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Changes of bedload characteristics along the Marsyandi River (central Nepal); implications for understanding hillslope sediment supply, sediment load evolution along fluvial networks, and denudation in active orogenic belts

Mikael Attal and Jerome Lave
Changes of bedload characteristics along the Marsyandi River (central Nepal); implications for understanding hillslope sediment supply, sediment load evolution along fluvial networks, and denudation in active orogenic belts (in Tectonics, climate, and landscape evolution, Sean D. Willett (editor), Niels Hovius (editor), Mark T. Brandon (editor) and Donald M. Fisher (editor))
Special Paper - Geological Society of America (2006) 398: 143-171

Abstract

Understanding and quantifying fluvial transport and bedrock abrasion processes have become major concerns in modeling landform response to tectonic and climatic forcing. Recent theoretical and experimental investigations have in particular stressed the importance of sediment supply and size in controlling bedrock incision rate. Many studies on the downstream evolution of pebble size have focused on unraveling the respective roles of selective sorting and abrasion, without paying much attention to sediment sources. In order to track sediment supply and characteristics from source to sink in an active tectonic setting, where long-term selective deposition can be excluded, we systematically measured sediment size and lithology on gravel bars along the Marsyandi River and its tributaries (Himalayas of central Nepal), and also in sediment source material from hillslopes (landslides, moraines, terrace deposits). The downstream evolution in lithological distribution is found to be in close agreement with common views on pebble abrasion and present views on denudation in the range: (1) pebbles from the more rapidly uplifted and eroded Higher Himalayan gneissic units are over-represented, due to their major contribution to sediment influx, (2) easily erodible lithologies such as schists, sandstones, and limestone are under-represented relative to resistant rock types like quartzite. More surprisingly, we observe a general downstream coarsening of gravel bar material along the middle and lower Marsyandi River, whereas downstream sediment fining is typical of most river systems. A simple integrative model that tracks pebbles from hillslope to the main stem of the river and includes abrasion coefficients for the different Himalayan lithologies and size distribution of hillslopes sediment supplies accounts for both changing lithologic proportion along the Marsyandi and for the downstream coarsening of gravel bar material. This coarsening mainly results from differences in sediment sources along the Marsyandi Valley, in particular from differences in size distributions of landslide and moraine material. However, the median pebble size of subsurface material in gravel bars is coarser than median size of the blocky material in the source. The choice of the measurement methods and their potential bias are discussed but cannot explain this surprising feature displayed by our measurements. We suspect that due to sediment transport madalities in active tectonic settings, the subpavement grain-size distribution on gravel bars is not representative of the average bedload size distribution. Consequently, pebble abrasion is more easily demonstrated by sescription of pebble lithology than by the downstreal evolution of pebble size. Our study also shows, in contrast with previous studies, that experimentally derived abrasion coefficients can account for the downstream evolution of pebbles without calling for additional fining processes. We conclude that the eroded lithology and hillslope sediment source exert a major influence on the downstream evolution of sediment characteristics, on bedload ratio, and probably on bedrock erosion efficiency. These conclusions have important implications in terms of river profile evolution, landscape denudation, internal erosion coupling, and the response of the fluvial network to glacial-interglacial fluctuations.


ISSN: 0072-1077
EISSN: 2331-219X
Coden: GSAPAZ
Serial Title: Special Paper - Geological Society of America
Serial Volume: 398
Title: Changes of bedload characteristics along the Marsyandi River (central Nepal); implications for understanding hillslope sediment supply, sediment load evolution along fluvial networks, and denudation in active orogenic belts
Title: Tectonics, climate, and landscape evolution
Author(s): Attal, MikaelLave, Jerome
Author(s): Willett, Sean D.editor
Author(s): Hovius, Nielseditor
Author(s): Brandon, Mark T.editor
Author(s): Fisher, Donald M.editor
Affiliation: Universite Joseph Fourier, Laboratoire de Geodynamique des Chaines Alpines, Grenoble, France
Affiliation: University of Washington, Department of Earth and Space Sciences, Seattle, WA, United States
Pages: 143-171
Published: 2006
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
Meeting name: Penrose conference on Tectonics, climate, and landscape evolution
Meeting location: Taroko Gorge, TWN, Taiwan
Meeting date: 20032003
References: 60
Accession Number: 2006-061382
Categories: Hydrogeology
Document Type: Serial Conference document
Bibliographic Level: Analytic
Annotation: Includes appendices
Illustration Description: illus. incl. 2 tables, geol. sketch maps
N27°30'00" - N29°00'00", E83°30'00" - E85°00'00"
Secondary Affiliation: University of Cambridge, GBR, United KingdomYale University, USA, United StatesPennsylvania State University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200635
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