In fluvial geomorphology, one of the most pervasive paradigms is that the size of the grains present in a river exercises an important effect on its character. In gravel-bed rivers, there is considerable scatter in the relations between so-called “representative grain sizes” and basic channel processes and morphologies. Under a grain size paradigm, our ability to rationalize the characteristics of a given channel and predict how it will respond to a change in conditions is limited. In this paper, I deconstruct this paradigm by exploring its historical origins in geomorphology and fluid dynamics, and identify three of its underlying premises: (1) the association between grain diameter and fluid drag derived from Nikuradse’s experiments with sand-coated surfaces; (2) the use of grain size by early process geomorphologists to describe general trends across large samples of sand-bed rivers; and (3) a classificatory approach to discerning bed structures originally developed for bed configurations found in sand-bed rivers. The conflation of sand- and gravel-bed rivers limits our ability to understand gravel-bed morphodynamics. Longstanding critique of the grain size paradigm has generated alternative ideas but, due to technological and conceptual limitations, they have remained unrealized. One such unrealized idea is the morphology-based definition of bed state – an important degree of freedom within fluvial systems, particularly in reaches where adjustments to planform are not easily achieved. By embracing recent advancements in fluid dynamics and remote sensing, I present an alternative or complementary concept of bed state based on the notion that fluvial systems act to maximize flow resistance. The proposed quantitative index represents the relative contribution of morphologic adjustments occurring at different spatial scales (discriminated using a wavelet transform) to a stable channel configuration. By explicitly acknowledging the complexity of bed adjustments we can move toward a more complete understanding of channel stability in gravel-bed rivers.

中文翻译：

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砾石层河流的河床状态形态动力学

在河流地貌学中，最普遍的范式之一是河流中颗粒的大小对其特征有重要影响。在砾石层河流中，所谓的“代表性粒度”与基本河道过程和形态之间的关系存在相当大的分散性。在粒度范式下，我们对给定通道的特征进行合理化并预测它将如何响应条件变化的能力是有限的。在本文中，我通过探索其在地貌学和流体动力学中的历史渊源来解构这一范式，并确定其三个基本前提：（1）从 Nikuradse 对砂涂层表面的实验得出的颗粒直径与流体阻力之间的关联；(2) 早期过程地貌学家使用粒度来描述大型沙床河流样本的总体趋势；(3) 一种识别河床结构的分类方法，最初是为在沙床河流中发现的河床配置而开发的。砂砾层河流的合并限制了我们理解砾石层形态动力学的能力。对粒度范式的长期批评已经产生了替代想法，但由于技术和概念的限制，它们仍未实现。其中一个未实现的想法是基于形态的河床状态定义——河流系统中的一个重要自由度，特别是在难以调整平面的河段。通过采用流体动力学和遥感方面的最新进展，我基于河流系统的作用是最大限度地提高流动阻力的概念，提出了一种替代或补充的床状态概念。提议的定量指数表示在不同空间尺度（使用小波变换区分）发生的形态调整对稳定通道配置的相对贡献。通过明确承认河床调整的复杂性，我们可以更全面地了解砾石河床的河道稳定性。