Downvalley fining of hillslope sediment in an alpine catchment: implications for downstream fining of sediment flux in mountain rivers,Earth Surface Processes and Landforms

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Downvalley fining of hillslope sediment in an alpine catchment: implications for downstream fining of sediment flux in mountain rivers
Earth Surface Processes and Landforms ( IF 2.8 ) Pub Date : 2020-03-18 , DOI: 10.1002/esp.4849
Leonard S. Sklar _{1,

2} , Clifford S. Riebe ₃ , Jennifer Genetti ₁ , Shirin Leclere ₁ , Claire E. Lukens _{3,

4}

Affiliation

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.

中文翻译：

高寒流域山坡沉积物的下谷细化：对山区河流底泥通量下游细化的影响

从山坡向河流输送的沉积物的大小分布会深刻影响河流的形态动力学，包括河流切入基岩和水生生境的质量。关于影响山坡风化产生的沉积物尺寸分布的因素知之甚少。我们目前对美国内华达山脉花岗岩基岩的陡峭流域Inyo Creek的山坡沉积物粒度分布进行现场研究。在除最粗粒度类别外的所有样本中，靠近主干流的山坡基部附近采样的颗粒均显示出沉积物大小明显减少，而样本高度降低。随着高度的降低，测得的尺寸分布变得越来越双峰，表现出粗糙，巨石模式不随海拔变化而变化，更丰富的精细模式从最高海拔的鹅卵石转变为中等海拔的砾石，最后转变为低海拔的沙石。我们解释山坡沉积物大小的这些海拔高度变化，以反映物理，化学和生物风化的变化，这可以用集水区的地形，气候和植被覆盖度的高度海拔梯度来解释。由于海拔高度和到出口的行进距离紧密耦合，因此，沉积物尺寸的高度趋势会沿着平行于干流的山坡使沉积物尺寸有系统地减小。我们将此现象称为“下谷式精炼”。前向模型表明，对于山坡和河道耦合且长期净沉积物沉降可忽略不计的山区景观，对下游粗粒沉积物的长期平均通量的下游精细化处理，需要对山坡沉积物进行下谷式精细化。该模型还表明，磨损在粗泥沙通量的下游细化中起次要作用，但在床荷与悬载之间的分配中起主要作用。在Inyo Creek观察到的模式可能在世界各地的山脉中广泛分布。©2020作者。约翰·威利父子有限公司（John Wiley＆Sons Ltd.）发布的《地球表面过程和地形》该模型还表明，磨损在粗泥沙通量的下游细化中起次要作用，但在床荷与悬载之间的分配中起主要作用。在Inyo Creek观察到的模式可能在世界各地的山脉中广泛分布。©2020作者。约翰·威利父子有限公司出版的《地球表面过程和地形该模型还表明，磨耗在粗泥沙通量的下游细化中起次要作用，但在床荷和悬载之间的分配中起主要作用。在Inyo Creek观察到的模式可能在世界各地的山脉中广泛分布。©2020作者。约翰·威利父子有限公司（John Wiley＆Sons Ltd.）发布的《地球表面过程和地形》

更新日期：2020-03-18

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