Downstream variations in the width of bedrock channels

Field surveys of channel width w and drainage area A in bedrock channel reaches reveal relationships where w 5 cA, similar to the classic hydraulic geometry of alluvial channels. Data from five mountain channel networks support the assumption used in many landscape evolution models that an alluvial hydraulic geometry relationship where b 5 0.3–0.5 holds for bedrock channel systems. Although there is substantial local variability in channel width in bedrock channel systems, there is no systematic difference in width versus drainage area relations for the surveyed bedrock and alluvial reaches in sedimentary lithologies in coastal Oregon and Washington. In contrast, bedrock channels were narrower, and therefore had deeper flow, than alluvial channels with equal drainage areas in the granite and limestone terrain of the Yuba River, California. In addition, data from the Mokelumne River show that bedrock channel width decreases substantially downstream at the contact between relatively weak limestone and more erosion-resistant granite, but that channel slope does not change appreciably across contacts between these two lithologies. Data from coastal Oregon drainage basins further show systematic channel widening after flood flows and debris flow impacts. We conclude that downstream variations in the width of bedrock channels generally follow traditional hydraulic geometry relations but also reflect the local influence of longitudinal patterns of bedrock erosivity and disturbance history.