HR: 0800h
AN: H41D-0764    [Abstracts]
TI: Valley Width Variation Controls on Riffle Location and Persistence on a Gravel Bed River
AU: * White, J Q
EM: jqwhite@ucdavis.edu
AF: Department of Land, Air, and Water Resources, University of California at Davis, One Shields Avenue, Davis, CA 95616-8627, United States
AU: Pasternack, G B
EM: gpast@ucdavis.edu
AF: Department of Land, Air, and Water Resources, University of California at Davis, One Shields Avenue, Davis, CA 95616-8627, United States
AB: Numerous studies have examined the effect of constrictions on the formation of riffles and pools in rivers. Low velocity backwater effects are created above the constriction, with steep water surface slopes and flow acceleration after it. Such flow conditions cause varying sediment transport capacity, influencing the formation of riffles and pools near constrictions. Studies have shown that local bedrock outcrops, alluvial fans, sediment bars, and wood are capable of functioning as constrictions, but few address channel-confining valley walls as potential constrictions. Can variations in valley width influence riffle location and persistence? To address this question, research was performed on a valley-confined wandering gravel-bed reach of the regulated lower Yuba River from the Narrows Pool to the Highway 20 bridge. This reach has an average valley width of 164 m (range of 102-314 m). During 1853-1884, hydraulic gold mining filled this 7-km reach with ~26 m of mixed coarse sediment (totaling ~9-18 million cubic meters), creating a dynamic fluvial landscape comparable to recently deglaciated wandering gravel-bed streams. The hypothesis of this study was that riffles that persist over decades on wandering gravel-bed rivers are controlled by valley width variations. The study objectives were to (1) assess planform and elevation changes over 22 years and (2) correlate locations of persistent riffles with valley- wall constrictions. ArcGIS was used to measure valley width and changes in planform wetted area in georectified photo sets of this reach for 1984, 1986, 1991, 1996, 2002, 2004, 2005, and 2006. Detailed river valley DEMs were also available for 1999 and 2006 for measuring the recent incision rate, and pattern of the valley fill. Riffle crest locations for each year were plotted along with the valley width versus distance upstream, and all riffles crests that persisted for the entire duration were identified. The persistent riffle crests were statistically analyzed to various metrics of their associated downstream valley wall constriction. The aerial photo analysis revealed that over the past 22 years, the river has experienced significant lateral change in response to blocked sediment input, and frequent floods. River valley DEM differencing revealed that over the 7-year period, 463,000 cubic meters of sediment were scoured out of the reach, with an overall lowering of valley fill elevation. Despite the dramatic changes that have occurred over the past two decades, 7 riffles persisted in the same location. Among the metrics analyzed, a strong linear correlation was found between the riffle crest location and the midpoint between downstream and upstream constrictions. The seven riffles crests, that persisted, were within an average of 0.9 channel widths (standard deviation of 0.6 channel widths) of the midpoint between the downstream and upstream constriction. Wandering gravel-bed rivers provide complex aquatic habitat, and identifying controls on riffle locations and persistence, such as valley constrictions, will aid in managing aquatic species.
DE: 1820 Floodplain dynamics
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2007 Fall Meeting