riffles

A riffle is a shallow, fast-moving section of a stream or river where the water surface appears broken and irregular as it flows over a bed of coarse material. Riffles are the opposite of pools and are commonly arranged in a sequence with runs and pools along a channel reach.

Riffles form where flow energy deposits coarser sediments, or where bed roughness from rocks or vegetation accelerates water over upper gravel surfaces. The local channel slope and channel geometry promote fragmentation of the bed, creating alternating shallow, turbulent reaches (riffles) and deeper, calmer sections (pools).

Hydraulically, riffles have higher velocity, greater shear stress, and more turbulence than pools. The bed is typically strewn with gravels and cobbles, which causes surface agitation and increases oxygen exchange with the atmosphere. Sediment transport tends to sort particles, with coarser material accumulating at the riffle crest and finer material mantling downstream.

Ecology: The enhanced oxygenation and habitat complexity of riffles support diverse macroinvertebrate communities and provide spawning habitat for many fish that require clean, well-oxygenated gravel, such as salmonids. The interstitial spaces between gravel grains offer shelter for larvae and eggs.

In stream restoration and hydrology, riffles are used as indicators of habitat quality and are often restored to reestablish natural pool–riffle sequences. Assessments may measure riffle frequency, width, depth, and bed material to evaluate habitat suitability.