5.3 Sediment deposition and alluvial systems

Sediment Deposition and Alluvial Systems

Rivers shape landscapes through erosion, transport, and deposition. Sediment deposition occurs when flow slows, causing particles to drop out 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 you interpret sedimentary structures and facies, which reveal the history of ancient river systems and their environments.

Sediment Deposition in Fluvial Systems

Mechanisms of Sediment Deposition

Sediment deposition happens when the transport capacity of a flow decreases, usually because velocity or turbulence drops. The Hjulström curve is the key tool here: it plots particle size against the critical velocities needed for erosion, transportation, and deposition. Notably, very fine particles (clays and silts) require surprisingly high velocities to erode because of cohesion, but once in transport they stay suspended even at low velocities.

Several mechanisms drive deposition in different parts of the fluvial system:

• Aggradation occurs when sediment supply exceeds transport capacity, leading to vertical accumulation in the channel and on the floodplain.
• Lateral accretion builds point bars along the inner banks of meander bends and drives channel migration over time.
• Overbank deposition happens during floods when sediment-laden water overtops channel banks and deposits fine material across the floodplain.
• Backwater effects from obstructions or base level changes reduce upstream flow velocities, inducing localized deposition.

Channel geometry, flow regime, sediment supply, and particle characteristics (size, shape, density) all influence where and how deposition occurs.

Types of Deposition and Their Effects

Different styles of deposition produce distinct landforms and sediment packages:

• Vertical accretion builds up the floodplain surface over time through repeated overbank flooding events. Each flood adds a thin layer of fine sediment.
• Lateral accretion creates point bars and scroll bars along the inner bends of meandering rivers as the channel migrates outward.
• Longitudinal accretion forms mid-channel bars and islands in braided river systems, where sediment accumulates parallel to flow.
• Deltaic deposition occurs where rivers enter standing water bodies (oceans, lakes), and flow velocity drops rapidly.
• Avulsion is an abrupt channel relocation that creates new depocenters (areas of active sediment accumulation) and leaves behind abandoned channels.
• Crevasse splay deposition forms fan-shaped deposits when floodwaters breach natural levees, spreading coarser sediment onto the floodplain.
• Backwater deposition occurs wherever flow velocity is locally reduced, such as at confluences or upstream of reservoirs.

Fluvial Depositional Landforms

Channel-Related Landforms

These landforms develop within or immediately adjacent to the active channel:

• Point bars are crescent-shaped deposits on the inside of river bends. Helical flow (a corkscrew-like secondary circulation) pushes slower water toward the inner bank, causing sediment to accumulate there. They typically fine upward from gravel at the base to sand and silt near the top.
• Natural levees are elevated ridges of coarser sediment flanking the channel. During floods, the coarsest suspended sediment drops out first as water spills over the banks, building these ridges over many flood cycles.
• Crevasse splays are fan-shaped deposits that form when floodwaters breach a natural levee. They spread coarser sediment across the floodplain in a lobate pattern.
• Oxbow lakes are curved water bodies left behind when a meander bend gets cut off from the main channel. Over time, they fill with fine sediments and organic material.
• Mid-channel bars and islands develop in braided river systems where high sediment loads and variable discharge create multiple shifting channels.
• Longitudinal bars form parallel to flow direction in straighter channel segments.
• Riffle-pool sequences alternate between shallow, fast-flowing sections (riffles) and deeper, slower sections (pools) in gravel-bed rivers. Their spacing is typically 5 to 7 channel widths apart.

Floodplain and Valley-Scale Landforms

At a larger scale, deposition builds landforms that define the shape of entire valleys:

• Floodplains are flat, low-lying areas adjacent to the channel that get periodically inundated during high flows. They receive sediment through overbank deposition and grow both vertically and laterally over time.
• Alluvial fans are cone-shaped deposits that form where steep mountain streams emerge onto flat plains. Flow suddenly spreads out and slows, dropping sediment in a radial pattern with distributary channels.
• Terraces are abandoned floodplain surfaces that sit above the current floodplain. They form when a river incises into its previous floodplain deposits, often in response to base level drop or reduced sediment supply.
• Paleochannels are remnant channel features preserved on floodplains that indicate past river courses. They're useful for reconstructing how a river system has migrated over time.
• Backswamps form in low-lying areas of the floodplain far from the main channel. They're often poorly drained and rich in organic matter because natural levees prevent easy drainage back to the river.
• Floodbasins are large, flat areas between river channels in multi-channel systems that accumulate fine-grained sediments.
• Alluvial ridges are elevated zones along river channels built up by repeated levee formation and vertical accretion. The channel actually sits higher than the surrounding floodplain, which is why avulsion is always a risk.

Factors Controlling Alluvial Systems

Geomorphological and Hydrological Factors

The character of an alluvial system depends on a set of interacting physical controls:

• Channel gradient determines flow energy and sediment transport capacity. Steeper gradients mean higher energy and coarser sediment transport; lower gradients favor deposition.
• Sediment supply (both quantity and grain size distribution) sets the balance between erosion and deposition. A river receiving abundant coarse sediment from its catchment will behave very differently from one with limited fine-grained input.
• Discharge variability strongly influences channel pattern. Highly variable flow regimes (with big floods and low baseflows) tend to produce braided or anastomosing patterns, while more steady discharge favors single-thread meandering channels.
• Tectonic activity and base level changes alter the river's longitudinal profile. Uplift steepens gradients and promotes incision; subsidence or sea level rise promotes aggradation.
• Valley confinement constrains how much a channel can migrate laterally and limits floodplain development. A river in a narrow bedrock valley behaves differently from one on a broad alluvial plain.
• Hydraulic geometry relationships link channel width, depth, and velocity to discharge and sediment load in predictable ways.
• Flow regime (perennial, intermittent, or ephemeral) influences channel stability and the continuity of sediment transport processes.

Environmental and Anthropogenic Factors

Beyond the physical framework, biological and human factors play major roles:

• Vegetation increases bank stability through root reinforcement, traps sediment, and adds flow resistance. Before land plants evolved in the Devonian, rivers were almost exclusively braided because there was nothing to stabilize banks.
• Climate controls precipitation patterns, runoff generation, and weathering rates, all of which influence how much sediment is produced and transported.
• Human activities like dam construction, channelization, and land-use changes significantly alter natural sediment budgets and channel morphology. Dams trap sediment upstream and starve downstream reaches, often causing channel incision below the dam.
• Bedrock lithology affects sediment supply (how easily rock weathers into transportable particles), channel resistance to erosion, and valley shape.
• Soil characteristics influence infiltration rates and runoff generation, which control how much water and sediment reach the channel from hillslopes.
• Glacial history shapes valley morphology (U-shaped valleys, moraines) and provides abundant sediment sources in previously glaciated regions.
• Wildfire regimes temporarily remove vegetation cover and alter soil properties, often causing dramatic spikes in sediment yield from burned watersheds.

Sedimentary Structures and Facies in Fluvial Environments

Sedimentary Structures

Sedimentary structures are the internal features preserved within deposits that record the conditions under which sediment was laid down. They're your primary tool for interpreting ancient fluvial environments.

• Cross-bedding is one of the most common structures in fluvial deposits. It forms from the migration of bedforms like dunes and bars, with inclined layers (foresets) recording the downstream face of the migrating feature. Trough cross-bedding indicates 3D dune migration; planar cross-bedding suggests 2D dune or bar migration.
• Fining-upward sequences are the signature of point bar deposits. Coarse gravel and sand at the base grade upward into finer sand and silt, reflecting the gradual decrease in flow energy as the channel migrates laterally.
• Massive bedding and parallel lamination in fine-grained sediments are typical of overbank (floodplain) deposits, laid down from suspension during floods.
• Scour-and-fill structures mark erosion episodes followed by deposition. They're often found at channel bases or associated with crevasse splays.
• Clast imbrication occurs in coarse-grained deposits when flat or elongate clasts stack up tilted against the current. This provides reliable information about paleoflow direction.
• Ripple marks form on bed surfaces under lower flow regime conditions and can indicate flow direction (asymmetric ripples) or oscillating currents (symmetric ripples).
• Mud cracks (desiccation cracks) develop in fine-grained sediments during subaerial exposure, confirming that the surface dried out between depositional events.

Facies Associations and Interpretations

A facies is a body of sediment with distinctive characteristics (grain size, structures, geometry) that reflect specific depositional conditions. In fluvial systems, facies group into associations that correspond to different parts of the river environment:

• Channel fill facies include trough cross-bedded sands and gravels, coarse lag deposits at the channel base, and clay plugs that fill abandoned channels.
• Point bar facies show epsilon cross-stratification (large-scale inclined surfaces dipping toward the channel center) and fining-upward sequences. These are the classic indicator of meandering river deposits.
• Crevasse splay facies consist of coarser-grained sediments with lobate (fan-shaped) geometries that thin away from the breach point.
• Floodplain facies are dominated by fine-grained sediments with horizontal lamination, along with evidence of soil development (paleosols).
• Bioturbation and pedogenic features such as root traces, burrows, and soil horizons are common in floodplain deposits. They indicate periods of subaerial exposure between flood events.

Facies models for different river types (meandering, braided, anastomosing) show characteristic vertical and lateral facies relationships. By matching observed facies associations to these models, you can reconstruct the type of river system that produced ancient deposits. Paleocurrent indicators like cross-bedding orientations and clast imbrication help reconstruct ancient flow directions and drainage patterns.