Slope canyons converging with equal elevations suggest channels obey Playfair's Rule

What is it about?

Landscapes in which tributary channels are 'hanging' (such as in glacial landscapes) imply that the last or most prominent erosion occurred in the main (trunk) channel. In the dendritic networks of channels in the USA Atlantic continental slopes, the tributaries were found to meet trunk channels at the same elevation, effectively obeying Playfair's Rule. Playfair originally noted this tendency in rivers, which he speculated was due to the enhanced erosion that would occur in a tributary channel to restore it to the trunk channel elevation. Here it implies either that erosion down the channels by sedimentary flows has led to this state or that erosion has reached beds of greater resistance.

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Photo by Leon Liu on Unsplash

Photo by Leon Liu on Unsplash

Why is it important?

By noting where submarine channels obey Playfair's Rule, the time-averaged erosion rates (if caused by on-going erosion) should be the same on each pair of channels meeting at the confluences. This allows techniques used in subaerial geomorphology to be applied to investigate the implied effects of channel catchment area and gradient, which were found to appear similar to those of rivers even though the erosion mechanisms are not necessarily similar.