitive areas (e.g. critical habitat, uncommon habitat types, areas particularly sensitive to disturbance). After minimizing the number of crossings and locating them away from sensitive areas, attention should be focused on the design of the crossing structure itself.
In order to preserve or restore all important elements of riverine ecosystems, crossing structures should be designed with the following objectives in mind:
Habitat quality: Avoid designs that will disrupt the natural hydraulic properties of the river or stream, and use of rip-rap or other armoring of channels or banks.
River and stream continuity: Maintain the continuity of the aquatic and benthic (stream bottom) elements of rivers and streams through the maintenance of appropriate substrates, water depths and flows. Provide appropriate bank-edge habitat for weak swimming and crawling animals.
Processes that maintain the channel and streambed: Maintain the natural slope of the stream bed and the capacity for sediment and woody debris to pass downstream through the crossing.
Fish and other aquatic organism passage: Facilitate movement for all fish species (including juvenile and relatively small resident fish) and other aquatic organisms such as amphibians, reptiles, and crayfish. Avoid physical barriers such as inlet or outlet drops, debris accumulation, weirs, baffles, or other structures that would block movement of aquatic organisms.
Wildlife passage: Facilitate movement of wildlife species including those primarily associated with river and stream ecosystems and others that may utilize riparian areas as movement corridors. Some species of wildlife, such as muskrat and stream salamanders, may benefit from the maintenance of river and stream continuity. Other species may require more open structures as well as dry passage along banks or within the streambed at low flow.