Questions and Answers
Where are braided streams found?
next to alluvial fans, melting glaciers, and rapidly eroding mountain ranges
What are braided streams characterized by?
broad, flat channels cut by many interwoven (anastomosing) sub channels separated by shallow areas called bars
What is a cross-section of a braided stream like?
shallow flow and irregular topography
Braided streams are dominated by?
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Meandering rivers carry a lot more?
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Braided streams require repeated?
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When a flood surge hits a braided stream, what generally happens?
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What can a stream become absolutely choked with?
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As floods wane or the slope decreases, what happens to the coarser sediment?
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What do initial coarse deposits nucleate?
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During bar formation, what is deposited first?
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Bars migrate in which two directions?
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What are the three main types of bars?
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Longitudinal bars migrate how?
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Transverse bars migrate at what angle?
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Lateral bars are attached to?
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How do bars migrate?
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Migrating bars produce what?
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As the floods end, what happens to point bars?
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Channels also?
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Because of channel bar migration, the cross-beds in these bars do what?
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What is the result of channel bar migration forming cross beds moving downstream and side to side?
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Trough cross bedding reflects what?
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Tabular cross bedding reflects what?
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Gravel deposits tend to be?
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Channel deposits tend to feature?
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Bar migration tends to leave behind?
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How thick are beds often?
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The occurrence of mud in braided streams is?
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Only braided streams ______ are dominated by gravels
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Typical in the geologic record are deposits dominated by?
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Melting of glacier ice leaves behind?
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Meltwaters can carry sediment off the ice and?
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Glacial till enters the rock record to produce?
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Glacial outwash is characterized by?
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Meltwaters carry away?
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Study Notes
Braided Streams Overview
- Located next to alluvial fans, glaciers, and rapidly eroding mountain ranges.
- Characterized by broad, flat channels intersected by interwoven subchannels, with shallow bars separating them.
Physical Characteristics
- Display shallow flow and irregular topography.
- Composed mainly of sand and gravel, with minimal silt and clay, which are carried to meandering rivers and deltas.
Sedimentation and Flood Dynamics
- Require repeated floods to transport significant sediment from source areas.
- Flood surges rework loose sediments, especially shallow bars.
Sediment Behavior
- Streams can become choked with sediment, approaching the physical limits of flow capacity.
- As floods recede, coarser sediment drops out where current slows, leading to nucleation of bar formation.
Bar Formation and Migration
- Coarsest material is deposited first, with faster currents creating anastomosing channels.
- Bars migrate both laterally and downstream based on currents from subchannels.
Types of Bars
- Longitudinal bars migrate straight downstream.
- Transverse bars migrate at angles relative to overall stream flow.
- Lateral bars are attached to the edge of the stream.
Bar Migration Mechanism
- Bars migrate similarly to dunes, with sediment cascading off the advancing edges.
- Migrating bars produce cross-beds, resembling flat-topped dunes.
Post-Flood Dynamics
- As floods end, point bars emerge, allowing for additional fine sediment deposition in subchannels.
- Channels typically fine upwards, creating varied sedimentation patterns.
Cross-Bedding Patterns
- Cross-beds may not only point downstream but also laterally due to bar migration.
- Results in a 180-degree spread of paleocurrent measurements, typically averaging downstream.
Bedding Features
- Trough cross bedding indicates erosion pulses reworking older deposits.
- Tabular cross bedding reflects calmer stream conditions.
Deposit Characteristics
- Gravel deposits often exhibit massive structures or crude planar beds.
- Channel deposits typically show trough cross-bedding, whereas bar migrations result in tabular beds.
Bed Thickness and Composition
- Typical bed thickness ranges from 2 to 5 meters.
- Rare occurrences of mud; braided streams are predominantly sand and gravel systems.
Proximity to Sources
- Only near-source braided streams are predominantly gravel-dominated.
- Typical geological records show deposits largely composed of sands with occasional gravel lenses.
Glacial Influence
- Melting glacier ice results in a blanket of till.
- Meltwaters transport sediment from glaciers, reworking till to form glacial outwash plains.
- Glacial till in the rock record creates unsorted, matrix-supported conglomerates.
Stream Deposits
- Meltwaters carry away finer materials, leaving behind gravel and sand deposits characteristic of braided streams.
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Description
Explore the fascinating world of braided streams, their physical characteristics, and the dynamics of sedimentation and flood behavior. This quiz highlights the unique features of these streams, including their formation and migration patterns. Test your understanding of how braided streams interact with their environment.
