How Do Streams Transport and Deposit Sediments?

Streams transport and deposit sediments through a complex interplay of solution, suspension, and bed load processes, governed by factors like velocity, gradient, discharge, and channel characteristics. These mechanisms redistribute Earth’s materials, shaping landscapes and influencing ecosystems. Deposition, triggered by energy loss, creates features like floodplains, deltas, and alluvial fans, while erosion and transport continuously reshape stream channels.

How streams carry sediment: dissolved minerals, suspended fine grains, and bed load moving by traction and saltation.

Mechanisms of Sediment Transport

Sediment transport in streams occurs through several physical mechanisms that are influenced by water velocity, turbulence, and channel characteristics. The primary mechanisms include:

1. Dissolved Load (Solution)

Process: Chemical weathering dissolves certain minerals in water. These dissolved ions (e.g., calcium, sodium, bicarbonate) are carried invisibly in solution.

Characteristics: This load, derived from soluble minerals such as those in limestone, does not contribute to the visible sediment load but is crucial in the chemical makeup of stream water.

2. Suspended Load

Process: Fine-grained particles like silt and clay are held in suspension by the energy of moving water. Increased turbulence and higher velocities can keep these particles aloft over long distances.

Characteristics: The suspended load increases with greater water turbulence, often contributing to the murky appearance of streams during high-flow events such as floods.

3. Bed Load (Traction and Saltation)

Process: Larger particles (sand, gravel, boulders) that are too heavy to remain in suspension are moved along the streambed. This transport occurs through:

• Traction: The rolling or sliding motion of the largest particles (gravel, cobbles, boulders) along the channel bottom, pushed by the force of the water. This requires the highest flow velocities and is most common during floods or in steep, high-energy stream reaches. The impacts between these large clasts can sometimes be heard during flood events.
• Saltation: The movement of smaller bed load particles, primarily sand, in a series of intermittent hops or bounces. Turbulent eddies lift particles from the bed; they travel a short distance downstream before gravity pulls them back down.

Characteristics: The bed load moves relatively slowly and requires high energy, such as that provided by fast-flowing water or flood events. The movement of these particles can be intense enough to produce audible sounds from collisions.

Factors Influencing Sediment Transport

Several factors determine the capacity of a stream to transport sediments:

Water Velocity: Faster-moving water has more kinetic energy, enabling it to carry larger and heavier sediments. Increased velocity also enhances turbulence, which is critical for suspending fine particles.

Stream Gradient (Slope): Steeper gradients produce higher velocities, enhancing sediment transport, particularly for larger particles.

Discharge (Water Volume): Greater volumes of water, as seen during flood events, increase the stream's capacity to erode and transport sediments.

Particle Size: Larger particles require more energy to be moved, whereas fine particles can be easily transported in suspension.

Channel Shape and Roughness: A smooth, straight channel generally allows for faster, more uniform flow, while rough or irregular channels create turbulence that can both trap and mobilize sediments.

Turbulence: While roughness can impede average flow velocity, it increases turbulence – the chaotic, unsteady, swirling motion within the water. Turbulence is a critical factor, especially for keeping finer sediment particles suspended within the water column, counteracting gravity's pull. Increased velocity and flow obstructions both contribute to higher levels of turbulence.

Sediment Deposition Processes

Deposition is the process by which a stream drops, or ceases to transport, the sediment it carries. This occurs whenever the stream's energy decreases to the point that its velocity is no longer sufficient to maintain transport for particles of a given size. Several common scenarios lead to sediment deposition:

Reduction in Stream Velocity: Any factor that causes the flow to slow will lead to deposition, typically beginning with the heaviest/largest particles and progressing to finer materials as velocity decreases further. Key causes include:

Decrease in Gradient: As a stream flows from a steeper slope onto a flatter area (e.g., from a mountain valley onto a plain or into a broader valley), its velocity naturally decreases, causing sediment deposition. Features like alluvial fans are formed this way.

Entry into Standing Water: When a stream enters a lake, reservoir, or the ocean, its velocity drops abruptly. This causes deposition of most of its sediment load, with coarser materials settling near the mouth and finer materials carried further out, often leading to the formation of deltas.

Widening of the Channel: If the stream channel widens significantly, the flow spreads out, depth decreases, and velocity reduces, often resulting in the formation of mid-channel bars or bank-attached deposits.

Flow Obstructions: Natural features (e.g., large boulders, log jams) or human-made structures (e.g., dams, bridge piers) can create localized zones of reduced velocity, inducing sediment deposition immediately upstream.

Inside Bends (Meanders): Water flows more slowly along the inner bank of a stream bend, leading to the deposition of sediment and the formation of point bars.

Reduction in Discharge: As stream flow diminishes (e.g., following a flood peak or during a dry season), the overall energy decreases, causing deposition of sediments that were transportable only at higher flows.

Flooding (Overbank Flow): While floods generate high energy within the channel, leading to erosion and transport, when water overtops the banks and spreads across the adjacent floodplain, it experiences a significant increase in width and frictional resistance. This causes a sharp decrease in velocity and the deposition of fine suspended sediments (silt and clay), which contributes to the fertility of floodplain soils.

Characteristics of Deposited Sediments

The process of deposition often leaves distinctive features in the resulting sediment accumulations:

Sorting: Because particles settle out of transport at different velocities based on their size, weight, and shape, stream deposits are often sorted. This means that a specific deposit may consist predominantly of particles of a similar size range (e.g., a layer of sand, a bed of gravel).

Graded Bedding: A single depositional event, such as a waning flood flow, can produce a layer of sediment that exhibits graded bedding. This is characterized by a gradation in particle size from coarsest at the bottom of the layer to finest at the top, reflecting the progressive decrease in transport energy during deposition.