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I I I I I I I I I I I I I I I I I I I GEOMORPHIC RISK Stream instability, either as lateral migration, deposition, or scour, is the prevailing geomorphic hazard for river corridors. The following procedure uses geomorphic relationships for stream form and channel geometry to determine the type and condition of the stream. The approach is to evaluate a river corridor at several scales, beginning with an assessment of large scale features and then focusing on local conditions. METHODOLOGY The methodology consists of three steps: 1) Identify the geomorphically active area; 2) Determine the chance of instability hazards; and 3) Assess the damages in the stream corridor. GEOMORPHICALL Y ACTIVE AREA. Data on the lateral extent of the geomorphically active stream environment can be gathered from published topographic, soils, and geology maps and other sources of earth surface data. Field inspection by a qualified professional with experience and technical knowledge of geomorphology, flood hydrology, and sedimentation is recommended. The stream form relationships presented in the previous section provide tools for estimating the lateral extent of the geomorphically active area. The longitudinal extent of the area must include significant changes in any of the following important variables: stream grade, water discharge, sediment load, channel bed and bank material, and geologic influences. The importance of stream grade is strongly emphasized. In most case, changes in water discharge, sediment load, bed and bank material, and geology are manifest in the stream grade. Changes in longitudinal grade should be carefully evaluated. Lane's basic relationship for channel stability aids in the interpretation of longitudinal variation in a stream. Q S . Q, D;o EQ.10 where Q is the water discharge, Q, is sediment transport, S is stream grade, and D;o is mean bed material size. The following table summarizes some of the possible causes for stream grade changes. 12