River bed sediments typically fine downstream, where fining of median grain sizes are often described as exponential, except where fine gravel abruptly transitions to sand. Across the gravel-sand transition, median grain sizes can reduce by more than 10 mm (>90%) over a distance of only a few channel widths. There are several viable theories for why the gravel-sand transition occurs, but they remain a matter of ongoing discussion in the literature. Here, we present a review of known morphological characteristics associated with gravel-sand transitions and the existing theories for their development (e.g., abrasion, size selective transport, washload deposition). This is combined with a global database of published gravel-sand transitions across a range of climatic, tectonic and geographic settings. We identify an absence of universal morphological characteristics associated with the transition. However, the position of the transition is relatively predictable, occurring either a small distance downstream of mountain ranges or at a characteristic backwater distance upstream of a base-level control. This supports previous findings where the position of the transition is sensitive to long-term changes in gravel runout distance (e.g., through changes in gravel supply, basin subsidence rate) and/or changes in base level (e.g., sea level rise). Both backwater effects and exhaustion of gravel supply generate a distinct and abrupt change in water surface slope between the gravel and sand reaches, suggesting this is a control on the location at which they develop. The abrupt nature of the gravel-sand transition is then considered in terms of the two theories that seem most able to explain the phenomenon at the granular scale. We also focus on the apparent absence of river beds with median bed grain diameters of ~1–10 mm, or within the ‘grain size gap’, to better understand how this relates to the development and nature of gravel-sand transitions. The absence of median bed grain sizes within this range may be a reflection of grain size statistics, where these grain sizes are actually present but never dominate bed material. Alternatively, these grain sizes may be absent from hillslope sediment sources. Finally, we consider how particle dynamics may prevent formation of a stable gravel bed with gap material. Even if these grain sizes are produced on hillslopes, particles may raft downstream over the sand bed and disperse. Research into how grain size gap material is generated, transported and deposited in river systems should be a future priority.

河床沉积物通常在下游很细，中值粒度的细化通常被描述为指数级，除非细砾石突然转变为沙子。在砾石 - 沙子过渡区，中值粒度可以在仅几个通道宽度的距离内减少超过 10 毫米 (>90%)。有几种可行的理论来解释为什么会发生砾石 - 沙子转变，但它们仍然是文献中持续讨论的问题。在这里，我们回顾了与砾石-砂过渡相关的已知形态特征及其发展的现有理论（例如，磨损、尺寸选择性运输、洗涤负载沉积）。这与在一系列气候、构造和地理环境中已发布的砾石-沙过渡的全球数据库相结合。我们发现缺乏与过渡相关的通用形态特征。然而，过渡的位置是相对可预测的，要么发生在山脉下游的一小段距离处，要么发生在基线控制上游的特定回水距离处。这支持了先前的发现，即过渡位置对砾石跳动距离的长期变化（例如，通过砾石供应、盆地沉降率的变化）和/或基准面的变化（例如，海平面上升）敏感。回水效应和砾石供应枯竭都会导致砾石和沙子河段之间的水面坡度发生明显而突然的变化，这表明这是对其发育位置的控制。然后根据似乎最能在颗粒尺度上解释这种现象的两种理论来考虑砾石 - 沙子转变的突然性质。我们还关注中值河床粒径为~1-10 毫米或在“粒径间隙”内的河床的明显缺失，以更好地了解这与砾石 - 沙子过渡的发展和性质之间的关系。在此范围内没有中值床粒度可能是粒度统计数据的反映，其中这些粒度实际上存在但从不支配床材料。或者，山坡沉积物源中可能不存在这些粒度。最后，我们考虑粒子动力学如何防止形成具有间隙材料的稳定砾石床。即使这些粒度是在山坡上生产的，颗粒可能会漂向下游的沙床并分散。研究粒度间隙材料如何在河流系统中产生、运输和沉积应该是未来的优先事项。