Scour pool (sculpted pool)
Pools may span the channel, hosting tranquil or standing flow at low-stage. Alluvial pools are alternating deep areas of channel along an undulating longitudinal bed profile. Pools tend to act as sediment storage zones. They tend to occur at characteristic locations, such as the concave bank of bends in sinuous alluvial channels. In fine-grained systems they also occur in expansion zones where flows exit from more confined, laterally stable sections or where flow converges at anabranch confluences.
At high flow stage, when flow converges through pools, decreased roughness and greater bed shear stresses induce scour and flushing of sediment stored on the bed. Subcritical flow occurs at low flow stage, when divergent flow occurs. Pool-infilling subsequently occurs, as pools act as areas of deep, low flow velocity and near-standing water conditions.