Particle Dynamics: Bedrock versus Alluvial River Segments

Abstract:

Many channels alternate longitudinally between bedrock and alluvial substrate. These alternations occur over a range of spatial scales and associated temporal scales. Transient bedrock and alluvial patches alternate over downstream distances of a few meters to hundreds of meters, whereas persistent bedrock and alluvial reaches alternate downstream over distances of kilometers to hundreds of kilometers. These longitudinal alternations are significant because of the differences in process and form between bedrock and alluvial reaches. Bedrock reaches limit the response of the channel and the greater drainage basin to relative base level fall. Alluvial reaches limit the rate and distance of particle movement downstream, as well as limiting the habitat available for riverine organisms, biogeochemical reactions and nutrient storage, and water quality. In both types of substrate, particle movement is a limiting factor. (Here, particles include mineral sediment and particulate organic matter.) In bedrock channels, particle movement largely governs the rate and manner of erosion. In alluvial channels, particle movement governs channel form and the stability of habitat. Fundamental research questions for both channel types center on particle dynamics: How do interactions among bedrock substrate, sediment supply, sediment transport, and hydraulics influence rates of bedrock erosion? How do interactions among sediment supply, sediment transport, and biota influence particle transport and residence time? Although bedrock channel segments likely exert a more fundamental influence on river response to relative base level change and landscape evolution, alluvial channel segments likely exert a stronger limiting effect on downstream fluxes of water, solutes, and particles, as well as more critical influences on riverine habitat.