On the stability of step-pool mountain streams

Although step-pools are generally considered to be stable bedforms, stability is not absolute, but depends on size, scale, and perspective. Hydraulic analysis of the stability of step-pool sequences in the Santa Monica Mountains, California, indicates that they are active channel features that are generally restructured within 5 to 100 years. The degree of mobility depends on step particle size. Steps are stable within small temporal and spatial scales, where they function as independent variables that dissipate stream energy and regulate channel hydraulics, but stability decreases at larger scales, where step pools become dependent variables that respond to discharge and sediment load. Hence the role of step-pools changes from energy dissipation to one of channel adjustment. These results underscore the need to consider larger spatial and temporal scales in order to reveal the complete function and significance of step-pools in mountain fluvial systems.