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. ("Large wood" is commonly defined to be pieces of wood more than 10 cm (4 in) in diameter and more than 1 m (3 ft 3 in) long.)^[1] Log jams in rivers and streams often span the entirety of the water's surface from bank to bank. Log jams form when trees floating in the water become entangled with other trees floating in the water or become snagged on rocks, large woody debris, or other objects anchored underwater. 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. Until they are dismantled by natural causes or humans, log jams can grow quickly, as more wood arriving from upstream becomes entangled in the mass. Log jams can persist for many decades, as is the case with the log jam in Spirit Lake.

Historically in North America, large natural "log rafts" were common across the continent prior to European settlement.^[2] The most famous natural wood raft is the Great Raft on the Red River in Louisiana, which prior to its removal in the 1830s affected between 390 and 480 km (240–300 mi) of the main channel.^[3] It has been suggested that such extensive log rafts may have been common in Europe in prehistory.^[4] Currently, the largest known log jam is over 3 million tonnes in the Mackenzie River in Canada's Northwest Territories.^[5] It contains more than 400,000 caches of wood and stores 3.4 million tons of carbon, equivalent to a year's emissions from 2.5 million cars. ^[6]

Log jams are not to be confused with man-made timber rafts created by loggers or the intentional release of large masses of trees into the water during a log drive to a sawmill.