The extent to which flows mix at confluences is pivotal for determining spatial patterns of water quality and biodiversity. Because mixing processes are complex, predicting rates and characteristic scales of mixing is difficult. Here we introduce a theory for confluence mixing dynamics of shallow flows in which the mixing process is controlled by the switching between two modes of behaviour—one similar to a wake behind an obstacle and the other similar to a mixing layer between two parallel flows. Dye visualizations of mixing in field-based controlled experiments agree with theoretical predictions and support transitional behaviour between the two mixing modes. According to our theoretical framework, the mixing interface can grow rapidly in wake mode, when large vortices are shed from a zone of stagnant flow within the confluence, but lateral shear between incoming flows is negligible. This rapid growth occurs even though flow curvature and shallowness inhibit growth through advective and turbulent lateral exchange of momentum and through bed friction. Our findings provide insight into the importance of different modalities of flow structure in controlling mixing at river confluences, thereby contributing to practical knowledge on the role of confluences in dispersal of contaminants in river systems.

流量在汇流处混合的程度对于确定水质和生物多样性的空间格局至关重要。由于混合过程很复杂，因此很难预测混合的速率和特征尺度。在这里，我们介绍了浅流汇合混合动力学的理论，其中混合过程由两种行为模式之间的切换控制——一种类似于障碍物后面的尾流，另一种类似于两个平行流之间的混合层。基于现场的受控实验中混合的染料可视化与理论预测一致，并支持两种混合模式之间的过渡行为。根据我们的理论框架，混合界面可以在尾流模式下快速增长，当大涡流从汇合区内的停滞流区域脱落时，但流入流之间的横向剪切可以忽略不计。即使流动曲率和浅度通过动量的平流和湍流横向交换以及床摩擦抑制生长，也会发生这种快速生长。我们的研究结果深入了解了不同模式的流动结构在控制河流汇流处混合的重要性，从而有助于了解汇流在河流系统中污染物扩散中的作用的实践知识。