Very stable, steep, ridge-like sequences formed by arrangements of boulders in irregular transverse ribs that partially or fully span the channel in bedrock-confined settings. Rapids in bedrock channels may be analogous with riffles in alluvial systems. Individual particles break the water surface at low flow stage.
Boulders are structurally realigned during high energy events to form stable transverse ribs that are associated with neither divergent nor convergent flow. Typically, 15-50 % of the stream demonstrates supercritical flow.