Author(s): Andrew W. Tranmer; Daniele Tonina; Peter Goodwin; Rohan Benjankar; Matthew G. Tiedemann
Keywords: Stream power; Canyon morphology; Floodplain; Flow energy; Thresholds
Abstract: Shear stress and stream power have long been metrics for the stability of alluvial streams, but little is still known about their variation within a watershed. Large spatial variability in longitudinal stream power has been reported for streams in the USA, Australia and UK. All these previous studies attribute the variability in stream power to controls imposed by the width of the alluvial valley and floodplain, however rely on field data that are difficult to obtain at high densities and are time consuming. This study examines stream flows that were modeled with a 1-dimensional hydraulic model supported with cross sections extracted from a LiDAR point cloud and spaced at approximately one channel width (30m). The dependence of the variation in longitudinal distribution of shear stress and unit stream power on valley width was tested in the Deadwood River, a laterally confined stream flowing within an alluvial canyon in central Idaho (USA). Results show that shear stress and unit stream power during an annual snowmelt season can attain 260 N/m2 and 600 W/m2, well in excess of the catastrophic criterion of 100 N/m2 and 300 W/m2 cited in the literature, yet local channel conditions allow for marginal floodplain formation within the fluvial system. Floodplain development and maintenance in sediment supply limited systems were found to fundamentally depend on sediment availability from tributary inputs and accommodation space provided by increased canyon width. These metrics are examined in relation to 16 identified geomorphic units and correlated with the presence or absence of floodplains to identify what the catastrophic values are for canyon environments and what conditions foster floodplain sustainability.