Canyons with undulating walls

Channels incised into cohesive substrates commonly develop undulating walls characterized by regular repetitive variations in channel width. Canyons w ith undulating walls are found in a variety of lithologic, climatic, and te ctonic regimes, and are most commonly present downstream from a knick zone of active incision. Flume experiments and natural channel morphologic patte rns lead us to infer that wall undulations are the remnants of breached, of fset 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, me found that intact rock strength, rock -mass strength, joint orientation and spacing, bedding, and porosity did no t differ significantly between channel reaches with straight walls, undulat ing walls, and meanders. The wavelength and amplitude of wall undulations a lso did not correlate with any substrate characteristic. These results lead us to conclude that the primary control on formation of undulating walls i s hydraulic processes. Along the study channels, deep, narrow reaches with undulating walls altern ate with wider straight or meandering reaches. Using flume simulations and one- and two-dimensional hydraulic modeling, we found that the wall undulat ions 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 undulat ions are preserved following knickpoint incision. The wall undulations then act to regulate downstream energy expenditure in a manner analogous to bed forms. This implies that generalized principles of uniform energy expenditu re developed for alluvial channels may also apply at the reach scale to bed rock channels with relatively homogeneous substrates.