Waterfall alteration of bedrock channel form in the San Gabriel Mountains

Waterfalls play a critical role in landscape evolution. Waterfalls can control the rate and style that new relief is introduced into a landscape, and, when waterfalls self-form, they can alter channel erosion rates and longitudinal profiles. Here we investigate how waterfalls alter channel form and whether waterfalls might alter erosional processes in adjacent (waterfall-free) reaches and hillslopes. We examine channels located upstream, at, and downstream of waterfalls using channel width, channel slope and waterfall locations extracted from 93 basins in the Front Range of the San Gabriel Mountains in California, USA. While these mountains are thought to be in a large-scale erosional steady state, we find that many channels are in a transient state of adjustment due to the presence of waterfalls. Our results show that waterfalls increase channel slope (up to a factor of 20) and narrow channel width (up to a factor of 3), not only at the waterfall itself, but also up to 100 m upstream and downstream of the waterfall.  We explore the influence of waterfall retreat rate on changes in channel form by examining waterfalls that occur across a range of drainage areas (which has been shown to scale with retreat rate). We demonstrate that waterfalls occurring at larger drainage areas have approximately twice the effect on changes in channel width, and may cause channel narrowing four times as far downstream as those at smaller drainage areas. These findings highlight the potential to estimate waterfall retreat rate from spatial changes in channel form.  Through examining the different ways that waterfalls alter channel processes, this research illustrates that rivers may be out of equilibrium on a small scale even when they are at a large-scale steady state.