The geomorphic significance of step–pools in mountain streams

This paper develops a conceptual model to define the changing role and significance of step–pools as energy dissipators in steep mountain channels. Although energy dissipation is a significant function of step–pools, this role changes over time with variations in discharge. Steps are effective in reducing stream energy at low flows, but their effectiveness diminishes with increasing stage. Accordingly, the manner in which energy dissipation occurs also varies. With increasing flows, spill resistance gives way to a dominance in form and grain resistance. The conceptual model for the changing role of step–pools is illustrated with data from hydraulic analysis and modeling of step–pools in the Santa Monica Mountains of California. The model points to the importance of the size of steps in determining their role in energy dissipation and in their interactions with channel hydraulics. The model offers a new articulation for the geomorphic significance of step–pools in mountain streams, and it serves as a useful template for a more complete understanding of step–pools over a longer time scale.