Riverine transport of silt and clay particles—or mud—builds continental landscapes and dominates the fluxes of sediment and organic carbon across Earth’s surface. Compared with fluxes of sand-sized grains, mud fluxes are difficult to predict. Yet, understanding the fate of muddy river sediment is fundamental to the global carbon cycle, coastal landscape resilience to sea-level rise, river restoration and river–floodplain morphodynamics on Earth and Mars. Mechanistic theories exist for suspended sand transport, but mud in rivers is often thought to constitute washload—sediment with settling velocities so slow that it does not interact with the bed, such that it depends only on upstream supply and is impossible to predict from local hydraulics. To test this hypothesis, we compiled sediment concentration profiles from the literature from eight rivers and used an inversion technique to determine settling rates of suspended mud. We found that mud in rivers is largely flocculated in aggregates that have near-constant settling velocities, independent of grain size, of approximately 0.34 mm s⁻¹, which is 100-fold faster than rates for individual particles. Our findings indicate that flocculated mud is part of suspended bed-material load, not washload, and thus can be physically described by bed-material entrainment theory.

河道中淤泥和粘土颗粒（或泥浆）的运输形成了大陆景观，并主导了整个地球表面的沉积物和有机碳通量。与沙粒的通量相比，泥浆的通量很难预测。然而，了解泥泞河流沉积物的命运对于全球碳循环，沿海景观对海平面上升的适应力，河流修复以及地球和火星的河漫滩形态动力学至关重要。存在用于悬浮砂运输的力学理论，但通常认为河流中的泥浆构成冲刷作用-沉积物的沉降速度是如此之慢，以至于它不会与床层相互作用，因此它仅取决于上游供应，无法通过当地水力系统进行预测。 。为了检验这个假设，我们根据文献从八条河流中收集了沉积物浓度剖面图，并使用反演技术确定了悬浮泥浆的沉降速率。我们发现，河流中的泥浆主要以聚集体的形式絮凝，其沉降速度与颗粒大小无关，接近恒定，约为0.34 mm s^-1，比单个粒子的速率快100倍。我们的研究结果表明，絮凝的泥浆是悬浮床料负荷的一部分，而不是洗涤负荷，因此可以用床料夹带理论进行物理描述。