18 Sediment storage and transport in coarse bed streams: scale considerations

Abstract Bedforms in gravel-bed rivers range in size from 10 −2 to 10 3  m and have a wide range of affects on sediment transport and channel stability. To study the affect of these bedforms on sediment transport a classification scheme is proposed that breaks the bedforms into micro, meso, macro and megaforms. Then using two creeks in British Columbia and one in California, the trend of sediment transport rates is related to the bed state, sediment and wood storage and the associated bedforms are discussed. At small spatial and temporal scales (micro/mesoform scales), variability in sediment transport rate can be ascribed to the changing state of the bed, which largely depends on the sediment supply regime. Stabilizing bedforms develop when sediment supply is low, and reduces the depth of the bed active layer and the mobility of the grains, thereby decreasing the sediment transport rate. Sediment rating relations in a low sediment supply channel are steep and are believed to be extremely sensitive to small changes in sediment supply and flow regime when compared to higher sediment supply systems. High sediment supply suppresses the development of stabilizing bedforms and increases the mobility of grains and the depth of the active layer. At the reach scale (macroform scale), high sediment mobility is shown to produce complex cycles of aggradation and degradation that can persist for decades. In-channel woody debris can strongly influence the timing and magnitude of these aggradation–degradation cycles and also has an important effect on the development of megaforms.