Step pool

Position: In-channel

 

Form

Formed on steep slopes (0.03-0.10 m m-1) channel-spanning stair-like features comprise bedrock, or boulder and cobble clasts or wood separated by areas of quieter flow in a plunge pool downstream. The risers of individual steps are generally made up of several large boulders, or keystones.

 

Process description

A cyclic pattern of acceleration and deceleration characterises the flow regime as water flows over or through the boulders forming each step before plunging into the pool below. Such tumbling flow is supercritical over the step and subcritical in the pool. Turbulent mixing results in considerable energy dissipation. Step development is strongly influenced by local sediment supply (availability of keystones) and transport conditions.