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Avulsion is characterized by the gradual shifting of a river's course over centuries.
Answer: False
Explanation: This statement is incorrect. Avulsion is defined as the rapid abandonment of an existing channel and the formation of a new one, rather than a gradual shift.
Gravity acts to encourage rivers to maintain their existing course, even when a shorter, steeper alternative path becomes available.
Answer: False
Explanation: This statement is incorrect. Gravity naturally favors the path of steepest descent; therefore, it encourages rivers to adopt shorter, steeper routes when available, potentially leading to avulsion.
Avulsions typically progress from upstream to downstream through the process of headward erosion.
Answer: False
Explanation: This statement is incorrect. Avulsions generally progress from downstream to upstream via headward erosion, where the new channel is incised back towards the source.
The primary driver for river avulsion is an increasing channel slope.
Answer: False
Explanation: This statement is incorrect. The primary driver for avulsion is a decreasing or gentle channel slope, which leads to instability and the search for a steeper path.
Gravity's role in avulsion is to resist the flow towards steeper paths.
Answer: False
Explanation: This statement is incorrect. Gravity's role is to promote flow towards the path of steepest descent, thereby encouraging avulsion when a steeper alternative exists.
Avulsions are typically associated with periods of low flow and channel stability.
Answer: False
Explanation: This statement is incorrect. Avulsions are typically triggered by high-energy events, such as floods, which provide the necessary force to breach channels, rather than periods of low flow and stability.
Avulsions progress from upstream to downstream via headward erosion.
Answer: False
Explanation: This statement is incorrect. Avulsions generally progress from downstream to upstream via headward erosion, where the new channel is incised back towards the source.
The principal cause of instability that precipitates river avulsion is a decreasing channel slope.
Answer: True
Explanation: The principal cause of instability leading to avulsion is a decreasing channel slope, which encourages the river to seek a steeper path.
Gravity's role in the process of avulsion is to favor the path of steepest descent.
Answer: True
Explanation: Gravity's role in avulsion is indeed to favor the path of steepest descent, encouraging the river to adopt a more direct and steeper route.
Headward erosion is the mechanism by which avulsions progress from downstream locations towards upstream segments of the river.
Answer: True
Explanation: Headward erosion is the process by which avulsions typically progress upstream from a point of breach.
What is the fundamental definition of avulsion within the discipline of fluvial geomorphology?
Answer: The rapid abandonment of an old channel and the formation of a new one.
Explanation: Avulsion is characterized by the rapid abandonment of an existing river channel and the establishment of a new course.
How do avulsions typically progress along a river system?
Answer: From downstream to upstream via headward erosion.
Explanation: Avulsions typically progress from downstream to upstream via headward erosion.
What is the primary driver responsible for causing instability in river channels that ultimately leads to avulsion?
Answer: A decreasing or low channel slope.
Explanation: The primary driver causing instability that leads to avulsion is a decreasing or low channel slope.
What is the definition of 'headward erosion' in the context of avulsion progression?
Answer: The process by which erosion moves upstream along a new channel path.
Explanation: Headward erosion is the process by which erosion advances upstream along a new channel path, characteristic of avulsion progression.
What is the role of gravity in the process of river avulsion?
Answer: Gravity pulls water towards the path of steepest descent, encouraging a new channel.
Explanation: Gravity plays a crucial role by favoring the path of steepest descent, encouraging a new channel when available.
What is the primary cause of instability in river channels that leads to avulsion?
Answer: Decreasing channel slope.
Explanation: The primary cause of instability leading to avulsion is a decreasing channel slope.
Rivers exhibit a greater propensity for avulsion when their channel slope is steep.
Answer: False
Explanation: This assertion is false. Rivers are more prone to avulsion when their channel slope is gentle or decreasing, as this leads to reduced flow energy and increased sediment deposition, destabilizing the channel.
A reduction in channel slope results in increased shear stress on the riverbed, thereby inhibiting sediment deposition.
Answer: False
Explanation: This statement is false. A reduced channel slope leads to decreased shear stress, which in turn promotes sediment deposition and the aggradation of the riverbed.
Rising sea levels can induce backwater conditions, leading to the infilling of existing channels and potentially obscuring the geological record of past avulsions.
Answer: False
Explanation: This statement is incorrect. Rising sea levels or subsidence can cause backwater conditions and deposition, which actually preserve or record past avulsions within the sedimentary strata, rather than obscuring them.
Avulsion is typically initiated when the river channel bed has aggraded to a level approximately one channel depth above the adjacent floodplain.
Answer: True
Explanation: Avulsion is typically triggered when the river channel bed has aggraded to a state of superelevation, reaching a level approximately one channel depth above the floodplain, thereby providing sufficient hydraulic head for levee breaches.
Avulsions are primarily triggered by conditions of low water levels and drought.
Answer: False
Explanation: This statement is incorrect. Avulsions are typically associated with high-energy events, such as major floods, which provide the necessary force to breach channels, rather than low water conditions.
Log-jams have not been investigated as potential causes for avulsions in deltaic channels.
Answer: False
Explanation: This statement is false. Log-jams have been investigated as potential causes for avulsions, acting as obstructions that can lead to channel overtopping and subsequent avulsion.
Log-jams can precipitate avulsions by functioning as obstructions that induce channel overtopping.
Answer: True
Explanation: Log-jams can indeed precipitate avulsions by acting as obstructions that lead to channel overtopping and subsequent course alteration.
Sediment deposition elevates a river channel's bed exclusively during periods of increasing slope.
Answer: False
Explanation: This statement is false. Sediment deposition, and subsequent bed aggradation, occurs when a river's slope *decreases*, leading to a loss of energy and the settling of transported sediment.
The removal of dams possesses the potential to induce avulsions through modifications in river flow regimes and sediment transport dynamics.
Answer: True
Explanation: The removal of dams can indeed lead to avulsions by altering river flow and sediment dynamics, potentially creating conditions conducive to channel switching.
Tectonic influences possess the capacity to trigger avulsions through the alteration of river gradients.
Answer: True
Explanation: Tectonic influences, such as crustal movements, can alter river gradients and thereby trigger avulsions.
Superelevation of a river channel is caused by the erosion of its banks.
Answer: False
Explanation: This statement is false. Superelevation of a river channel, where the bed is raised above the floodplain, is primarily caused by aggradation (sediment deposition), not bank erosion.
What is the consequence of reduced shear stress on a riverbed, resulting from a decreasing channel slope?
Answer: Increased sediment deposition, raising the channel bed.
Explanation: Reduced shear stress on the riverbed due to a decreasing slope leads to increased sediment deposition and channel aggradation.
What specific condition pertaining to channel bed elevation is cited as a primary trigger for avulsion?
Answer: The channel bed aggrading to be one channel depth above the floodplain.
Explanation: Avulsion is typically triggered when the channel bed has aggraded to a level approximately one channel depth above the floodplain.
Which of the following is identified as a common trigger for river avulsions?
Answer: Removal of dams.
Explanation: The removal of dams is identified as a common trigger for river avulsions.
Beyond channel slope, what factors have been investigated as potential causes for avulsions in deltaic and coastal plain channels?
Answer: Tectonic influences and log-jams.
Explanation: Tectonic influences and log-jams have been investigated as potential causes for avulsions in deltaic and coastal plain channels.
How does sediment deposition contribute to the elevation of a river channel's bed?
Answer: By depositing sediment when the river's slope decreases and it loses energy.
Explanation: Sediment deposition contributes to raising a river channel's bed when the river's slope decreases, causing it to lose energy and deposit sediment.
What is the definition of 'superelevation' in relation to river channels and the phenomenon of avulsion?
Answer: The condition where the channel bed is significantly higher than the surrounding floodplain due to aggradation.
Explanation: Superelevation refers to the condition where a river channel's bed is significantly higher than the surrounding floodplain due to aggradation.
An erosional avulsion is characterized by a river establishing a new course that is significantly less steep than its original channel.
Answer: False
Explanation: This statement is incorrect. An erosional avulsion typically occurs when the new path offers a significantly steeper gradient, facilitating rapid erosion and channel switching.
In the context of a partial avulsion, the river completely abandons its original channel.
Answer: False
Explanation: This statement is false. A partial avulsion occurs when the river splits its flow between the original channel and a newly formed channel, rather than completely abandoning the former.
A meander cutoff typically occurs when the potential new channel slope is substantially gentler than the existing channel slope.
Answer: False
Explanation: This statement is false. Meander cutoffs are favored when the potential new channel offers a steeper or significantly shorter path, which gravity will exploit.
Stream capture is a geomorphological process wherein a river diverts its flow into the drainage basin of another river.
Answer: True
Explanation: Stream capture is indeed a geomorphological process where a river diverts its flow into another river's drainage basin.
For meander cutoffs, avulsion is likely if the existing channel slope is significantly less than the potential new slope.
Answer: False
Explanation: This statement is false. A meander cutoff is likely when the potential new channel slope is significantly greater than the existing slope, or the new path is substantially shorter, making it more favorable.
Stream capture is unrelated to the process of river course alteration.
Answer: False
Explanation: This statement is false. Stream capture, like avulsion, represents a significant alteration of a river's course, involving the diversion of flow into a new drainage basin.
A meander cutoff is classified as a major avulsion that results in the complete redirection of the river's flow.
Answer: False
Explanation: This statement is false. A meander cutoff is generally considered a minor form of avulsion, where the river bypasses a meander loop, rather than a complete redirection of the entire river system.
A critical factor for the occurrence of meander cutoffs is the ratio between the existing channel slope and the potential slope of a new, shorter path, often cited as approximately 1/5.
Answer: True
Explanation: A critical factor for meander cutoffs is indeed the ratio between the existing channel slope and the potential slope of a new path; a ratio less than approximately 1/5 indicates a high likelihood of cutoff.
A partial avulsion is characterized by the new channel possessing a significantly steeper slope than the original channel.
Answer: False
Explanation: This statement is false. A partial avulsion typically occurs when the new channel has a slope comparable to the old channel, leading to flow splitting, rather than a significantly steeper slope.
Under which slope condition is an erosional avulsion most likely to be favored?
Answer: When the new channel slope is significantly greater than the original.
Explanation: An erosional avulsion is favored when the new channel offers a significantly steeper gradient than the original channel.
What characteristic defines a partial avulsion?
Answer: The river splits its flow between the old and new channels.
Explanation: A partial avulsion is defined by the river splitting its flow between the original channel and a newly formed channel.
What is the critical condition regarding the ratio of channel slopes for a meander cutoff to occur?
Answer: The ratio of existing slope to potential new slope must be less than 1/5.
Explanation: For meander cutoffs, a critical factor is the ratio of existing slope to potential new slope being less than approximately 1/5.
What is the relationship between a meander cutoff and the broader process of avulsion?
Answer: A meander cutoff is a type of minor avulsion.
Explanation: A meander cutoff is considered a type of minor avulsion.
What is the significance of the ratio between a river's existing channel slope and the potential slope of a new path in the context of meander cutoffs?
Answer: The ratio helps determine if the new path is sufficiently shorter or steeper to be favored.
Explanation: The ratio between channel slopes is significant for meander cutoffs as it helps determine if the new path is sufficiently shorter or steeper to be favored.
Avulsion is a prevalent phenomenon in river deltas, primarily attributed to the characteristically low channel gradients present in these environments.
Answer: True
Explanation: Avulsion is indeed a common occurrence in river deltas due to their typically low channel gradients, a condition exacerbated by sediment deposition.
Within deltaic geomorphological contexts, the process of avulsion is frequently termed 'delta switching'.
Answer: True
Explanation: In deltaic settings, avulsion is commonly referred to as 'delta switching,' emphasizing the shift in the river's depositional locus.
The extension of a deltaic lobe into the sea typically results in an increase in the river channel's slope, thereby enhancing its stability.
Answer: False
Explanation: This statement is false. As a deltaic lobe extends, the channel lengthens without a proportional increase in elevation change, leading to a decrease in channel slope and reduced stability, which promotes avulsion.
The occurrence of repeated avulsions can result in a mature delta characterized by a simple, singular channel.
Answer: False
Explanation: This statement is false. Repeated avulsions typically lead to complex deltaic systems with multiple, shifting channels and abandoned lobes, rather than a simple, single channel.
The Mississippi River's bird's-foot delta advances into the sea as a result of sediment deposition, a process that decreases channel slope and consequently promotes avulsion.
Answer: True
Explanation: The Mississippi River's bird's-foot delta advances into the sea due to sediment deposition, which decreases channel slope and promotes avulsion.
Avulsions are less likely to occur in river deltas when contrasted with other riverine environments.
Answer: False
Explanation: This statement is false. River deltas, due to their low gradients and high sediment loads, are particularly prone to avulsions, making them more likely than in many other riverine settings.
A river plume is defined as the sediment deposited at the river mouth, which subsequently forms the delta.
Answer: False
Explanation: This statement is false. A river plume is the mixture of river water and seawater where the river enters a larger body of water; sediment deposition at the mouth forms the delta.
A river plume constitutes the mixture of river water and denser seawater observed at the confluence where a river enters a larger body of water.
Answer: True
Explanation: A river plume is indeed the mixture of river water and denser seawater observed where a river meets a larger body of water.
In which environments are avulsions most commonly observed, attributable to specific geomorphological conditions?
Answer: In river deltas with very small channel gradients.
Explanation: Avulsions are frequently observed in river deltas due to their characteristically low channel gradients.
What term is employed synonymously with avulsion within deltaic environments?
Answer: Delta switching
Explanation: In deltaic settings, avulsion is commonly referred to as 'delta switching'.
How does the progressive extension of a deltaic lobe influence the river channel's characteristics?
Answer: It decreases the channel slope, making it less stable.
Explanation: The extension of a deltaic lobe decreases the river channel's slope, rendering it less stable and more prone to avulsion.
What is the typical fate of an abandoned channel and its associated deltaic lobe following an avulsion event?
Answer: They are abandoned and tend to subside over time.
Explanation: Following an avulsion, abandoned channels and deltaic lobes are typically left to subside over time.
According to the provided text, what is the role of sediment plumes emanating from the Mississippi River delta?
Answer: They build the delta, causing it to advance and promoting avulsion conditions.
Explanation: Sediment plumes from the Mississippi River delta build the delta, causing it to advance and promoting conditions conducive to avulsion.
What is a river plume primarily composed of?
Answer: A mixture of river water and denser seawater.
Explanation: A river plume is primarily composed of a mixture of river water and denser seawater.
What is the significance of the Mississippi River's 'bird's-foot' delta in relation to fluvial processes?
Answer: Its long, finger-like lobes extend far into the sea, contributing to avulsion conditions.
Explanation: The Mississippi River's bird's-foot delta, with its extensive lobes reaching into the sea, contributes to conditions that promote avulsion.
What is a 'river plume,' and what is its function within deltaic processes?
Answer: The area where river water mixes with seawater, carrying sediment that builds the delta.
Explanation: A river plume is the area where river water mixes with seawater, carrying sediment that is crucial for building the delta.
The 2006 Suncook River avulsion event is classified as an instance of a partial avulsion.
Answer: False
Explanation: This statement is incorrect. The 2006 Suncook River avulsion is documented as an example of an erosional avulsion, not a partial one.
Sediment mobilized by the 2006 Suncook River avulsion contributed to the aggradation of the downstream riverbed, thereby facilitating a meander cutoff.
Answer: True
Explanation: Sediment from the 2006 Suncook River avulsion contributed to the aggradation of the downstream riverbed, raising it to near floodplain level around a meander bend and thereby facilitating a meander cutoff.
The Cheslatta River experienced avulsions in 1961 and 1972 as a direct consequence of natural meandering processes.
Answer: False
Explanation: This statement is incorrect. The Cheslatta River's avulsions were primarily caused by increased water discharge resulting from the creation of the Nechako Reservoir, not natural meandering.
Following the 1972 avulsion of the Cheslatta River, a cofferdam was employed to reinstate the river's original channel course.
Answer: True
Explanation: Subsequent to the 1972 avulsion of the Cheslatta River, a cofferdam was constructed with the objective of restoring the river to its original channel alignment, thereby managing the erosional consequences of reservoir releases.
The Cheslatta Fan is a geomorphological feature formed by the Cheslatta River avulsing into a region characterized by aridity.
Answer: False
Explanation: This statement is incorrect. The Cheslatta Fan is a sediment deposit formed by the Cheslatta River's avulsions into the Nechako River system, not necessarily a drier region.
The 2006 Suncook River avulsion is cited as an example of which specific type of avulsion?
Answer: Erosional avulsion
Explanation: The 2006 Suncook River avulsion is documented as an example of erosional avulsion.
What was a notable downstream consequence of the sediment mobilized during the 2006 Suncook River avulsion?
Answer: It contributed to a meander cutoff further downstream.
Explanation: Sediment from the 2006 Suncook River avulsion contributed to the aggradation of the downstream riverbed, facilitating a meander cutoff.
What external factor precipitated the significant erosional avulsions experienced by the Cheslatta River?
Answer: Increased water discharge from the Nechako Reservoir.
Explanation: The Cheslatta River's erosional avulsions were precipitated by increased water discharge from the Nechako Reservoir.