Log jam
Encyclopedia
A log jam is an accumulation of Large Woody Debris
(logs more than four inches in diameter and over six feet long) that can span an entire stream
or river
channel.
hydraulics, which control characteristics of scour and deposition. Thus, log jams become an important factor in the geomorphology
of wooded streams and rivers.
Water is often impounded on the upstream side of a log jam, creating a pool
. Water draining from this pool often creates a downstream "plunge pool
". Decrease in water velocity in the pool often results in deposition of sand
or gravel
in the pool bottom. This may progress into "bar" formation.
Log jams also cause shifts in stream flow. They often redirect flow into former channels or spur creation of new channels
.
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.
Pool and bar formation by log jams also provides prime habitat for riparian forests.
Large Woody Debris
Large woody debris 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...
(logs more than four inches in diameter and over six feet long) that can span an entire stream
Stream
A stream is a body of water with a current, confined within a bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as a branch, brook, beck, burn, creek, "crick", gill , kill, lick, rill, river, syke, bayou, rivulet, streamage, wash, run or...
or river
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
channel.
Effects on stream/river geomorphology
Log jams alter flowStreamflow
Streamflow, or channel runoff, is the flow of water in streams, rivers, and other channels, and is a major element of the water cycle. It is one component of the runoff of water from the land to waterbodies, the other component being surface runoff...
hydraulics, which control characteristics of scour and deposition. Thus, log jams become an important factor in the geomorphology
Geomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...
of wooded streams and rivers.
Water is often impounded on the upstream side of a log jam, creating a pool
Stream pool
A stream pool, in hydrology, is a stretch of a river or stream in which the water depth is above average and the water velocity is quite below average.-Formation:...
. Water draining from this pool often creates a downstream "plunge pool
Plunge pool
A plunge pool can be a natural hydrologic fluvial landform feature or a constructed recreational garden feature...
". Decrease in water velocity in the pool often results in deposition of sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
or gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...
in the pool bottom. This may progress into "bar" formation.
Log jams also cause shifts in stream flow. They often redirect flow into former channels or spur creation of new channels
Channel (geography)
In physical geography, a channel is the physical confine of a river, slough or ocean strait consisting of a bed and banks.A channel is also the natural or human-made deeper course through a reef, sand bar, bay, or any shallow body of water...
.
Effects on ecology
Log jams provide important fishFish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate 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...
habitat. The pools created and sediment deposited by formation of log jams create prime spawning grounds for many species of salmon
Salmon
Salmon is the common name for several species of fish in the family Salmonidae. Several other fish in the same family are called trout; the difference is often said to be that salmon migrate and trout are resident, but this distinction does not strictly hold true...
. 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.
Pool and bar formation by log jams also provides prime habitat for riparian forests.