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.
Riffles are vital for aquatic life, supporting a wide range of species that depend on them in different ways. The abundance of benthic macroinvertebrates in riffles draws fish, which wait downstream to exploit this food source.