This effect is most noticeable where water velocities are high, as at bridges where most of the channel is blocked by drift. Drift accumulations originate at the water surface, but ultimately become part of the streambed. As the water level rises during a flood, drift already on the bridge generally remains in place as new drift continues to be added at the water surface. When the water level falls, accumulated drift typically slides downward on the pier or piers in contact with it until it rests on the bed. Most accumulations do not float to the surface during subsequent floods, but form a solid mass with irregular protrusions around the base of the pier or piers against which they rest.
Sediment fills some of the remaining voids in the accumulation.
This mass of wood and sediment may persist indefinitely, potentially forming a base for further accumulations.