Downvalley fining of hillslope sediment in an alpine catchment: implications for downstream fining of sediment flux in mountain rivers. Issue 8 (18th March 2020)

• Record Type: Journal Article
• Authors: Sklar, Leonard S.; Riebe, Clifford S.; Genetti, Jennifer; Leclere, Shirin; Lukens, Claire E.
• Is Part Of: Earth surface processes and landforms. Volume 45:Issue 8(2020)
• Page Start: 1828
• Page End: 1845
• Publication Date: 2020-03-18
• Subjects: downvalley fining -- sediment size distributions -- river bedload -- physical weathering -- chemical weathering -- landscape evolution; Geomorphology -- Periodicals; 551.4
• DOI: 10.1002/esp.4849
• Languages: English
• ISSNs: 0197-9337

Abstract

Abstract: The size distributions of sediment delivered from hillslopes to rivers profoundly influence river morphodynamics, including river incision into bedrock and the quality of aquatic habitat. Yet little is known about the factors that influence size distributions of sediment produced by weathering on hillslopes. We present results of a field study of hillslope sediment size distributions at Inyo Creek, a steep catchment in granitic bedrock of the Sierra Nevada, USA. Particles sampled near the base of hillslopes, adjacent to the trunk stream, show a pronounced decrease in sediment size with decreasing sample elevation across all but the coarsest size classes. Measured size distributions become increasingly bimodal with decreasing elevation, exhibiting a coarse, bouldery mode that does not change with elevation and a more abundant finer mode that shifts from cobbles at the highest elevations to gravel at mid elevations and finally to sand at low elevations. We interpret these altitudinal variations in hillslope sediment size to reflect changes in physical, chemical, and biological weathering that can be explained by the catchment's strong altitudinal gradients in topography, climate, and vegetation cover. Because elevation and travel distance to the outlet are closely coupled, the altitudinal trends in sediment size produce a systematic decrease in sediment size along hillslopes parallel to the trunk stream. We refer to this phenomenon as 'downvalley fining.' Forward modeling shows that downvalley fining of hillslope sediment is necessary for downstream fining of the long‐term average flux of coarse sediment in mountain landscapes where hillslopes and channels are coupled and long‐term net sediment deposition is negligible. The model also shows that abrasion plays a secondary role in downstream fining of coarse sediment flux but plays a dominant role in partitioning between the bedload and suspended load. Patterns observed at Inyo Creek may be widespread in mountain ranges around the world. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. Abstract : We measured hillslope sediment size distributions across a steep, rapidly eroding catchment. Particle size becomes increasingly bimodal and decreases downvalley due to altitudinal shifts in the sizes that make up the finer mode. We interpret this to reflect altitudinal variations in physical, chemical, and biological weathering, which are likely common in mountain landscapes around the world. Forward modeling shows that downvalley fining in hillslope sediment dominates over abrasion in causing downstream fining in the long‐term average coarse sediment flux of steep mountain rivers.