Nehalem River Watershed Assessment 1 6.0 Sediment Sources

Fish and other aquatic life have evolved to deal with a certain amount of sediment within streams. Some erosion occurs naturally within a watershed. The amount of erosion and the subsequent delivery of sediment to streams varies considerably during the year. The largest amount of sediment is moved during high flow events. Land-form changes can occur during heavy precipitation and snowmelt events such as in the February, 1996 flooding. The challenge is to separate human-caused erosional processes from those that would naturally occur within a watershed. For this assessment, potential sources of sediment were evaluated. Rural road instability, slope instability (not related to roads), and rural road runoff were identified as potential sediment sources. It was determined, according to GWEB guidelines, that surface erosion from range land and crop land was not an issue since all the areas of these land uses are located in low sloped areas (all less than 50 % slope). Also, since the total urban area in the watershed is only 0.5 % and no major tributaries receive drainage mostly from urban sources, urban runoff was determined as very low priority and not an issue for this assessment