Most such trees fall into the stream as a result of bank erosion, while others are felled by wind throw, ice, disease, or old age.
Flood plains can provide drift to the channel under some circumstances.
Channel characteristics that determine the rate of bank erosion, such as bank height, bank angles, bank materials, erosion rates, and drift concentrations, vary from point to point within a given basin. Trees rooted in alluvium stabilize banks by binding sediment together with their roots. When stream banks erode, tree bases typically remain attached and project into the channel beyond the retreating bank. Growth of roots continues in the bank, while the exposed parts of the root mass become burly, develop rougher bark, and are often scarred by the impact of drift. The trunk leans toward the stream as it is deprived of support on that side, while the top of the tree grows straight upward.