Thresholds in River Regimes

The bed of natural stream channels can be composed of bedrock, fine-grained alluvium, or a coarse-grained gravel pavement. Alluvial channels are characterized by one of two types of beds: oarse gravel or cobbles that move only near bankfull discharge, and sand-bed channels in which transport occurs at all but perhaps the lowest flows. The role of discharge in sediment transport and bed erosion will be represented by a dominant discharge, that is, a steady discharge that performs same action as the natural sequence of discharges. Natural stream channels are primarily alluvial, that is, they have at low flow stages a bed composed of unconsolidated sediment transported into place by the stream. The response of rivers to a natural or man-induced change in the hydraulic regime will depend upon preexisting nature of the bed. Under certain circumstances a small or gradual change in hydraulic regime can trigger a change to a different channel type.

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