Rivers and streams are more than just conduits for water and fish. They are long, linear ecosystems made up of the physical environment, communities of organisms, and a variety of ecological processes that shape and maintain these ecosystems over time. The long-term conservation of important aquatic resources (such as fish) requires the maintenance of healthy and ecologically viable ecosystems. Road crossings have the potential to undermine the ecological integrity of river and stream systems in a number of ways. To ensure the productivity and viability of river and stream ecosystems we must protect the quality of the physical environment (habitat), maintain intact communities of aquatic organisms, and take care not to disrupt critical ecological processes.