Rivers commonly exhibit substantial variability in suspended‐sand concentration, even at constant water discharge. Here we derive an approach for evaluating how much of this variability arises from mean bed‐sand grain size. We apply this approach to the Colorado River in Grand Canyon, where discharge‐independent concentration of suspended sand varies by more than a factor of 23 (N = 1.4 × 10⁶). Theory predicts that where concentration is controlled by bed‐sand grain size, concentration and grain size in suspension will be inversely correlated (i.e., coarsening of the bed causes suspended sand to become coarser in grain size and lower in concentration). Although the observed correlation is negative, riverbed grain size accounts for only 40% of the variability in concentration. The residuals vary by an order of magnitude; they arise from other processes, such as changes in topography or distribution of sand that cause shear stress to change at constant discharge, changes in the fine tail of bed‐sand grain sizes or changing bedforms. Both bed sand and the other factors influence concentration for durations from less than 1 day to several years. Predictions of concentration based on bed‐sand grain size (N = 4 × 10⁴) are less accurate than predictions based on suspended‐sand grain size, probably because suspended sand is a natural integrator of sand‐transporting processes, giving more weight to those areas of the bed that exchange more sand with the flow. Although the causes of variability vary from one river to another, the approach illustrated here is applicable to any river in which concentration varies at constant water discharge.

中文翻译：

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大峡谷科罗拉多河中悬浮沙浓度变化的成因分析

即使在恒定的排水量下，河流的悬浮砂浓度通常也存在很大的变化。在这里，我们推导了一种方法，用于评估这种变化的多少是由平均床砂粒度引起的。我们将这种方法应用于大峡谷的科罗拉多河，其中与排放无关的悬浮砂浓度变化超过23倍（N  = 1.4×10 ⁶）。理论预测，在浓度受床砂粒度控制的情况下，悬浮液中的浓度和粒度将成反比关系（即，床层粗化会导致悬浮砂的粒度变大而浓度降低）。尽管观测到的相关性为负，但河床粒度仅占浓度变化的40％。残差变化一个数量级。它们是由其他过程引起的，例如地形变化或沙粒分布的变化，这些变化会导致剪应力在恒定流量下发生变化，床砂细粒度的尾部变化或床形变化。床砂和其他因素都会影响浓度，持续时间从不到1天到几年。基于床砂粒度的浓度预测（N = 4×10 ⁴）的准确性不及基于悬浮砂粒度的预测，这可能是因为悬浮砂是输沙过程的自然整合者，赋予床层与流交换更多砂的区域更大的权重。尽管变化的原因在一条河流与另一条河流之间各不相同，但此处说明的方法适用于在恒定排水量下浓度变化的任何河流。