A Stream Channel Stability Assessment Methodology By

Various definitions of stream channel stability are presented including "the natural stable channel", the graded river, dynamic equilibrium, and regime channels, and a quantitative assessment methodology is presented that distinguishes between stability states. The assessment procedure involves a stream channel stability prediction and validation methodology on a hierarchical framework. The stream channel stability method develops fieldmeasured variables to assess: 1) Stream state or channel condition variables, 2) Vertical stability (degradation/aggradation), 3) Lateral stability, 4) Channel patterns, 5) Stream profile and bed features, 6) Channel dimension factor, 7) Channel scour/deposition (with competence calculations of field verified critical dimensionless shear stress and change in bed and bar material size distribution), 8) Stability ratings (modified Pfankuch method) adjusted by stream type, 9) Dimensionless ratio sediment rating curves by stream type and stability ratings, and 10) Selection of position in stream type evolutionary scenario as quantified by morphological variables by stream type to determine state and potential of stream reach. The stability assessment is conducted on reference reach (stable) reaches and a departure analysis is performed when compared to an unstable reach of the same stream type. The assessment procedure utilizes various hierarchical levels for prediction and subsequent validation. Changes in the variables controlling river channel form, primarily streamflow, sediment regime, riparian vegetation, and direct physical modifications can cause stream channel instability. Separating the difference between anthropogenic versus geologic processes in channel adjustment is a key to prevention/mitigation/restoration of disturbed systems The adverse consequence of stream channel instability (dis-equilibrium) is associated with increased sediment supply, land productivity change, land loss, fish habitat deterioration, changes in both short and long-term channel evolution and loss of physical and biological function.