The depth of a blockage is limited by the depth of flow. Several observed drift accumulations extended upward nearly to the maximum flood stage even after the flood receded. Allowing for compression of the jam as the water level dropped, these accumulations likely occupied the entire depth of flow. Accumulations may be irregular, but most large accumulations are similar in shape. Logs are initially trapped perpendicular to approaching flow in most accumulations, but as accretion continues, logs are added parallel to the upstream edge of the raft.
Accretion is fastest where the path of most drift in the river intersects the accumulation.
The net result is often an accumulation with a curved upstream edge, and with the upstream nose of the raft near the thalweg. Drifting logs encountering the nose, where the edge of the raft is perpendicular to the flow, are more likely to be trapped at the upstream edge of the raft, or swept under it, than logs that encounter the edges diagonal to the approaching flow. The sides of an accumulation may be trimmed by the breakage of protruding logs. 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.