Reach-scale bankfull channel types can exist independently of catchment
hydrology
Abstract
Reach-scale morphological channel classifications are underpinned by the
theory that each channel type is related to an assemblage of reach- and
catchment-scale hydrologic, topographic, and sediment supply drivers.
However, the relative importance of each driver on reach morphology is
unclear, as is the possibility that different driver assemblages yield
the same reach morphology. Reach-scale classifications have never needed
to be predicated on hydrology, yet hydrology controls discharge and thus
sediment transport capacity. The scientific question is: do two or more
regions with quantifiable differences in hydrologic setting end up with
different reach-scale channel types, or do channel types transcend
hydrologic setting because hydrologic setting is not a dominant control
at the reach scale? This study answered this question by isolating
hydrologic metrics as potential dominant controls of channel type. Three
steps were applied in a large test basin with diverse hydrologic
settings (Sacramento River, California) to: (1) create a reach-scale
channel classification based on local site surveys, (2) categorize sites
by flood magnitude, dimensionless flood magnitude, and annual hydrologic
regime type, and (3) statistically analyze two hydrogeomorphic linkages.
Statistical tests assessed the spatial distribution of channel types and
the dependence of channel type morphological attributes by hydrologic
setting. Results yielded ten channel types. Nearly all types existed
across all hydrologic settings, which is perhaps a surprising
development for hydrogeomorphology. Downstream hydraulic geometry
relationships were statistically significant. In addition,
cobble-dominated uniform streams showed a consistent inverse
relationship between slope and dimensionless flood magnitude, an
indication of dynamic equilibrium between transport capacity and
sediment supply. However, most morphological attributes showed no
sorting by hydrologic setting. This study suggests that median hydraulic
geometry relations persist across basins and within channel types, but
hydrologic influence on geomorphic variability is likely due to local
influences rather than catchment-scale drivers.