If you are ever looking for visual examples of fluid mechanics in action, rivers are a good place to start. Natural riverbeds, which would be classified under open channels in TAM 335, are typically heterogenous and can cause various forms of turbulence.
A bluff is a body that separates flow over a significant part of its surface (as opposed to a streamlined body, which is designed to lower the drag force occurring between itself and the fluid it passes through). When fluid flows over a bluff, such as a boulder, it creates vortices, or flow that circles around an axis. See NASA’s drag coefficients for general shapes here.
The conglomeration of bluff bodies in riverbeds in the form of rocks, logs, and other natural debris is such that river flow experiences random separation and interference. Flow separation can lead to vortex shedding, in which each section of flow creates a vortex pattern after passing over the body. In a constrained flow channel (like a river), the flow separation in one area of current can easily interfere with that of another, creating unique interference patterns.
As a side note, river rafters are cautioned against passing near bridge pylons in strong currents. The pylons disrupt the flow, creating a net change in velocity from the boundary layer to flow farther away from the pylon. In aggressive waters, the gradient can easily be extreme enough to flip a raft without warning.