The role of topographic variability in river channel classification

Abstract

To date, subreach-scale variations in flow width and bed elevation have rarely been included in channel classifications. Variability in topographic features of rivers, however, in conjunction with sediment supply and discharge produces a mosaic of channel forms that provides unique habitats for sensitive aquatic species. In this study we investigated the utility of topographic variability attributes (TVAs) in distinguishing channel types and dominant channel formation and maintenance processes in montane and lowland streams of the Sacramento River basin, California, USA. A stratified random survey of 161 stream sites was performed to ensure balanced sampling across groups of stream reaches with expected similar geomorphic settings. For each site surveyed, width and depth variability were measured at baseflow and bankfull stages, and then incorporated in a channel classification framework alongside traditional reach-averaged geomorphic attributes (e.g., channel slope, width-to-depth, confinement, and dominant substrate) to evaluate the significance of TVAs in differentiating channel types. In contrast to more traditional attributes such as slope and contributing area, which are often touted as the key indicators of hydrogeomorphic processes, bankfull width variance emerged as a first-order attribute for distinguishing channel types. A total of nine channel types were distinguished for the Sacramento Basin consisting of both previously identified and new channel types. The results indicate that incorporating TVAs in channel classification provides a quantitative basis for interpreting nonuniform as well as uniform geomorphic processes, which can improve our ability to distinguish linked channel forms and processes of geomorphic and ecological significance.

Integrated analysis of flow, form, and function for river management and design testing

“…Specific geomorphic attribute values were accurately represented by the synthetic morphologies, including channel dimensions, cross‐sectional geometry, depth and width variability, sinuosity, and slope (Figure ). The flow convergence routing mechanism was shown to occur in the pool–riffle archetype but not in the plane bed one, confirming that the two morphologies were capturing distinct geomorphic maintenance processes as distinguished by the Sacramento Basin channel classification (Lane, Pasternack, et al, ). Once a person understands how to produce a synthetic DTM with the required subreach‐scale variability to drive geomorphic processes and ecological functions, then the software implements that very quickly.…”

“…Two archetypal river corridor morphologies distinguished in the Sacramento Basin channel classification by Lane, Pasternack, Dahlke, and Sandoval‐Solis () were considered in this study as a proof of concept: semiconfined plane bed and semiconfined pool–riffle. These morphologies were selected for their common occurrence in midelevation montane environments (Montgomery & Buffington, ; Wohl & Merritt, ) and their similar channel dimensions and slopes contrasted by major differences in subreach‐scale topographic variability.…”

“…These morphologies were selected for their common occurrence in midelevation montane environments (Montgomery & Buffington, ; Wohl & Merritt, ) and their similar channel dimensions and slopes contrasted by major differences in subreach‐scale topographic variability. An existing field data‐driven channel classification for the Sacramento Basin (Lane, Pasternack, et al, ) provided the parameter values needed to synthesize the two archetypal morphologies, quantified as the median field‐surveyed values for each channel type.…”

“…The Cartesian stationing was scaled by w BF so that the actual distance was given by x i = x r * w BF . The sinusoidal function alignment parameters were adjusted iteratively to achieve desired values for h BF , width‐to‐depth ratio ( w / h BF ), sinuosity, and the coefficient of variation (CV) of w BF and h BF based on plane bed and pool–riffle channel classification archetypes (Lane, Pasternack, et al, ). Floodplain confinement, the bankfull to floodplain width ratio, was used to set valley width and overbank topography.…”

“…Because river classifications traditionally aim to capture the central tendency of river types at the reach scale, they contain little to no information on subreach‐scale topographic variability and landform patterning (Lane, Pasternack, et al, ). This study used outputs from a channel classification that was unique in including statistical characterization of subreach width and depth variability using the metric of CV based on the average and standard deviation of field‐derived values (Lane, Pasternack, et al, ). However, there remained numerous landform patterning permutations using the control function parameters of Equation that could yield those CV values, many associated with profoundly different geomorphic processes.…”