Slide 6 of 30
Notes:
Simple theoretical arguments would suggest that mixed bedrock-alluvial channels should be rare. In this diagram the gradient of bedrock and alluvial channels (for a given upstream supply of water and sediment) is plotted as a function of the rate of local downcutting of the channel. The gradient of alluvial channels is not a strong function of gradient, but that of bedrock channels should be if erosion is proportional to shear stress or stream power per unit width. This suggests that areas of rapid erosion and steep gradients should be bedrock (C), and areas of low gradient and low erosion rates should be alluvial(A), with only a narrow zone (B) in which mixed alluvial, bedrock channels should be expected. But, in fact, mixed bedrock alluvial channels are common, and often the occurrence of bedrock and alluvial channel sections is reversed from the above expectation, as is shown in the following slides.
