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The Influence of Large Landslides on River Incision in a Transient Landscape

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The Influence of Large Landslides on River Incision in a Transient Landscape The infl uence of large landslides on river incision in a transient landscape: Eastern margin of the Tibetan Plateau (Sichuan, China) William B. Ouimet† Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Kelin X. Whipple School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA Leigh H. Royden Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Zhiming Sun Zhiliang Chen Chengdu Institute of Geology and Mineral Resources, Chengdu, Sichuan Province, China ABSTRACT along a river course, and thus the magnitude common, landslides dominate hillslope erosion of the landslide infl uence, is set by two funda- (Schmidt and Montgomery, 1995; Burbank et Deep landscape dissection by the Dadu mental timescales—the time it takes to erode al., 1996), and large landslides often fi ll river and Yalong rivers on the eastern margin of landslide deposits and erase individual dams valleys, forming landslide dams that inundate the Tibetan plateau has produced high-relief, and the recurrence interval of large landslides upstream channels with water and sediment narrow river gorges and threshold hillslopes that lead to stable dams. Stable, gradually (Costa and Schuster, 1988). The formation and that frequently experience large landslides. eroding landslide dams create mixed bedrock- failure of dams resulting from large mass wast- Large landslides inundate river valleys and alluvial channels with spatial and temporal ing events such as catastrophic landslides, rock- overwhelm channels with large volumes (>105 variations in incision, ultimately slowing long- falls, and rock avalanches are well-documented m3) of coarse material, commonly forming term rates of river incision, thereby reducing and have been studied extensively around the stable landslide dams that trigger extensive the total amount of incision occurring over world (Costa and Schuster, 1991). Landslide and prolonged aggradation upstream. These a given length of river. A stronger landslide dam stability and the style of dam failure (i.e., observations suggest that strong feedbacks effect implies that a higher percentage of catastrophic or gradual) depend on the size and among hillslope processes, channel morphol- channel length is buried by landslide-related geometry of the blockage and material char- ogy, and incision rate are prevalent through- sediment, leading to reduced river incision acteristics such as texture and sorting. Not all out this landscape and are likely characteris- effi ciency. The longer it takes a river channel landslide dams fail quickly and/or catastrophi- tic of transient landscapes experiencing large to incise into a landslide dam and remove all cally; many stabilize and block river valleys for increases in local relief, in general. Landslide landslide-related sediment, the more control hundreds to tens of thousands of years, with sig- effects are a by-product of rapid incision these events have on the evolution of the river nifi cant consequences for river morphology. initiated by regional uplift. However, over profi le and landscape evolution. This can be In this paper, we explore the infl uence of large timescales relevant to landscape evolution the result of slow erosion of stable dams, or a landslides and landslide dams in the context of (>104 yr), large landslides can also act as a higher frequency of large events. bedrock river incision and landscape evolution. primary control on channel morphology and Recent studies on this topic have emphasized longitudinal river profi les, inhibiting incision Keywords: Asia, Tibetan plateau, geomorphol- the enduring infl uence of large, catastrophic and further preventing the complete adjust- ogy, river incision, landslides, transient rivers, landslides on deposition and erosion in the Indus ment of rivers to regional tectonic, climatic, landscape evolution, Dadu River River within the northwest Himalaya (Hewitt, and lithologic forcing. 1998; 2006), and the effects of large landslides We explore a probabilistic, numerical INTRODUCTION and landslide dams on the channel morphology model to provide a quantitative framework and longitudinal profi les of rivers in southwest for evaluating how landslides infl uence bed- Landslides are an important erosional pro- New Zealand, the Swiss Alps, Tien Shan, and rock river incision and landscape evolution. cess in all landscapes with moderate to steep the Himalayas (Korup, 2005; 2006; Korup et al., The time-average number of landslide dams hillslopes. In rapidly uplifting and incising fl u- 2006). Both of these authors point out that land- vial landscapes, where high-relief, threshold slide effects are superimposed onto the other †[email protected] hillslopes, and relatively narrow river gorges are controls of river profi le and landscape evolution, GSA Bulletin; November/December 2007; v. 119; no. 11/12; p. 1462–1476; doi: 10.1130/B26136.1; 10 fi gures; 1 table; Data Repository item 2007216. 1462 For permission to copy, contact [email protected] © 2007 Geological Society of America Infl uence of large landslides on river incision: Eastern Tibet 90°E 10 0°E A B M i N 500 km n llow R. ° R Ye 40 N 40°N Fig. 3 . Y a Qiadam l o Basin n a' g R D . a Yellow R. d u Yangtze R. R Mek . Tibetan ong R Y . D Plateau a 1B a a d Y n Salween R. al u o g n R g t R . M z . e Sichuan e Y ° a . k R n 30°N Tsangpo-Brahmaputra R. Basin 30 N R o . g ze t gt n z an a e Y g y R. H i m a l a R . R y d Mongolia d a Low-relief relict topography dissected w 6 by major river gorges a earl R. r P Ν aa' r R I ed 40∞ R China . 4 India 2 Mekong Ν Yalong Elevation (km) Elevation Yangtze Dadu 20∞ South 0 Bay of China 0 200 400 600 20°N Bengal 90∞ Ε Sea C Distance (km) 90°E 100°E Figure 1. (A) Regional topography and rivers of the eastern margin of the Tibetan Plateau, a transient landscape characterized by deep river gorges cut into an uplifted, low-relief relict landscape. (B) Patches of low-relief, rel- ict landscape preserved at high elevations. For a detailed description of the criterion used to identify patches of relict landscape, see Clark et al., 2006. (C) Topographic cross-section (path a to a′ indicated in B), extracted from the ~1-km resolution GTOPO30 Digital Elevation Model (U.S. Geological Survey, 1993). such as tectonics, climate, lithology, and base- Overview of Stable Landslide Dam river channels narrow. Upstream of landslide level fall, and that a better understanding of the Formation and Effects on Rivers dams, river gradients are low, and fi ne-grained infl uence of landslides is needed before valley lake sediments and alluvial gravels accumulate. landforms and channel morphology can be used A stable landslide dam is any landslide dam In conjunction with fi lling within the trunk river, to infer the effects of these external forcings. that stabilizes and blocks a river valley for hun- side tributaries may become fi lled with alluvial Deep landscape dissection on the eastern dreds to tens of thousands of years, either initially fans graded to the high fi ll level of the trunk margin of the Tibetan Plateau has produced or after some degree of catastrophic failure and stream. The abrupt change in slope associated high-relief, narrow river gorges and thresh- dam-outburst fl ood erosion. Landslide deposits with the transition from upstream, low-gradient old hillslopes that frequently experience large that fi ll river valleys and consist of a high per- fi lls to steep, dramatic rapids through the land- landslides, making the entire region highly centage of large (>2 m) boulders or defl ect rivers slide deposits creates signifi cant knickpoints susceptible to landslide dams. Here, we use over bedrock ridges often lead to the formation with a drop in elevation of up to 100- to 300-m key examples from the Dadu and Yalong river of stable landslide dams. Once upstream lake (Korup, 2006). gorges (Fig. 1) to highlight the infl uence of large levels reach the top of the landslide dam and The integrated effects of large landslides on landslides and the interaction between landslide streamfl ow over landslide deposits begins, ero- river channels completely prohibit rivers from dams, channel morphology, longitudinal river sion-resistant boulders armor the channel bed as eroding their bed and incising over the length of profi les, and river incision. We then develop a fi ner material is washed downstream. This pro- the landslide mass and associated fi ll deposits. numerical model to simulate the occurrence and cess condenses the original landslide material This period of local non-incision continues for erosion of landslide dams along a length of river, to a smaller mass composed of large boulders, the entire duration of a landslide dam event, from and examine long-term controls on the degree stabilizing the landslide dam, and protecting the emplacement of the dam, to complete inci- to which channels may be buried by landslide- the top of the initial deposit from further ero- sion through landslide deposits and some por- related sediment and inhibit river incision. Our sion. These large boulders are not easily moved tion of the associated upstream fi ll. During this model is an important step toward quantify- in even large fl oods, and serve to roughen the time, the long-term evolution of the river profi les ing the infl uence of large landslides in actively bottom of the channel. There is often evidence and landscape evolution are affected in two main incising landscapes and incorporating this infl u- of advanced fl uvial sculpting of boulders, which ways: (1) downstream reaches continue to incise, ence into models of bedrock river incision and attests to long periods of boulder stability.
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