Hydrologic regime influences on the style of riffle-pool, plane bed, and step-pool sequences

Riffle-pool, plane bed, and step-pool geomorphic units represent archetypical, instream river forms that exist across a number of hydrologic settings. Given the common existence of these geomorphic units, previous research has identified distinct relationships between channel attributes with these characteristic forms. However, contrasting hydrologic settings likely reflect different flood and disturbance regimes important to form and its maintenance. Therefore, variability in archetypical form may reflect the influence of the defining hydrologic regime. The objective of this study was to determine if stream channel attributes under differing hydrologic regimes within a single basin exhibit statistical differences for riffle-pool, plane bed, and step-pool sequences. Sites from five different hydrologic regimes within the Sacramento River Basin, California, USA, were classified as riffle-pool, plane bed, or step-pool sequences based on statistically significant differences in surveyed channel attributes. The channel attributes within each respective geomorphic unit type were then tested for statistical independence by hydrologic classification. While variability of channel attributes between geomorphic unit types is dominant (e.g. plane bed compared to riffle-pool), a number of statistically significant differences were found within geomorphic unit types of differing hydrology (e.g. snowmelt vs. winter rainfall step-pool streams). Results from this study suggest hydrologic regime can inform the understanding of certain channel attributes of characteristic river forms; however, the attributes are no doubt also influenced by local and basin physiographic settings.