Avulsion (river)
Encyclopedia
In sedimentary geology and fluvial geomorphology
, avulsion is the rapid abandonment of a river channel and the formation of a new river channel. Avulsions occur as a result of channel slopes that are much lower than the slope that the river could travel if it took a new course.
settings, where sediment
deposits
as the river enters the ocean and channel gradient
s are typically very small. This process of avulsion in deltaic settings is also known as delta switching.
Deposition from the river results in the formation of an individual deltaic lobe that pushes out into the sea. An example of a deltaic lobe is the bird's-foot delta
of the Mississippi River
, pictured at right with its sediment
plumes. As the deltaic lobe advances, the slope
of the river channel becomes lower because the river channel is longer but has the same change in elevation (see slope
or gradient
). As the slope of the river channel decreases, it becomes unstable for two reasons. First, water under the force of gravity will tend to flow in the most direct course downslope. If the river could breach its natural levees (i.e., during a flood), it would spill out onto a new course with a shorter route to the ocean, thereby obtaining a more stable steeper slope. Second, as its slope is reduced, the amount of shear stress on the bed will decrease, resulting in deposition of sediment within the channel and raising of the channel bed relative to the floodplain. This will make it easier for the river to breach its levees and cut a new channel that enters the ocean at a steeper slope.
When this avulsion occurs, the new channel carries sediment out to the ocean, building a new deltaic lobe., The abandoned delta eventually subsides. Because this process results in the formation of a series of lobes, it is also known as delta switching.
This process is also related to the distributary network of river channels that can be observed within a river delta. When the channel does this, some of its flow can remain in the abandoned channel. When these channel switching events happen repeatedly over time, a mature delta will gain a distributary network.
Subsidence
of the delta and/or sea-level rise can further cause backwater
and deposition in the delta. This deposition fills the channels and leaves a geologic record of channel avulsion in sedimentary basin
s. On average, an avulsion will occur every time the bed of a river channel aggrades
enough that the river channel is superelevated above the floodplain by one channel-depth. In this situation, enough hydraulic head
is available that any breach of the natural levees will result in an avulsion.
in New Hampshire
, in which heavy rains caused flow levels to rise. The flow level rise was pronounced behind an old mill dam, which produced a shallowly-sloping pool that overtopped a sand and gravel quarry, connected with a downstream section of channel, and cut a new (and shorter) channel at a rate of 25–50 meters per hour. Sediment mobilized by this erosional avulsion produced a depositionally-forced meander cutoff further downstream by superelevating the bed around the meander bend to nearly the level of the floodplain.
, where the high-sinuosity
meander
bend is abandoned in favor of the high-slope. This occurs when the ratio between the channel slope and the potential slope after an avulsion is less than 1/6.
Avulsions usually occur as a downstream to upstream process via head cutting erosion. If a bank of a current stream is breached a new trench will be cut into the existing floodplain. It either cuts through floodplain deposits or reoccupies an old channel.
Geomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...
, avulsion is the rapid abandonment of a river channel and the formation of a new river channel. Avulsions occur as a result of channel slopes that are much lower than the slope that the river could travel if it took a new course.
Deltaic and net-depositional settings
Avulsions are common in deltaicRiver delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...
settings, where sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
deposits
Deposition (geology)
Deposition is the geological process by which material is added to a landform or land mass. Fluids such as wind and water, as well as sediment flowing via gravity, transport previously eroded sediment, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of...
as the river enters the ocean and channel gradient
Gradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
s are typically very small. This process of avulsion in deltaic settings is also known as delta switching.
Deposition from the river results in the formation of an individual deltaic lobe that pushes out into the sea. An example of a deltaic lobe is the bird's-foot delta
River delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...
of the Mississippi River
Mississippi River
The Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...
, pictured at right with its sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
plumes. As the deltaic lobe advances, the slope
Slope
In mathematics, the slope or gradient of a line describes its steepness, incline, or grade. A higher slope value indicates a steeper incline....
of the river channel becomes lower because the river channel is longer but has the same change in elevation (see slope
Slope
In mathematics, the slope or gradient of a line describes its steepness, incline, or grade. A higher slope value indicates a steeper incline....
or gradient
Gradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
). As the slope of the river channel decreases, it becomes unstable for two reasons. First, water under the force of gravity will tend to flow in the most direct course downslope. If the river could breach its natural levees (i.e., during a flood), it would spill out onto a new course with a shorter route to the ocean, thereby obtaining a more stable steeper slope. Second, as its slope is reduced, the amount of shear stress on the bed will decrease, resulting in deposition of sediment within the channel and raising of the channel bed relative to the floodplain. This will make it easier for the river to breach its levees and cut a new channel that enters the ocean at a steeper slope.
When this avulsion occurs, the new channel carries sediment out to the ocean, building a new deltaic lobe., The abandoned delta eventually subsides. Because this process results in the formation of a series of lobes, it is also known as delta switching.
This process is also related to the distributary network of river channels that can be observed within a river delta. When the channel does this, some of its flow can remain in the abandoned channel. When these channel switching events happen repeatedly over time, a mature delta will gain a distributary network.
Subsidence
Subsidence
Subsidence is the motion of a surface as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation...
of the delta and/or sea-level rise can further cause backwater
Backwater (river)
A backwater is a part of a river in which there is little or no current. It refers either to a branch of a main river which lies alongside it and then rejoins it or to a body of water in a main river which is backed up by an obstruction such as the tide or a dam.-Alternative channel:If a river has...
and deposition in the delta. This deposition fills the channels and leaves a geologic record of channel avulsion in sedimentary basin
Sedimentary basin
The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...
s. On average, an avulsion will occur every time the bed of a river channel aggrades
Aggradation
Aggradation is the term used in geology for the increase in land elevation due to the deposition of sediment. Aggradation occurs in areas in which the supply of sediment is greater than the amount of material that the system is able to transport...
enough that the river channel is superelevated above the floodplain by one channel-depth. In this situation, enough hydraulic head
Hydraulic head
Hydraulic head or piezometric head is a specific measurement of water pressure above a geodetic datum. It is usually measured as a water surface elevation, expressed in units of length, at the entrance of a piezometer...
is available that any breach of the natural levees will result in an avulsion.
Erosional avulsions
Rivers can also avulse due to the erosion of a new channel that creates a straighter path through the landscape. This can happen during large floods in situations in which the slope of the new channel is significantly greater than that of the old channel. Where the new channel's slope is about the same as the old channel's slope, a partial avulsion will occur in which both channels are occupied by flow. An example of an erosional avulsion is the 2006 avulsion of the Suncook RiverSuncook River
The Suncook River is a river located in central New Hampshire in the United States. It is a tributary of the Merrimack River, which flows to the Gulf of Maine....
in New Hampshire
New Hampshire
New Hampshire is a state in the New England region of the northeastern United States of America. The state was named after the southern English county of Hampshire. It is bordered by Massachusetts to the south, Vermont to the west, Maine and the Atlantic Ocean to the east, and the Canadian...
, in which heavy rains caused flow levels to rise. The flow level rise was pronounced behind an old mill dam, which produced a shallowly-sloping pool that overtopped a sand and gravel quarry, connected with a downstream section of channel, and cut a new (and shorter) channel at a rate of 25–50 meters per hour. Sediment mobilized by this erosional avulsion produced a depositionally-forced meander cutoff further downstream by superelevating the bed around the meander bend to nearly the level of the floodplain.
Meander cutoffs
An example of a minor avulsion is known as a meander cutoffMeander cutoff
A meander cutoff occurs when a meander bend in a river is breached by a chute channel that connects the two closest parts of the bend. This causes the flow to abandon the meander and to continue straight downslope...
, where the high-sinuosity
Sinuosity
Sinuosity or sinuosity index is a measure of deviation of a path between two points from the shortest possible path...
meander
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...
bend is abandoned in favor of the high-slope. This occurs when the ratio between the channel slope and the potential slope after an avulsion is less than 1/6.
Occurrence
Avulsion typically occurs during large floods which carry the power necessary to rapidly change the landscape.Avulsions usually occur as a downstream to upstream process via head cutting erosion. If a bank of a current stream is breached a new trench will be cut into the existing floodplain. It either cuts through floodplain deposits or reoccupies an old channel.
See also
- AggradationAggradationAggradation is the term used in geology for the increase in land elevation due to the deposition of sediment. Aggradation occurs in areas in which the supply of sediment is greater than the amount of material that the system is able to transport...
- River deltaRiver deltaA delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...
- Alluvial fanAlluvial fanAn alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial...
- MeanderMeanderA 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...
- Stream capture