Channel types
Encyclopedia
A wide variety of river
and stream
channel types exist in limnology
. All these can be divided into two groups by using the water-flow gradient
as either low gradient channels for streams or rivers with less than two percent (2%) flow gradient, or high gradient channels for those with greater than a 2% gradient.
s, wandering rivers, single thread sinuous
rivers (meandering), and anastomosing
rivers. The channel type developed depends on stream gradient, riparian vegetation and sediment supply. Braided river
s tend to occur on steeper gradients where there is a large supply of sediment for braid bar
s, while single thread sinous channels occur where there is a lower sediment supply for point bar
s. Anastomosing channels are multithreaded, but are much more stable than braided channels and commonly have thick clay and silt bank
s and occur at lower gradients of stream bed
. Wandering rivers fall between sinuous single thread and braided streams and are relatively stable multi-channel gravel bed rivers.
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...
and 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...
channel types exist in limnology
Limnology
Limnology , also called freshwater science, is the study of inland waters. It is often regarded as a division of ecology or environmental science. It covers the biological, chemical, physical, geological, and other attributes of all inland waters...
. All these can be divided into two groups by using the water-flow gradient
Stream gradient
Stream gradient is the grade measured by the ratio of drop in a stream per unit distance, usually expressed as feet per mile or metres per kilometre.-Hydrology and geology:A high gradient indicates a steep slope and rapid flow of water Stream gradient is the grade (slope) measured by the ratio of...
as either low gradient channels for streams or rivers with less than two percent (2%) flow gradient, or high gradient channels for those with greater than a 2% gradient.
Low gradient channels
Low gradient channels of rivers and streams can be divided into braided riverBraided river
A braided river is one of a number of channel types and has a channel that consists of a network of small channels separated by small and often temporary islands called braid bars or, in British usage, aits or eyots. Braided streams occur in rivers with high slope and/or large sediment load...
s, wandering rivers, single thread sinuous
Meander
A meander in general is a bend in a sinuous watercourse. A meander is formed when the moving water in a stream erodes the outer banks and widens its valley. A stream of any volume may assume a meandering course, alternately eroding sediments from the outside of a bend and depositing them on the...
rivers (meandering), and anastomosing
Anastomosis
An anastomosis is the reconnection of two streams that previously branched out, such as blood vessels or leaf veins. The term is used in medicine, biology, mycology and geology....
rivers. The channel type developed depends on stream gradient, riparian vegetation and sediment supply. Braided river
Braided river
A braided river is one of a number of channel types and has a channel that consists of a network of small channels separated by small and often temporary islands called braid bars or, in British usage, aits or eyots. Braided streams occur in rivers with high slope and/or large sediment load...
s tend to occur on steeper gradients where there is a large supply of sediment for braid bar
Braid bar
Braid Bars, or mid-channel bars, are landforms in a river that begin to form when the discharge is low and the river is forced to take the route of less resistance by means of flowing in locations of lowest elevation. Over time, the river begins to erode the outer edges of the bar, causing it to...
s, while single thread sinous channels occur where there is a lower sediment supply for point bar
Point bar
A point bar is a depositional feature of streams. Point bars are found in abundance in mature or meandering streams. They are crescent-shaped and located on the inside of a stream bend, being very similar to, though often smaller than towheads, or river islands.Point bars are composed of sediment...
s. Anastomosing channels are multithreaded, but are much more stable than braided channels and commonly have thick clay and silt bank
Bank (geography)
A geographic bank has four definitions and applications:# Limnology: The shoreline of a pond, swamp, estuary, reservoir, or lake. The grade can vary from vertical to a shallow slope....
s and occur at lower gradients of stream bed
Stream bed
A stream bed is the channel bottom of a stream, river or creek; the physical confine of the normal water flow. The lateral confines or channel margins, during all but flood stage, are known as the stream banks or river banks. In fact, a flood occurs when a stream overflows its banks and flows onto...
. Wandering rivers fall between sinuous single thread and braided streams and are relatively stable multi-channel gravel bed rivers.
High gradient channels
High gradient channels of rivers and streams have been divided into riffle-pool (which can cover all of the low gradient channel morphologies discussed above), rapid/plane bed, step-pool and cascade unit morphologies.- Riffle-pool sequenceRiffle-Pool SequenceIn a flowing stream, a riffle-pool sequence develops as a stream's hydrological flow structure alternates from areas of relatively shallow to deeper water. This sequence is present only in streams carrying gravel or coarser sediments. Riffles are formed in shallow areas by coarser materials such...
channels are composed of migrating poolsStream poolA 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:...
and transfers barsBar (river morphology)A bar in a river is an elevated region of sediment that has been deposited by the flow. Types of bars include mid-channel bars , point bars , and mouth bars...
called riffleRiffleA Riffle is a short, relatively shallow and coarse-bedded length of stream over which the stream flows at higher velocity and higher turbulence than it normally does in comparison to a pool....
s and occur on gradients less than 1-2 percent. - RapidRapidA rapid is a section of a river where the river bed has a relatively steep gradient causing an increase in water velocity and turbulence. A rapid is a hydrological feature between a run and a cascade. A rapid is characterised by the river becoming shallower and having some rocks exposed above the...
s (also called plane bed, but not to be confused with the plane beds described in sand bed rivers) lack distinct pools and bars but commonly have stone cells or clusters and occur on gradients in the range of 1-5 percent, and have "whitewaterWhitewaterWhitewater is formed in a rapid, when a river's gradient increases enough to disturb its laminar flow and create turbulence, i.e. form a bubbly, or aerated and unstable current; the frothy water appears white...
". - Step-pools are composed of channel-spanning pools and boulderBoulderIn geology, a boulder is a rock with grain size of usually no less than 256 mm diameter. While a boulder may be small enough to move or roll manually, others are extremely massive....
/cobble steps that cause subcritical flow in the pool and supercritical flowSupercritical flowA supercritical flow is when the flow velocity is larger than the wave velocity. The analogous condition in gas dynamics is supersonic....
over the steps. They occur in gradients in the range of 5 and 20%. - CascadeWaterfallA waterfall is a place where flowing water rapidly drops in elevation as it flows over a steep region or a cliff.-Formation:Waterfalls are commonly formed when a river is young. At these times the channel is often narrow and deep. When the river courses over resistant bedrock, erosion happens...
units exist at steeper gradients (approx > 10–15 percent) where the channel is dominated by boulders and cobbles and channel spanning pools do not exist. Pocket pools are common. In all four channel types large woody debrisLarge Woody DebrisLarge 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...
may strongly influence the channel type.
See also
- Relief ratioRelief ratioThe relief ratio is a number calculated to describe the Grade of a river or stream.The calculation is just the difference in elevation between the river's source and the river's confluence or mouth divided by the total length of the river or stream. This gives the average drop in elevation per...
- Stream gradientStream gradientStream gradient is the grade measured by the ratio of drop in a stream per unit distance, usually expressed as feet per mile or metres per kilometre.-Hydrology and geology:A high gradient indicates a steep slope and rapid flow of water Stream gradient is the grade (slope) measured by the ratio of...
- Stream restorationStream restorationStream restoration or river restoration, sometimes called river reclamation in the UK, describes a set of activities that help improve the environmental health of a river or stream. Improved health may be indicated by expanded habitat for diverse species and reduced stream bank erosion...
- Riparian zone restorationRiparian zone restorationRiparian zone restoration is the ecological restoration of riparian zone habitats of streams, rivers, springs, lakes, floodplains, and other hydrologic ecologies.Riparian zones have been degraded throughout much of the world...
- Riparian zone restoration