The hydrodynamics of river confluences is a fascinating subject having attracted the attention of inquisitive minds for centuries (da Vinci & MacCurdy, 2009). The coherent flow structures generated within a confluence are key drivers of mixing and sediment transport. The roles of shear-induced vertically orientated Kelvin-Helmholtz (KH) instabilities (

A greater understanding of these structures is therefore of obvious interest. Four structures are often discussed: helical cells (secondary flow at the scale of the tributaries' widths), vertically orientated Kelvin-Helmholtz (KH) vortices (shear-induced instabilities along the mixing interface), episodic pulses (origins still not fully understood, see Sabrina et al. (2021)) and streamwise orientated vortices (SOVs). SOVs were first discovered in numerical models as a pair of back-to-back, counter-rotating SOVs flanking each side of the mixing interface (Constantinescu et al., 2011) and their development is generally attributed to the downwelling of superelevated water, in a process strengthened by planform curvature (Sukhodolov & Sukhodolova, 2019). Recently, however strongly coherent SOVs were directly observed in turbidity currents at the Coaticook-Massawippi confluence (Duguay et al., 2022), and numerical modeling showed these SOVs to be gravity currents, caused by a small density gradient Δρ of ≈0.5 kg/m 3 , being confined between the converging flows (Duguay et al., 2022). Gaining knowledge of how greater magnitudes of Δρ and/or a reversal in the direction of Δρ influence these SOVs is the first step towards a full understanding of these intriguing flow structures.Density gradients develop when differences in temperature, dissolved minerals, or suspended sediment concentrations are present across the mixing interface. Density gradients commonly occur (

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Confluences are sites of intense turbulent mixing in fluvial systems. The large-scale turbulent structures largely responsible for this mixing have been proposed to fall into three main classes: vertically orientated (Kelvin-Helmholtz) vortices, secondary flow helical cells and smaller, strongly coherent streamwise-orientated vortices. Little is known concerning the prevalence and causal mechanisms of each class, their interactions with one another and their respective contributions to mixing. Historically, mixing processes have largely been interpreted through statistical moments derived from sparse pointwise flow field and passive scalar transport measurements, causing the contribution of the instantaneous flow field to be largely overlooked. To overcome the limited spatiotemporal resolution of traditional methods, herein we analyse aerial video of large-scale turbulent structures made visible by turbidity gradients present along the mixing interface of a mesoscale confluence and complement our findings with eddy-resolved numerical modelling. The fast, shallow main channel (Mitis) separates over the crest of the scour hole's avalanche face prior to colliding with the slow, deep tributary (Neigette), resulting in a streamwise-orientated separation cell in the lee of the avalanche face. Nascent large-scale Kelvin-Helmholtz instabilities form along the collision zone and expand as the high-momentum, separated near-surface flow of the Mitis pushes into them. Simultaneously, the strong downwelling of the Mitis is accompanied by strong upwelling of the Neigette. The upwelling Neigette results in $50% of the Neigette's discharge crossing the mixing interface over the short collision zone. Helical cells were not observed at the confluence. However, the downwelling Mitis, upwelling Neigette and separation cell interact to generate considerable streamwise vorticity on the Mitis side of the mixing interface. This streamwise vorticity is strongly coupled to the large-scale Kelvin-Helmholtz instabilities, which greatly enhances mixing. Comparably complex interactions between large-scale Kelvin-Helmholtz instabilities and coherent streamwise vortices are expected at other typical asymmetric confluences exhibiting a pronounced scour hole.

A fish ladder designed to facilitate fish passage at the outlet end of perched culverts is investigated with a 3D computational fluid dynamics model. The fish ladder consists of a series of alternating arch baffles with geometries providing options for fish passage over varying flow and debris conditions within the ladder. At high flows, the baffle’s protruding center arch increases pool depth, reducing the volumetric bulk turbulence of the pools and improving fish habitat. The arch baffle is compared to a standard baffle design currently in use and demonstrates potential advantages for fish passage. A recirculation zone of low velocity occupies a large volume of the pool believed to provide appropriate hydraulic habitat for resting and staging jump attempts upstream. This numerical study provides an acceptable design for future physical prototype testing in the laboratory or field to verify hydraulics and evaluate fish passage effectiveness.