Riffles are biologically very important, because many aquatic species rely on them in one way or another. Many benthic macroinvertebrates are present in riffles, so fish often locate just downstream of a riffle to wait for these small creatures to drift down to them.
In the context of aquatic ecosystems, riffles are essential habitats that support a variety of species. Fish often congregate just downstream of riffles to capitalize on the abundant small organisms that drift down from these areas.