Channels incised into cohesive substrates commonly develop undulating walls characterized by regular, repetitive variations in channel width. Canyons with undulating walls are found in a variety of lithologic, climatic, and tectonic regimes, and are most commonly present downstream from a knick zone of active incision. Flume experiments and natural channel morphologic patterns lead us to infer that wall undulations are the remnants of breached, offset potholes or of sinuous longitudinal grooves formed during incision of the knick zone. Substrate characteristics seem to have little influence on these undulations. Using measurements from eight channels incised into the Navajo Sandstone of southern Utah, we found that intact rock strength, rock-mass strength, joint orientation and spacing, bedding, and porosity did not differ significantly between channel reaches with straight walls, undulating walls, and meanders. The wavelength and amplitude of wall undulations also did not correlate with any substrate characteristic. These results lead us to conclude that the primary control on formation of undulating walls is hydraulic processes.
Along the study channels, deep, narrow reaches with undulating walls alternate with wider straight or meandering reaches. Using flume simulations and one- and two-dimensional hydraulic modeling, we found that the wall undulations act to reduce interreach (spatial) variability in energy expenditure, and to minimize energy expenditure within a reach. We propose that the wall undulations and flow hydraulics create a feedback whereby the wall undulations are preserved following knickpoint incision. The wall undulations then act to regulate downstream energy expenditure in a manner analogous to bedforms. This implies that generalized principles of uniform energy expenditure developed for alluvial channels may also apply at the reach scale to bedrock channels with relatively homogeneous substrates.