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
Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin ...
" is commonly defined as pieces of wood more than in diameter and more than long)
Log jams in rivers and streams often span the entirety of the water's surface from
bank
A bank is a financial institution that accepts deposits from the public and creates a demand deposit while simultaneously making loans. Lending activities can be directly performed by the bank or indirectly through capital markets.
Because ...
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
Large woody debris (LWD) are the logs, sticks, branches, and other wood that falls into streams and rivers. This debris can influence the flow and the shape of the stream channel. Large woody debris, grains, and the shape of the bed of the stream ...
, 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 exponentially 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. The most famous natural wood raft is the
Great Raft
The Great Raft was a gigantic log jam or series of "rafts" that clogged the Red and Atchafalaya rivers and was unique in North America in terms of its scale.
Origin
The Great Raft probably began forming in the 12th century. It grew from its u ...
on the
Red River in
Louisiana
Louisiana , group=pronunciation (French: ''La Louisiane'') is a state in the Deep South and South Central regions of the United States. It is the 20th-smallest by area and the 25th most populous of the 50 U.S. states. Louisiana is borde ...
, which prior to its removal in the 1830s affected between of the main channel. It has been suggested that such extensive log rafts may have been common in Europe in prehistory.
Log jams are not to be confused with man-made
timber rafts created by
loggers
Lumberjacks are mostly North American workers in the logging industry who perform the initial harvesting and transport of trees for ultimate processing into forest products. The term usually refers to loggers in the era (before 1945 in the Unite ...
or the intentional release of large masses of trees into the water during a
log drive to a
sawmill
A sawmill (saw mill, saw-mill) or lumber mill is a facility where logs are cut into lumber. Modern sawmills use a motorized saw to cut logs lengthwise to make long pieces, and crosswise to length depending on standard or custom sizes (dimensi ...
.
Effects on river geomorphology
![DeadWoodGoodell Creek Log Jam](https://upload.wikimedia.org/wikipedia/commons/1/11/DeadWoodGoodell_Creek_Log_Jam.jpg)
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.
Effects on ecology
Log jams provide important
fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...
habitat. The pools created and sediment deposited by formation of log jams create prime spawning grounds for many species of
salmon
Salmon () is the common name for several commercially important species of euryhaline ray-finned fish from the family Salmonidae, which are native to tributaries of the North Atlantic (genus ''Salmo'') and North Pacific (genus '' Oncorhy ...
. 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 log jams are an aspect of trees acting as ecosystem engineers to alter river habitats to promote tree growth. In dynamic
braided rivers, such as the
Tagliamento River
The Tagliamento () is a braided river in north-east Italy, flowing from the Alps to the Adriatic Sea at a point between Trieste and Venice.
The Tagliamento river is considered as the last morphologically intact river in the Alps. (Its ...
in Italy where the dominant tree species is the
black poplar, fallen trees form log jams when they are deposited on bars, fine sediment is deposited around these log jams and sprouting seedlings are able to stabilise
braid bar
Braid bars, or mid-channel bars, are river landforms typically present in braided river channels. These formations have many names, including medial, longitudinal, crescentic, and transverse bars, as well as the more colloquial sandflat. Braid ...
s 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 log jams.
In large rivers in the
Pacific Northwest
The Pacific Northwest (sometimes Cascadia, or simply abbreviated as PNW) is a geographic region in western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Tho ...
of the United States, it has been shown there is a lifecycle 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 log jam formation. These log jams act as hard points resisting further erosion and channel migration. The areas of floodplain behind these log jams then become stable enough for more large trees to grow, which can in turn become potential log jam anchor points in the future.
Metaphorical usage
"Logjam" or "log jam" can be used metaphorically to mean "deadlock" or "impasse." It can be used either more literally, to mean a physical impasse, or more metaphorically, to mean an impasse in a process due to differing opinions, legal or technical issues, etc.
Here are two example sentences:
* "The presence of an ambulance on the side of the highway created a logjam of rubberneckers who just had to have a look" (more literal).
* "He was called in to try to break the logjam in the negotiations" (more metaphorical).
See also
*
Beaver dam
A beaver dam or beaver impoundment is a dam built by beavers to create a pond which protects against predators such as coyotes, wolves and bears, and holds their food during winter. These structures modify the natural environment in such a way t ...
, a wooden dam created by
beavers
*
Great Raft
The Great Raft was a gigantic log jam or series of "rafts" that clogged the Red and Atchafalaya rivers and was unique in North America in terms of its scale.
Origin
The Great Raft probably began forming in the 12th century. It grew from its u ...
*
River morphology
The terms river morphology and its synonym stream morphology are used to describe the shapes of river channels and how they change in shape and direction over time. The morphology of a river channel is a function of a number of processes and envir ...
*
Stream restoration
*
1886 St. Croix River log jam
References
{{Authority control
Aquatic ecology
Geomorphology
Rivers