Very stable, coarse-grained or bedrock features observed in steep, bedrock-confined settings. Comprise longitudinally and laterally disorganised bed material, typically cobbles and boulders. Flow cascades over large boulders in a series of short steps about one clast diameter high, separated by areas of more tranquil flow of less than one channel width in extent.
More than 50 % of the stream area is characterised by supercritical flow. Typically associated with downstream convergence of flow. Near-continuous tumbling/turbulent and jet-and-wake flow over and around large clasts contributes to energy dissipation. Finer gravels can be stored behind larger materials or wood. During moderate flow events, finer bedload materials are transported over the more stable clasts that remain immobile. Local reworking may occur in high magnitude-low frequency events.