What is avulsion in the context of sedimentary geology and fluvial geomorphology?

Avulsion is defined as the rapid abandonment of a river channel and the subsequent formation of a new river channel. This phenomenon is driven by conditions where the existing channel slope is less steep than a potential new course the river could take.

What is the primary reason cited for avulsions occurring in river deltas?

Avulsions are common in river deltas primarily because the channel gradients there are typically very small. This is due to the deposition of sediment as the river enters a larger body of water, which reduces the slope of the river channel.

What is another term used for avulsion in deltaic environments?

In deltaic settings, avulsion is also known as delta switching. This term highlights the process where a river shifts its course, creating a new deltaic lobe.

How does the advancement of a deltaic lobe contribute to the conditions for avulsion?

As a deltaic lobe extends further into the sea, the river channel's slope decreases because the channel becomes longer while the elevation change remains the same. This reduced slope makes the channel less stable and more prone to avulsion.

According to the text, what are the two main reasons why a river channel with a decreasing slope becomes unstable and promotes avulsion?

A river channel with a decreasing slope becomes unstable for two primary reasons: First, gravity naturally pulls water along the most direct, steepest path downslope; if the river breaches its levees during a flood, it will seek a shorter, steeper course to the ocean. Second, a reduced slope leads to less shear stress on the riverbed, causing more sediment deposition, which raises the channel bed relative to the floodplain, making it easier for the river to breach its banks and form a new channel.

What happens to the river and the landform after an avulsion occurs?

When an avulsion occurs, the river establishes a new channel that carries sediment to the ocean, thereby building a new deltaic lobe. The previously active channel and its associated deltaic lobe are eventually abandoned and tend to subside over time.

How can repeated channel switching events shape a mature delta?

When channel switching events, or avulsions, happen repeatedly over time, a mature delta develops a complex network of distributary channels. Some flow may continue in the abandoned channels, contributing to this intricate pattern.

What geological processes can lead to backwater conditions and deposition in a delta, potentially leaving a record of avulsion?

Subsidence of the delta itself or rising sea levels can cause backwater conditions and increased deposition within the delta. This deposition can fill existing channels, leaving behind a geological record of past channel avulsions within sedimentary basins.

What specific condition related to channel bed elevation is mentioned as a trigger for avulsion?

An avulsion typically occurs when the bed of a river channel has aggraded sufficiently to be superelevated above the floodplain by a distance equal to one channel depth. At this point, enough hydraulic head is available for any breach of the natural levees to result in an avulsion.

What is an erosional avulsion, and what conditions favor its occurrence?

An erosional avulsion happens when a river erodes a new, straighter path across the landscape, often during large floods. This type of avulsion is favored when the slope of the new channel is significantly greater than that of the original channel.

What distinguishes a partial avulsion from a complete avulsion?

A partial avulsion occurs when the slope of a newly eroded channel is roughly the same as the slope of the original channel. In this scenario, the river splits its flow between both the original and the newly formed channel.

Can you describe the 2006 avulsion of the Suncook River in New Hampshire as an example of erosional avulsion?

Yes, the 2006 Suncook River avulsion was an example of erosional avulsion. Heavy rains caused the river to overtop its banks and cut a new, shorter channel through a sand and gravel quarry. This demonstrated an erosional avulsion process, rapidly changing the river's course.

What was the impact of the Suncook River avulsion on the downstream channel?

The sediment mobilized by the Suncook River's erosional avulsion in 2006 contributed to a meander cutoff further downstream. This occurred because the superelevated riverbed around the meander bend was raised to nearly the level of the floodplain.

How was the Cheslatta River in British Columbia involved in an erosional avulsion?

The Cheslatta River experienced erosional avulsion due to significant water discharge from the Nechako Reservoir, which was created in the 1950s. The increased flow caused major erosion in the upper valley, with sediment deposited downstream. This led to the river avulsing in 1961 and again in 1972, carving new routes to the Nechako River and depositing sediment fans.

What measure was taken to restore the Cheslatta River to its original course after its avulsions?

Following the 1972 avulsion of the Cheslatta River, a cofferdam was constructed to restore the river to its original course. This intervention aimed to manage the erosional impacts caused by the reservoir spills.

What is a meander cutoff, and under what slope condition does it typically occur?

A meander cutoff is a minor form of avulsion where a river breaches a pronounced meander, creating a straighter channel that bypasses the loop. This typically happens when the ratio between the existing channel slope and the potential slope of the new, shorter course is less than approximately 1/5.

What are the typical conditions under which avulsions occur?

Avulsions typically occur during large floods, which provide the necessary energy to rapidly alter the river's course. Additionally, the removal of dams could potentially lead to avulsions.

How does avulsion generally progress from downstream to upstream?

Avulsions usually progress in a downstream-to-upstream manner through a process known as headward erosion. When a bank of the existing stream is breached, a new trench is cut into the floodplain, either through existing deposits or by reoccupying an old channel.

What factors have been investigated as potential causes for avulsions in deltaic and coastal plain channels?

Avulsions in deltaic and coastal plain channels have been investigated in relation to obstructions such as log-jams and also due to possible tectonic influences. These factors can disrupt the natural flow and stability of river channels.

What is the significance of the image caption regarding the Mississippi River delta?

The image caption describes plumes of sediment entering the ocean from the Mississippi River's bird's-foot delta. It explains that this sediment builds the delta and causes it to advance into the sea, leading to a decrease in channel slope and bed aggradation, which are conditions that promote avulsion.

What is aggradation in the context of river channels?

Aggradation is the process by which a river channel's bed is raised due to the deposition of sediment. This is a key factor contributing to channel instability and the potential for avulsion.

What is a river delta, and how is it related to avulsion?

A river delta is a landform created by the deposition of sediment where a river enters a larger body of water, such as an ocean or lake. Avulsions are common in deltas because the deposition of sediment reduces the channel slope and can lead to the river switching to a new course.

What is a river plume, and how does it relate to deltaic processes?

A river plume is the mixture of fresh river water and denser seawater that spreads out as the river enters the ocean. The sediment carried within these plumes is responsible for building deltas and can influence the conditions that lead to avulsion.

What is a floodplain, and how does a river's elevation relative to it play a role in avulsion?

A floodplain is the flat area of land adjacent to a river that is subject to flooding. Avulsions can occur when a river channel aggrades to the point where it is significantly elevated above its floodplain, creating a steeper potential gradient for a new channel.

What is stream capture, and how might it be related to avulsions?

Stream capture is a geomorphological process where a river erodes into another river's drainage basin, diverting its flow. While not explicitly detailed in the main text, it is listed as a related concept, suggesting a similarity in how river courses can be altered.

What is the primary driver for avulsion in river systems?

The primary driver for avulsion is the instability of a river channel caused by a low gradient slope. This instability encourages the river to seek a new course with a steeper gradient, often facilitated by floods or sediment deposition.

What is the role of gravity in the process of avulsion?

Gravity plays a crucial role by naturally favoring the path of steepest descent. When a river channel becomes too shallow or obstructed, gravity will pull the water towards a more direct and steeper route, potentially leading to an avulsion.

What is the concept of "delta switching"?

Delta switching is another term for avulsion, particularly used in the context of river deltas. It describes the process where a river abandms its current channel and begins flowing through a new channel, effectively switching the location of its deltaic deposition.

What is the "Mississippi River bird's-foot delta" mentioned in the text?

The Mississippi River bird's-foot delta is a specific type of river delta characterized by its long, finger-like lobes that extend far into the sea. The image caption notes that sediment plumes from this delta are responsible for its growth and the conditions that promote avulsion.

What is a "head cut" in the context of river erosion and avulsion?

A head cut, or headward erosion, is a process where erosion moves upstream along a river channel or a newly formed channel. This is the mechanism by which avulsions typically progress from the downstream point of breach towards the upstream parts of the river.

What role do natural levees play in the process of avulsion?

Natural levees are raised banks formed by sediment deposition along the sides of a river during floods. Avulsions often occur when a river overtops or breaches these natural levees, allowing water to escape the main channel and establish a new course.

What is the "Cheslatta Fan"?

The Cheslatta Fan is a deposit of sediment in the Nechako River, British Columbia, formed by the Cheslatta River after it experienced major erosional avulsions. These avulsions were caused by large reservoir spills from the Nechako Reservoir, which significantly increased the river's discharge and erosive power.

What is a "meander cutoff" and how does it relate to avulsion?

A meander cutoff is a specific type of minor avulsion where a river cuts across the narrow neck of a meander loop, creating a straighter channel. This happens when the river finds a shorter path, often facilitated by erosion during floods.

What is the significance of the ratio between channel slope and potential slope in meander cutoffs?

For meander cutoffs, a minor type of avulsion, the ratio between the existing channel slope and the potential slope of the new, straighter course is critical. If this ratio is less than approximately 1/5, a meander cutoff is likely to occur.

What is the role of sediment deposition in raising the channel bed relative to the floodplain?

As a river's slope decreases, it loses energy and deposits more sediment within its channel. This continuous deposition gradually raises the channel bed over time, making it higher than the adjacent floodplain and increasing the potential for the river to breach its banks.

What is meant by "channel slope" in the context of river avulsion?

Channel slope refers to the gradient of the riverbed, indicating how steeply the river descends from its source to its mouth. Avulsions occur when this slope becomes too gentle, prompting the river to seek a new course with a steeper gradient.

How can dam removal potentially lead to river avulsion?

Dam removal can lead to avulsion by altering the natural flow regime and sediment transport dynamics of a river. The sudden release of water and sediment, or the subsequent adjustment of the riverbed to a new equilibrium, can create conditions conducive to channel switching.

What is the significance of "log-jams" in relation to river avulsions?

Log-jams can act as obstructions within a river channel, causing water to back up and potentially overtop the banks. This blockage can initiate erosion and lead to the formation of a new channel, thus triggering an avulsion.

What is a "river plume" and how does it relate to the Mississippi River delta?

A river plume is the area where river water mixes with ocean water. The Mississippi River's extensive sediment plumes are crucial for building its bird's-foot delta, and as these plumes extend into the ocean, they contribute to the decreasing channel slope that promotes avulsion.

What is the "International scale of river difficulty" mentioned in the navbox?

The International Scale of River Difficulty is a system used to classify the challenges and hazards of navigating rivers, particularly for activities like whitewater rafting or kayaking. While not directly explained in the main text, it relates to the physical characteristics of rivers, such as rapids and flow.

What is a "point bar" in river morphology?

A point bar is a depositional feature found on the inside bend of a meander in a river. It forms as slower water on the inside of the bend deposits sediment, contrasting with the faster water on the outside bend that erodes the cut bank.

What is a "braided river"?

A braided river is characterized by multiple, smaller channels that split and rejoin around mid-channel bars or islands. This pattern often occurs in areas with high sediment loads and erodible banks, and can be related to avulsion processes.

What is "floodplain restoration" in the context of river engineering?

Floodplain restoration involves efforts to return a floodplain to a more natural state, often by reconnecting it with the river. This can help manage floodwaters, improve ecological habitats, and potentially influence river channel behavior, though it's distinct from avulsion itself.

What is "sediment transport" in the context of rivers?

Sediment transport refers to the movement of solid particles (sediment) by flowing water. This process is fundamental to river geomorphology, as it builds landforms like deltas and floodplains, and its dynamics are directly linked to the causes and effects of avulsion.

What is a "river valley"?

A river valley is the landform surrounding a river, typically carved by the river's erosive action over time. It encompasses the river channel, floodplain, and surrounding slopes, and its shape can influence where avulsions might occur.

What is a "river mouth"?

A river mouth is the point where a river flows into a larger body of water, such as an ocean, sea, lake, or another river. It is often characterized by depositional features like deltas and is a common location for avulsions to occur.

What is the "Suncook River Avulsion" of May 2006?

The Suncook River Avulsion of May 2006 was an event where heavy rains caused the river to overtop its banks and cut a new, shorter channel through a sand and gravel quarry. This demonstrated an erosional avulsion process, rapidly changing the river's course.

What is a "partial avulsion" and when does it occur?

A partial avulsion is a type of avulsion where the new channel has a slope similar to the old channel. In this scenario, the river splits its flow between both the original and the newly formed channel.

What is the relationship between aggradation and the superelevation of a river channel?

Aggradation, the process of raising the riverbed through sediment deposition, directly leads to the superelevation of the channel. When the aggraded bed is raised sufficiently above the surrounding floodplain, it creates the conditions for an avulsion.

What is the significance of "tectonic influences" in relation to river avulsions?

Tectonic influences, such as crustal movements or faulting, can alter the landscape and river gradients. These geological forces can create conditions that promote or trigger river avulsions by changing the relative elevations and slopes of the land.

What is the primary cause of instability in river channels that leads to avulsion?

The primary cause of instability leading to avulsion is a decrease in the river channel's slope. When the slope becomes too gentle, the river is more likely to seek a new, steeper path.

Avulsion River Wiki: River Realignments: The Dynamics of Avulsion

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What is Avulsion?

Definition and Core Concept

In the fields of sedimentary geology and fluvial geomorphology, avulsion signifies the rapid abandonment of an existing river channel and the concurrent establishment of a new one. This phenomenon is driven by the river's tendency to seek the path of least resistance, typically dictated by gravitational forces acting upon its flow. Essentially, avulsion occurs when the existing channel slope becomes less steep than a potential alternative course the river could adopt.

The Driving Force: Slope Instability

The fundamental principle behind avulsion lies in channel slope instability. When a river's gradient becomes too gentle, its capacity to transport sediment and maintain flow efficiency diminishes. This instability prompts the river to seek a shorter, steeper path to the sea or a lower base level, leading to the formation of a new channel and the abandonment of the old one.

A Fundamental Fluvial Process

Avulsion is not an anomaly but a natural and recurring process in the evolution of river systems, particularly evident in depositional environments like river deltas. It plays a critical role in shaping landscapes over geological timescales, influencing sediment distribution and the development of complex fluvial networks.

Avulsion in Deltaic Environments

Delta Switching Explained

Avulsions are particularly prevalent in river deltas, a process often termed delta switching. Here, rivers deposit significant amounts of sediment as they meet a standing body of water, such as an ocean or lake. This deposition naturally reduces the channel slope over time.

As a deltaic lobe, like the Mississippi River's bird's-foot delta, advances seaward, the river channel lengthens. This extension, coupled with continuous sediment deposition at the mouth, causes the channel slope to decrease. This reduction in gradient makes the channel unstable for two primary reasons:

Gravitational Preference: Water naturally seeks the most direct downslope path. During floods, if the river breaches its natural levees, it can spill onto a new course offering a shorter, steeper route to the sea.
Aggradation and Superelevation: A reduced slope leads to increased sediment deposition within the channel itself (aggradation). This raises the channel bed relative to the surrounding floodplain. Eventually, the channel becomes superelevated—higher than the floodplain—making it easier for floodwaters to breach the levees and establish a new channel with a steeper gradient.

When such an avulsion occurs, the new channel begins building a new deltaic lobe, while the previously active lobe gradually subsides and is eventually submerged.

Building Complex Deltas

The repeated cycle of deltaic lobe advancement, channel aggradation, avulsion, and subsidence is responsible for the intricate, branching patterns observed in many large river deltas. These cycles create a complex distributary network over time, reflecting the river's continuous search for an efficient pathway.

The Role of Sediment Load

The volume and type of sediment carried by a river are crucial factors. Rivers with high sediment loads, like the Mississippi, are more prone to rapid aggradation and subsequent avulsion because sediment deposition quickly modifies the channel's slope and bed elevation. This dynamic interplay between water discharge and sediment transport is central to delta formation and evolution.

Erosional Avulsions

Cutting New Paths

Unlike depositional avulsions common in deltas, erosional avulsions occur when a river erodes a new, straighter channel that offers a significantly steeper slope compared to its existing meandering path. This process is often triggered by major flood events that can carve new pathways through the landscape.

Partial vs. Full Avulsion

In situations where the newly eroded channel's slope is comparable to the old channel's slope, a partial avulsion may occur. In this scenario, the river's flow divides, with a portion continuing in the original channel while the remainder shifts to the new one. A full avulsion happens when the new channel completely captures the river's flow.

Suncook River, New Hampshire (2006): Heavy rainfall caused floodwaters to overtop a quarry, creating a shorter, steeper channel that bypassed a section of the existing river. This event also led to downstream deposition, causing a meander cutoff.
Cheslatta River, British Columbia: Altered flow regimes due to reservoir management caused significant erosion in the upper valley. Subsequent deposition in the lower valley led to avulsions in 1961 and 1972, creating a sediment fan in the Nechako River. A cofferdam was later built to restore the original course.

Meander Cutoffs: Minor Avulsions

Shortening the Path

A meander cutoff represents a minor form of avulsion. It occurs when a river flowing through a pronounced meander bend erodes through the narrow neck of land separating the two closest points of the bend. This breach creates a new, straighter channel, effectively shortening the river's course.

The Slope Ratio Threshold

Meander cutoffs typically happen when the ratio between the existing channel slope and the potential slope of the new, cut-off channel is less than approximately 1/5. This condition indicates that the new path offers a significantly more efficient route for the water flow, promoting the avulsion event.

When and How Avulsions Occur

Triggered by Floods

Avulsions are most commonly triggered during significant flood events. The increased water discharge and energy during floods provide the necessary force to breach channel banks, erode new pathways, and overcome the stability constraints of the existing channel.

The Head-Cutting Process

Avulsions typically propagate upstream through a process known as head-cutting erosion. When a breach occurs, erosion begins at the point of entry into the floodplain, gradually cutting backward (upstream) into the existing floodplain deposits or reoccupying abandoned channel segments. This process continues until the new channel is fully established and captures a significant portion of the river's flow.

Other Influences

While floods are the primary trigger, other factors can contribute to or influence avulsion events. These include obstructions such as large log jams within the channel and potential tectonic influences that might alter local base levels or create new topographic gradients.

Related Fluvial Concepts

Aggradation

Aggradation refers to the process of increasing land elevation due to the deposition of sediment. In rivers, it often leads to a rise in the channel bed, contributing to the superelevation that can trigger avulsion.

Stream Capture

Stream capture (or river capture) is a geomorphological process where a river erodes into an adjacent river system, diverting its flow. While distinct from avulsion, both involve a change in a river's course and are driven by erosional processes and slope dynamics.

Alluvial Fans

Alluvial fans are fan-shaped deposits of sediment formed where a river emerges from a confined valley onto a plain. The processes that form alluvial fans, including sediment deposition and channel shifting, are closely related to avulsion dynamics.

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References

Easterbrook, Don J.Surface Processes and Landforms Second EditionPrentice Hall, New Jersey: 1999.

A full list of references for this article are available at the Avulsion (river) Wikipedia page

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Important Notice

This content has been generated by an Artificial Intelligence, drawing upon established scientific literature. It is intended for educational and informational purposes, providing a structured overview of river avulsion. While efforts have been made to ensure accuracy and clarity, this resource is based on available data and may not encompass all nuances or the very latest research findings.

This is not professional geological or hydrological advice. The information presented here should not substitute consultation with qualified geomorphologists, geologists, or hydrologists for specific projects or analyses. Always consult with experts and refer to primary scientific sources for critical applications.

The creators of this page are not liable for any errors, omissions, or consequences arising from the use of this information.

River Realignments

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What is Avulsion? ℹ️

➡️ Definition and Core Concept

⚖️ The Driving Force: Slope Instability

🌍 A Fundamental Fluvial Process

Avulsion in Deltaic Environments 🌊

🏝️ Delta Switching Explained

🏗️ Building Complex Deltas

📉 The Role of Sediment Load

Erosional Avulsions ✂️

⛰️ Cutting New Paths

↔️ Partial vs. Full Avulsion

Meander Cutoffs: Minor Avulsions ➰

🔄 Shortening the Path

📏 The Slope Ratio Threshold

When and How Avulsions Occur ⏳

⚡ Triggered by Floods

⬆️ The Head-Cutting Process

🚧 Other Influences

Related Fluvial Concepts 🔗

📈 Aggradation

🗺️ Stream Capture

🌳 Alluvial Fans

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📜 Important Notice

Dive in with Flashcard Learning!

What is Avulsion?

Definition and Core Concept

The Driving Force: Slope Instability

A Fundamental Fluvial Process

Avulsion in Deltaic Environments

Delta Switching Explained

Building Complex Deltas

The Role of Sediment Load

Erosional Avulsions

Cutting New Paths

Partial vs. Full Avulsion

Meander Cutoffs: Minor Avulsions

Shortening the Path

The Slope Ratio Threshold

When and How Avulsions Occur

Triggered by Floods

The Head-Cutting Process

Other Influences

Related Fluvial Concepts

Aggradation

Stream Capture

Alluvial Fans

Teacher's Corner

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Discover other topics to study!

References

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Disclaimer

Important Notice