The shape of alluvial channels in relation to sediment type

Abstract

The weighted mean percent silt-clay in the channel and banks of stable alluvial stream channels is used as a parameter (M) descriptive of the physical characteristics of sediment. Silt-clay is defined as alluvial material smaller than 0.074 mm. As the percentage of silt and clay in banks and channel increases, the shape of stream channels expressed as a width-depth ratio (F) varies according to the equation, F=255 M-J.Os Neither mean annual discharge nor the mean annual flood significantly affects this relation in spite of the importance of discharge to the absolute width and depth of a channel. Downstream changes in width and depth of a stream channel are greatly influenced by sediment type. As M increases downstream along a given river, the depth increases more rapidly and the width less rapidly with discharge than if M was constant, and width-depth ratio decreases. Conversely, as M decreases downstream the depth increases less rapidly and the width more rapidly with discharge than if M was constant, and width-depth ratio increases. The downstream changes in width, depth and width-depth ratio along the Smoky Hill-Kansas River system is presented as an example of the importance of sediment type to stream regimen.