Downstream Morphological and Sedimentary Transformations in Modern Continental-Scale Rivers

Abdullah M. Wahbi, Michael D. Blum
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Abstract

Morphological characteristics in river systems, including channel dimensions and river gradients, scale to drainage basin area, which provides the means for such elements to be predicted, measured and modeled. Moreover, recent studies interpret downstream changes in channel morphological and sedimentary characteristics to be the product of changing flow hydraulics as rivers transit from the normal flow to the backwater reach and approach the coastal ocean. This paper quantifies how large modern rivers undergo morphological and sedimentary transformations in response to normal flow to backwater transition. Morphologies adapting to such backwater hydraulic conditions is a potential for further investigation. With applications in modeling of modern river systems, this also provides the means for paleoenvironment reconstructions based on changing morphological characteristics since such quantitative framework is grounded by similar depositional processes. Building on previous studies, we construct river-long profiles, estimate backwater lengths, measure the ratio between channel-belt and channel widths (BChB/BCh), and measure the ratio between sand-rich to mud-dominated environments of deposition (S/M ratio) in five large modern river systems. We use results from >55,000 measurements of morphological and lithological characteristics from ~3,850 valley cross-sections over ~5,500 river kilometers to show that: (a) channel gradients decrease by ~30-50% as the channel goes through the normal flow to backwater transition, whereas (b) BChB/BCh decreases by >~60% and (c) S/M ratios decrease by ~35-90% within the upper backwater reach. These values further decrease in the lower backwater reach and approach unity (BChB/BCh = 1; S/M = 0) as the gradient reaches zero (sea level). Such systematic transformations in morphologic and sedimentary characteristics indicate they are both inherent and predictable, and can be used to interpret normal flow vs. backwater hydraulics in ancient fluvial deposits.
现代大陆尺度河流的下游形态和沉积转变
河流系统的形态特征,包括河道尺寸和河流坡度,与流域面积成比例关系,这为预测、 测量和模拟这些要素提供了手段。此外,最近的研究将河道形态和沉积特征的下游变化解释为河流从正常流向回水河段和 接近沿岸海洋时水流水力学变化的产物。本文定量分析了现代大河是如何在正常水流向回水过渡的过程中发生形态和沉积变 化的。适应这种回水水力条件的形态有可能得到进一步研究。在现代河流系统建模中的应用,也为基于形态特征变化的古环境重建提供了方法,因为这种定量框架是以类似的沉积过程为基础的。在以往研究的基础上,我们在五条大型现代河流水系中构建了河流长剖面图,估算了回水长度,测量了河道带与河道宽的比例(BChB/BCh),并测量了富沙环境与以泥为主的沉积环境的比例(S/M 比)。我们利用对约 5,500 公里河流中约 3,850 个河谷断面进行的 >55,000 次形态和岩性特征测量结果表明(a) 当河道经过正常流向背水的过渡时,河道坡度降低约 30-50%,而 (b) BChB/BCh 降低 >~60%,(c) S/M 比值在背水上游降低约 35-90%。这些数值在下游回水河段进一步降低,并在梯度为零(海平面)时接近统一(BChB/BCh = 1;S/M = 0)。形态和沉积特征的这种系统性转变表明,它们既是固有的,也是可预测的,可以用来解释古河道沉积中的正常流与回水水力学。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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