Sediment Transport and River Types Overview

Announcements

• Labs will meet at the usual location and then walk to the water tank with the TA.
• No lab scheduled for next week.
• First exam on September 26th, during class; will be paper-based, no phones allowed.

Class Outline (September 14, 2023)

• Previous week covered siliciclastic sediment transport.
• Current topics include bedform stability, environments of deposition, rivers, and deltas.
• Next week’s focus: coastal environments.

River Types Overview

• Rivers adapt to topography and landscape gradients.
• Meandering rivers: High sinuosity and fine sediment load; cohesive banks resistant to erosion.
• Braided rivers: Numerous channels separated by bars; characterized by low cohesion banks and coarser sediment load.

Meandering Rivers

• Lateral accretion occurs on point bars (inner bends) and erosion on cut banks (outer bends).
• Important mechanisms include helical flow and decreased energy at point bars.
• Facies distribution is determined by lateral migration of the channel.
• Leave behind fining-upwards deposits as they migrate.

Braided Rivers

• Exhibit multiple channels with low cohesion banks, which are easily eroded.
• Channels show variability in morphology and deposits.
• Typically found in high gradient settings with heavy sediment loads.

Fluvial Deposits Characteristics

• Absence of marine fossils; often contains roots, mudcracks, and tracks.
• Typically red in color with a downstream decrease in grain size, indicating downstream flow patterns.

Coastal Environments: Deltas

• Deltas form at river mouths where water and sediment meet another body of water (ocean, lake).
• Major global river deltas have distinct classifications based on sediment dispersal processes: river-dominated, tide-dominated, and wave-dominated.

Delta Types and Characteristics

• River-dominated deltas: Distributary channels with mouth bars (e.g., Mississippi Delta).
• Tide-dominated deltas: Exhibit normal tidal channels; sediments transport bidirectionally due to tides.
• Wave-dominated deltas: Sediment reworked into bars and influenced by longshore drift (e.g., Sao Francisco Delta).

Delta Formation Processes

• Factors determining delta morphology include river outflow density and interaction with marine processes.
• Delta environments consist of delta plain (fluvial processes), delta front (fluvial & marine), and prodelta (marine processes).
• Progradation and lobe-switching are key features of delta development.

General Delta Characteristics

• Deltas can vary greatly in size—from less than 1 km² to over 100,000 km².
• Sediment tends to fine distally; a coarsening upward succession often observed.
• Classic facies models predict lithology based on depositional environments and sea-level changes.

Key Takeaways

• Bedform stability depends on grain size and flow velocity.
• Understanding river dynamics is crucial in predicting sediment distribution and delta morphology.
• Deltas are complex systems influenced by both fluvial and marine processes, leading to diverse sedimentary environments.