Logjam

A log jam is an accumulation of large wood (commonly defined as pieces of wood more than 10 cm (4 in) in diameter and more than 1 m (3 ft 3 in) long also commonly called large woody debris) that can span an entire stream or river channel.

Historically in North America, large "log rafts" were common across the continent prior to European settlement. 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. It has been suggested that such extensive log rafts may have been common in Europe in prehistory.

Log jams alter flow hydraulics by diverting flow towards the bed or banks, increasing flow resistance and creating upstream pools, diverting flow onto the floodplain and damming the channel causing water to spill over the structure. These altered channel hydraulics change local patterns of erosion and deposition, which can create greater variety in local geomorphology and thus create provision and variety of habitat for instream living organisms. 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, the formation of channel-spanning log jams can lead to the formation of an upstream pool, water spilling over the structure generating a "plunge pool" immediately downstream.

The hydraulic and geomorphological effects of log jams are highly dependent on the slope of the river (and thus the potential power of the stream); in steep channels, log jams tend to form channel-spanning steplike structures with an associated downstream scour pool, whereas, in large lowland rivers with low slopes, log jams tend to be partial structures primarily acting to deflect flow with minimal geomorphological change.

Log jams provide important fish habitat. The pools created and sediment deposited by formation of log jams create prime spawning grounds for many species of salmon. These pools also provide refuge for fish during low water levels when other parts of a stream may be nearly dry. Log jams can provide refuge, as velocity shelters, during high-flow periods.

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