The role of waterfalls and knickzones in controlling the style and pace of landscape adjustment in the western San Gabriel Mountains, California

Abstract

Bedrock rivers set the pace of landscape adjustment to tectonic and climatic forcing by transmitting signals of base-level change upstream through the channel network and ultimately to hillslopes. River incision is typically modeled as a monotonic function of bed shear stress or stream power, modulated by sediment tools and cover effects, but these models do not apply in channels with steep or vertical bedrock reaches due to changes in fl ow dynamics, hydraulic geometry, and bed cover. Here, we investigate how such knickzones (oversteepened channel reaches often containing waterfalls) infl uence the propagation of slope-break knickpoints that separate relict from adjusting topography, and thus the response times of landscapes to external forcing. We use a conceptual long-profi le model to explore the consequences of waterfalls and knickzones on channel response and compare predictions to light detection and ranging (LiDAR) topography, fi eld observations, and cosmogenic radionuclide data from Big Tujunga Creek, a 300 km 2 watershed in the San Gabriel Mountains, California. Three prominent knickzones along Big Tujunga Creek, characterized by numerous waterfalls, show contrasting behavior. For the upper knickzone, waterfalls align with bands of harder rock exposed on adjacent hillslopes, and between waterfalls, the channel is mantled by large (>2 m) boulders, indicating knickzone retreat is slow compared to predictions of slope-break knickpoint retreat from stream-power models, enhancing the preservation of an upstream relict landscape. The middle knickzone shows evidence for both fast and slow knickzone retreat, as well as signifi cant deviations from predictions of uniform tributary knickpoint elevations derived from stream-power models. The lower knickzone is characterized by a waterfall and knickzone within an incised inner gorge that provide evidence of rapid retreat relative to background channel incision. Overall, we fi nd a pattern of decreasing knickzone and waterfall retreat rate with distance upstream of the range front, beyond decreases predicted by simple area-dependent celerity models. Our results highlight that waterfalls and knickzones can both enhance and inhibit landscape adjustment, leading to divergent controls on the pace of landscape evolution.

Bedload transport controls bedrock erosion under sediment-starved conditions

“…For comparable situations where mean values of bedload volumes and erosion rate on bare bedrock sections are available, this model can be easily calibrated by adjusting its prefactor K. Potential applications are steep bedrock channels in detachment-starved catchments (e.g., Wohl, 1998Wohl, , 1999, channel knickpoint sections with exposed bedrock such as waterfalls (e.g., Miller, 1991;Wohl et al, 1994;Cook et al, 2013;Mackey et al, 2014;DiBiase et al, 2015), high lateral bedrock sections above the channel or its banks (e.g., Hartshorn et al, 2002;Turowski et al, 2008) or even hydropower facilities that have to cope with natural sediment flux such as sediment bypass tunnels (Jacobs and Hagmann, 2015).…”

River profile response to normal fault growth and linkage: an example from the Hellenic forearc of south-central Crete, Greece

“…All of the waterfalls occur in Mesozoic carbonate bedrock that is massive or has bedding that gently dips upstream. One possible explanation for why these steep river channel segments appear to incise less efficiently than lower-gradient channels (e.g., n < 1) is that the mechanics of river incision change when vertical waterfalls form (Lamb et al, 2007;DiBiase et al, 2015). Previous research has shown that waterfall formation may either enhance or inhibit river incision rates and would result in an apparent n > 1 or n < 1, respectively.…”

“…This process can help explain why generation 2 knickpoints are not observed in basins 6-9; river channel segments have been oversteepened such that a change in rock uplift rate is insufficient to generate a new, lower knickpoint. If correct, this interpretation means that quantitative information on the timing of uplift events and uplift rates from river profiles can be obscured or erased from the river network as a result of changing incision processes due to waterfall formation (DiBiase et al, 2015).…”

“…It has been argued that these oversteepened sections of rivers may act to enhance or inhibit river incision, as well as to obscure tectonic and climatic information that may be encoded in river systems (Haviv et al, 2006;Lamb et al, 2007). For example, DiBiase et al (2015) showed that the development of waterfalls associated with transient slope-break knickpoints (Kirby and Whipple, 2012) related to increases in uplift rate observed along Big Tujunga Creek in the San Gabriel Mountains of California generally acted to slow knickpoint retreat rates at higher elevations in the catchment relative to predictions from detachmentlimited stream power models; in contrast, some low elevation waterfalls appear to travel faster than theoretical expectations.…”

Quantifying the controls on potential soil production rates: a case study of the San Gabriel Mountains, California

“…Clair et al, 2015). Given the pervasively fractured nature of bedrock in the SGM (e.g., Dibiase et al, 2015), I assume that changes in the stress state of bedrock or intact regolith near ridgetops lead to the opening of preexisting fractures (i.e., an increase in the bulk porosity of bedrock or intact regolith) rather than the fracturing of intact rock. I adopt the analytic solutions of Savage and Swolfs (1986), who solved for the topographic modification of regional compressive stresses beneath ridges and valleys oriented perpendicular to the most compressive-stress direction.…”