Development of river morphologic stability index (RMSI) to assess mountain river systems

Study region: Five mountain rivers (Rasau, Congkak, Sekayu, Nal, and Lata Payung) in Malaysia. Study focus: Stability of mountain rivers is essential to maintain the natural landscape of fluvial morphology amid climate variability and anthropogenic stresses. However, rapid assessment tools for characterizing the channel stability of mountain river systems are lacking. In this study, a River Morphologic Stability Index (RMSI) is developed and used to evaluate the stability of 32 cross-sections in five mountain rivers in Malaysia. The performance of this index is compared with that of five conventional indexes. New hydrological insights for the region: The analysis indicates that ten crucial parameters (and their weightages) are key to represent the dynamic changes of river landscapes: channel form (0.16), channel dimension (0.14), substrate (0.13), channel pattern (0.12), bank slope (0.09), bank erosion (0.08), channel adjustment (0.08), artificial feature and structure (0.07), degree of constriction (0.07), and vegetative cover (0.06). These parameters formed the basis of the development of RMSI. For each parameter, its ratings and weightages were defined to differentiate between the excellent and worst river conditions. The final RMSI score was grouped into four categories: highly stable (0 %–25 %), moderately stable (26 %–50 %), moderately unstable (51 %–75 %), and extremely unstable (76 %–100 %). Results indicated that 22 of all the 32 cross-sections concerned were within the ‘very stable’ category.