5.3 Sediment deposition and alluvial systems

Rivers shape landscapes through erosion, transport, and deposition. Sediment deposition occurs when flow slows, dropping particles based on size. This process creates diverse landforms like point bars, natural levees, and floodplains.

Alluvial systems are complex, influenced by factors like channel gradient, sediment supply, and climate. Understanding these processes helps us interpret sedimentary structures and facies, revealing the history of ancient river systems and their environments.

Sediment Deposition in Fluvial Systems

Mechanisms of Sediment Deposition

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• Sediment deposition occurs when transport capacity of flow decreases due to reduced velocity or turbulence
• Hjulström curve shows relationship between particle size and critical velocities for erosion, transportation, and deposition
• Deposition influenced by channel geometry, flow regime, sediment supply, and particle characteristics (size, shape, density)
• Aggradation results from sediment supply exceeding transport capacity leads to vertical accumulation in channel and floodplain
• Lateral accretion forms point bars and contributes to channel migration and meandering
• Overbank deposition happens during floods when sediment-laden water overtops channel banks deposits fine material on floodplain
• Backwater effects from obstructions or base level changes induce localized deposition by reducing upstream flow velocities

Types of Deposition and Their Effects

• Vertical accretion builds up floodplain surface over time through repeated overbank flooding events
• Lateral accretion creates point bars and scroll bars along inner bends of meandering rivers
• Longitudinal accretion forms mid-channel bars and islands in braided river systems
• Deltaic deposition occurs where rivers enter standing bodies of water (oceans, lakes)
• Avulsion involves abrupt channel relocation creates new depocenters and abandoned channels
• Crevasse splay deposition forms fan-shaped deposits when floodwaters breach natural levees
• Backwater deposition occurs in areas of reduced flow velocity (confluences, reservoirs)

Fluvial Depositional Landforms

Channel-Related Landforms

• Point bars form crescent-shaped deposits on inside of river bends due to helical flow patterns and decreased velocities
• Natural levees develop as elevated ridges of coarser sediment along river banks during flood events
• Crevasse splays create fan-shaped deposits when floodwaters breach natural levees depositing coarser sediment on floodplain
• Oxbow lakes result from curved water bodies formed when meander bends cut off from main channel fill with fine sediments
• Mid-channel bars and islands emerge in braided river systems with multiple channels and high sediment loads
• Longitudinal bars form parallel to flow in straight channel segments
• Riffle-pool sequences alternate between shallow (riffle) and deep (pool) sections in gravel-bed rivers

Floodplain and Valley-Scale Landforms

• Floodplains comprise flat, low-lying areas adjacent to river channels periodically inundated during high flows receive sediment via overbank deposition
• Alluvial fans develop as cone-shaped deposits where steep mountain streams emerge onto flat plains characterized by radial distributary channels
• Terraces represent abandoned floodplain surfaces formed by river incision into previous floodplain deposits
• Paleochannels appear as remnant channel features preserved on floodplains indicate past river courses
• Backswamps form in low-lying areas of floodplains away from main channel often poorly drained and rich in organic matter
• Floodbasins occur in large, flat areas between river channels in multi-channel systems accumulate fine-grained sediments
• Alluvial ridges develop as elevated zones along river channels due to repeated levee formation and vertical accretion

Factors Controlling Alluvial Systems

Geomorphological and Hydrological Factors

• Channel gradient determines flow energy, sediment transport capacity, and depositional patterns
• Sediment supply (quantity and grain size distribution) influences balance between erosion and deposition
• Discharge variability affects channel patterns more variable regimes often lead to braided or anastomosing patterns
• Tectonic activity and base level changes alter river longitudinal profiles impact erosional and depositional behavior
• Valley confinement constrains lateral channel migration and floodplain development
• Hydraulic geometry relationships link channel dimensions to discharge and sediment load
• Flow regime (perennial, intermittent, ephemeral) influences channel stability and sediment transport patterns

Environmental and Anthropogenic Factors

• Vegetation impacts bank stability, sediment trapping, and flow resistance affects channel morphology and floodplain development
• Climate controls precipitation patterns, runoff generation, and weathering rates influence sediment production and transport
• Human activities (dam construction, channelization, land-use changes) significantly alter natural processes and morphology
• Bedrock lithology affects sediment supply, channel resistance to erosion, and valley morphology
• Soil characteristics influence infiltration rates, runoff generation, and sediment production in catchments
• Glacial history shapes valley morphology and provides sediment sources in previously glaciated regions
• Wildfire regimes impact vegetation cover, soil properties, and sediment yield in affected watersheds

Sedimentary Structures and Facies in Fluvial Environments

Sedimentary Structures

• Cross-bedding commonly occurs in fluvial deposits reflects migration of bedforms (dunes, bars)
• Fining-upward sequences characterize point bar deposits represent gradual decrease in flow energy during lateral channel migration
• Massive bedding and parallel lamination in fine-grained sediments typify overbank deposits on floodplains
• Scour-and-fill structures indicate erosion episodes followed by deposition often associated with channel bases or crevasse splays
• Imbrication of clasts in coarse-grained deposits provides information about paleoflow direction in ancient fluvial systems
• Ripple marks form on bed surfaces in lower flow regime conditions
• Mud cracks develop in fine-grained sediments during subaerial exposure and desiccation

Facies Associations and Interpretations

• Channel fill facies include trough cross-bedded sands and gravels, lag deposits, and clay plugs
• Point bar facies exhibit epsilon cross-stratification and fining-upward sequences
• Crevasse splay facies consist of coarser-grained sediments with lobate geometries
• Floodplain facies comprise fine-grained sediments with horizontal lamination and soil development
• Bioturbation and pedogenic features (root traces, soil horizons) common in floodplain deposits indicate subaerial exposure periods
• Facies models for different river types (meandering, braided, anastomosing) show characteristic vertical and lateral facies relationships
• Paleocurrent indicators (cross-bedding, clast imbrication) help reconstruct ancient flow directions and drainage patterns