Channelized and hillslope sediment transport and the geomorphology of mountain belts

Abstract

This paper uses the results of landscape evolution models and morphometric data from the Andes of northern Peru and the eastern Swiss Alps to illustrate how the ratio between sediment transport on hillslopes and in channels influences landscape and channel network morphologies and dynamics. The headwaters of fluvial-and debris-flow-dominated systems (channelized processes) are characterized by rough, high-relief, highly incised surfaces which contain a dense and hence a closely spaced channel network. Also, these systems tend to respond rapidly to modifications in external forcing (e.g., rock uplift and/ or precipitation). This is the case because the high channel density results in a high bulk diffusivity. In contrast, headwaters where landsliding is an important sediment source are characterized by a low channel density and by rather straight and unstable channels. In addition, the topographies are generally smooth. The low channel density then results in a relatively low bulk diffusivity. As a consequence, response times are greater in headwaters of landslide-dominated systems than in highly dissected drainages. The Peruvian and Swiss case studies show how regional differences in climate and the litho-tectonic architecture potentially exert contrasting controls on the relative importance of channelized versus hillslope processes and thus on the overall geomorphometry. Specifically, the Peruvian example illustrates to what extent the storminess of climate has influenced production and transport of sediment on hillslopes and in channels, and how these differences are seen in the morphometry of the landscape. The Swiss example shows how the bedding orientation of the bedrock drives channelized and hillslope processes to contrasting extents, and how these differences are mirrored in the landscape.

Relationships between landscape morphology, climate and surface erosion in northern Peru at 5°S latitude

“…8f that dissects an east-facing hillslope in a straight way, which is characteristic for a channel-forming debris flow event. In contrast, the Sechura desert did not experience gullying and substantial erosion despite the facts that (1) this region experiences maximum precipitation during El Niños, (2) this area is made up of a thick layer of unconsolidated sediment prone to erosion (Schneider et al 2008) and that (3) this segment generally displays a high surface roughness (caused by e.g., the dune fields). We explain this lack of response compared to the uplands by the absence of a linked channel network.…”

“…Area B is located in the mountainous region to the north-east of the Meseta, where a network of deeply dissected channels is visible (Fig. 1b, Schneider et al 2008). This segment shows high magnitudes in the local mean difference map (Fig.…”

“…The upper termination of the headwaters is defined by a sharp scarp-line, which-at closer inspection-is made up of coalescing concave geometries. These features were considered by Schneider et al (2008) to have been formed by landsliding (Fig. 6c).…”

“…Mass movements were the origin of granular flows that are a common sediment transport mechanism in this part of the Andes. Because these erosional processes have only affected the regolith, and since exposure of bedrock is rarely observed, Schneider et al (2008) assigned a transport-limited sediment flux regime to the channel network in this uppermost segment.…”

“…In recent publications, Schneider et al (2008) used geomorphic maps and morphometric data to reconstruct the nature of surface erosion and sediment transport in the Piura drainage basin. According to these authors and observations in the field, the headwaters and the transverse tributaries of this drainage basin comprise three areas that differ in regolith thickness and intensities of weathering, the nature of surface erosion and sediment transport, and the drainage architecture (Fig.…”