Scientists generally agree that one of the biggest factors influencing stream temperatures is the removal of vegetation, in particular trees, from beside the stream. The riparian area, the upland area next to a stream, has a critical effect on the quality of the habitat in the stream. In the northwest, upland areas next to streams have been vegetated, and in most areas, forested, for thousands of years. Trees and other vegetation in the riparian zone provide nutrients in the form of leaves and insects. The roots of plants in the riparian zone reduce the erosive force of the water, and stabilize streambanks.
Trees shade the stream, and help maintain cool water temperatures.
Wetlands in the riparian area store nutrients, store flood waters, filter out pollutants, and provide refuges for small fish during storms and floods, when the water flow in the channel is too strong for them to withstand. In short, the riparian area is a very important component of the entire stream landscape. Alteration of a riparian area will almost invariably have an effect on the stream itself. To say it another way, the way we have used watersheds has significantly reduced their ability to sustain natural salmon populations. Usually one single action is not to blame, but rather the cumulative effects of all the activities in the watershed are to blame.