Log jam

A log jam is a naturally occurring phenomenon characterized by a dense accumulation of tree trunks and pieces of large wood across a vast section of a river, stream, or lake.

They can build up slowly over months or years, or they can happen instantaneously when large numbers of trees are swept into the water after natural disasters.

A notable example caused by a natural disaster is the log jam that occurred in Spirit Lake following a landslide triggered by the eruption of Mount St. Helens.

Unless they are dismantled by natural causes or humans, log jams can grow quickly, as more wood arriving from upstream becomes entangled in the mass.

[9] The formation of a log jam against one bank typically concentrates flow in the wood-free portion of the channel, increasing velocity through this section and promoting scour of the riverbed.

These stable islands are then prime areas for establishment of seedlings and further vegetation growth, which in turn can eventually provide more fallen trees to the river and thus form more log jams.