A log jam is an accumulation of large wood (commonly defined as pieces of wood more than 10 cm in diameter and more than one metre 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 Louisiana's Red River, which prior to removal in the 1830s affected between 390–480 km of the main channel. It has been suggested that such extensive log rafts may have been common in Europe in pre-history.
Effects on stream/river geomorphology
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 in stream biota. The formation of a logjam against one bank typically concentrates flow in the wood free portion of the channel, increasing velocity through this section and promoting scour of the river bed, the formation of channel spanning logjams 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 logjams are highly dependent on the slope of the river (and thus the potential power of the stream), in steep channels logjams tend to form channel-spanning step-like structures with an associated downstream scour pool, whereas in large lowland rivers with low slopes logjams tend to be partial structures primarily acting to deflect flow with minimal geomorphological change.
Effects on ecology
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.
It has been suggested that logjams are part of trees acting as ecosystem engineers to alter river habitats to promote tree growth. In dynamic braided rivers such as the Tagliamento River in Italy, where the dominant tree species is Black Poplar fallen trees form logjams when they are deposited on bars, fine sediment is deposited around these logjams and sprouting seedlings are able to stabilise braid bars and promote the formation of stable islands in the river. 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 logjams.
In large rivers in the American Pacific Northwest it has been shown there is a life-cycle of tree growth and river migration, with large trees falling into the channel as banks erode, then staying in place and acting as focal points for logjam formation. These logjams acts as "hard points" resisting further erosion and channel migration. The areas of floodplain behind these logjams then become stable enough for more large trees to grow which in turn can potentially be logjam anchor points in the future.