The riparian zone refers to the border of moist soils and plants that exist next to a body of water. The riparian zone can be composed of gently sloping shores, steep banks, or other types of terrain. From the perspective of a bird traveling down the Columbia, the riparian zone would change many times. These changes come about because of geologic differences, altitude, varied river flow, and the types of organisms and vegetation that can survive and thrive from area to area. As these forces interact, the nature of the river and the species which migrate to it change in concert with one another.
As the Columbia and its tributaries move downstream, the channel widens, the gradient flattens, and the water slows. In these areas more permanent plant species can survive, such as tree and shrub communities and specialized grasses and forbs (non-grasslike herbs). These plant species, in turn, provide food and shelter for the rich diversity of wildlife living along the riverbank.
Elk, deer, bear, sheep, and mountain lions are examples of animals that feed in these relatively lush riparian zones. Other smaller animals that live and feed along this biologically vital corridor include birds (like the ring-necked pheasant, grouse, geese, falcons, great blue herons, hummingbirds and warblers), small mammals (such as longtail weasel and striped skunk), reptiles (garter snake and the western painted turtle), and amphibians (red-legged frog and the Pacific giant salamander). Many threatened, endangered, or sensitive species are also frequent visitors to riparian zones. Examples include the bald eagle, peregrine falcon, and kit fox.
The existence of riverside vegetation is also important to the health of the species that live within the river. Specifically, this vegetation helps maintain a river’s health by influencing the amount and kind of sediment in the river. Riverside vegetation does this by anchoring soil, catching silt, filtering out pollutants, and absorbing nitrogen and phosphorus. Such vegetation also helps provide shade that cools the water and provides habitat for insects and their predators.
The effect of too much sedimentation can be seen when vegetation along riverbanks is removed by flooding or another event. Sediment washes back into the water, causing turbidity. Turbidity occurs when sediment is stirred up and suspended in water, and in a river can impair the respiration of fish or other aquatic organisms. Turbid conditions can also cause sediment to cover gravel used for fish-spawning, raise the temperature of the water, and bury submerged plants.