The river flow velocity never exceeds non-scouring velocity of
the soil forming the river bed. If this condition is violated, bed
scour and river meandering begin. It proceeds until the length of
the river has sufficiently increased and its hydraulic gradient and
velocity have decreased to restore the equilibrium in the interaction
between the water flow shear action and the resistance of
a river bed to that shear. The mean water flow velocity in alluvial
rivers usually varies within the range of 0.3–0.8 m/s and seldom
exceeds 1.0 m/s.