Fluvial erosion, transport and deposition (continued)

• Mixed bedrock-alluvial channels. Channels with thin, often discontinuous alluvial cover

  • Issues:
    • What controls overall rate of erosion? Rate of bedrock erosion or sediment flux divergence?
    • What determines channel gradient? Necessity to erode bedrock or necessity to transport sediment?
    • Importance of locally-derived coarse alluvium

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Many natural stream channels, primarily ones transporting mostly gravel, are mixed bedrock-alluvial channels, which are defined as a thin, often discontinuous alluvial cover only a few grain diameters thick over bedrock. The important issues include: is the overall rate of erosion, as well as the channel gradient, determined primarily by the need to transport the alluvial cover or by the necessity to erode bedrock? Locally derived, coarse alluvium shed from valley walls or steep tributaries commonly dominates the channel bed in mixed bedrock-alluvial systems. The discussion here largely follows the more extended presentation in Howard (1998, op. Cit.).