Most of the trees that become drift are undermined by bank erosion. Rivers with unstable channels have the most bank erosion and the most drift, but some drift is present in most rivers during floods. Floating drift is concentrated along the thread of the stream and moves at about the average flow velocity. Logs longer than the width of the channel accumulate in jams, or are broken into shorter pieces. Sunken woody debris moves more slowly and tends to accumulate in and along the channel, rather than being transported downstream to bridges.
Drift accumulates against obstacles such as bridge piers that divide the flow at the water surface.
Groups of obstacles separated by narrow gaps trap drift most effectively. Drift accumulation begins at the water surface, but accretion can cause an accumulation to grow downward to the streambed. An accumulation on a single pier grows no wider than the length of the longest logs it contains. The gap between two piers is not blocked by drift unless individual logs can reach from pier to pier. Drift damages bridges mostly through local and contraction scour.