Unit 5: Fluvial and glacial processes - Subjective Questions

Fluvial processes are the geomorphological activities performed by running water in rivers and streams. They shape the landscape through four major processes:

• Erosion: Removal of soil and rock by hydraulic action, abrasion, attrition, and solution.
• Transportation: Movement of sediment as traction, saltation, suspension, and solution.
• Deposition: Accumulation of sediment when the river loses energy.
• Weathering and mass movement: These processes supply material to river channels and valley slopes.

The intensity of fluvial activity depends on discharge, velocity, channel gradient, sediment load, rock resistance, and vegetation cover. Over time, rivers produce valleys, waterfalls, meanders, floodplains, oxbow lakes, levees, alluvial fans, and deltas.

River erosion occurs through several mechanisms:

• Hydraulic action: The force of moving water loosens and removes particles from the bed and banks. Air trapped in cracks may increase pressure and weaken the rock.
• Abrasion or corrasion: Sediments carried by the river strike and scrape the channel bed and banks.
• Attrition: Rock fragments collide with one another, becoming smaller and more rounded.
• Solution or corrosion: Soluble minerals are dissolved by slightly acidic river water.

These processes are strongest where velocity and turbulence are high, especially during floods, on steep slopes, and around waterfalls. Hydraulic action and abrasion commonly deepen and widen river valleys.

Rivers transport sediment in four principal ways:

• Traction: Large boulders and pebbles roll or slide along the channel bed.
• Saltation: Sand-sized particles bounce or hop along the bed.
• Suspension: Fine silt and clay are carried within the water column by turbulence.
• Solution: Dissolved minerals are transported invisibly within the river water.

The amount and type of load depend on river velocity, discharge, particle size, channel gradient, and turbulence. As velocity decreases, the river usually deposits its largest particles first, followed by smaller sediments.

River velocity is the speed at which water flows, while discharge is the volume of water passing a particular point per unit time. It may be expressed as:

where is discharge, is the cross-sectional area of the channel, and is mean velocity.

• Greater velocity increases the river's ability to erode and transport sediment.
• Greater discharge usually provides more energy and allows a larger sediment load to be carried.
• When velocity or discharge decreases, the river loses competence and capacity, causing deposition.
• Competence refers to the largest particle size a river can transport.
• Capacity refers to the total quantity of sediment a river can carry.

Floods substantially increase both discharge and sediment-carrying ability.

In the upper course, a river flows over a steep gradient and has limited discharge. Vertical erosion is dominant, especially through hydraulic action and abrasion. The river cuts downward into its bed, producing a narrow V-shaped valley.

Weathering weakens the steep valley sides, and mass movement causes loosened material to move toward the channel. This widens the valley slightly while maintaining its V-shaped profile. Where the river follows a winding route around resistant rocks, projecting ridges called interlocking spurs extend into the valley. The river has insufficient energy to cut through these spurs, so it flows around them.

Waterfalls commonly develop where hard rock overlies softer rock or where a geological fault creates a sudden change in elevation.

• The softer rock is eroded more rapidly by hydraulic action and abrasion.
• A plunge pool forms at the base because falling water and swirling sediment intensify erosion.
• Undercutting creates an overhang of resistant rock.
• The overhang eventually collapses because of gravity and weathering.
• The waterfall retreats upstream, leaving behind a narrow, steep-sided gorge.

Repeated cycles of undercutting, collapse, and upstream retreat produce a gorge below the waterfall.

A meander is a sinuous bend in a river channel. It forms because variations in velocity cause erosion and deposition across the channel.

• The fastest flow, called the thalweg, usually moves toward the outer bank.
• Hydraulic action and abrasion erode the outer bank, forming a river cliff or cut bank.
• Slower flow on the inner bank deposits sediment, forming a slip-off slope or point bar.
• Helicoidal flow transfers water and sediment across the channel, encouraging lateral migration.
• Over time, the meander becomes more pronounced and shifts across the floodplain.

Meanders are most common in the middle and lower courses, where lateral erosion is important.

An oxbow lake develops when a meander loop becomes cut off from the main river channel.

1. Erosion continues on the outer banks, narrowing the meander neck.
2. During a flood, the river may breach the narrow neck and take a straighter route.
3. Deposition occurs at the entrances to the abandoned loop.
4. Continued deposition seals off the loop from the main channel.
5. The isolated water body forms an oxbow lake.
6. Sediment and organic material gradually fill the lake, which may eventually become a marsh or meander scar.

This process shortens the river channel and increases the straightness of its course.

A floodplain is a broad, flat area bordering a river that is periodically covered by floodwater. It forms through lateral erosion, meander migration, and repeated deposition.

• During floods, water spreads beyond the channel and loses velocity.
• Coarse sediment is deposited near the channel, while finer silt is carried farther across the floodplain.
• Repeated flooding builds a layer of fertile alluvium.
• Meander migration, channel shifting, and the formation of oxbow lakes enlarge the floodplain.

Natural levees are raised banks formed when coarse sediment is deposited immediately beside the river channel during floods. Finer sediments settle farther away, producing a gradual decrease in surface height away from the channel.

Both alluvial fans and deltas are depositional landforms formed when a river loses velocity, but they develop in different settings.

• Alluvial fan: Forms where a steep stream emerges from a mountain valley onto a relatively flat plain. The sudden reduction in gradient causes coarse sediment to be deposited in a cone-shaped fan. Deposition is usually poorly sorted and may occur through several shifting channels.
• Delta: Forms where a river enters a lake or sea and loses velocity. Sediment accumulates at the river mouth, often creating distributaries, levees, mudflats, and delta lobes. Delta sediments are commonly layered and become finer away from the river mouth.

Thus, alluvial fans form at the base of slopes, whereas deltas form at standing-water bodies.

A glacier is a persistent mass of compacted snow and ice that flows slowly under gravity. Glaciers form where annual accumulation of snow exceeds annual melting and ice loss.

• Accumulation is the gain of snow and ice. It occurs through snowfall, avalanching, freezing rain, and wind-blown snow.
• Ablation is the loss of ice through melting, evaporation, sublimation, and calving of icebergs.

The boundary between the accumulation zone and the ablation zone is called the equilibrium line. If accumulation exceeds ablation, the glacier advances or thickens. If ablation exceeds accumulation, the glacier retreats, although the ice may continue flowing forward.

Glacial erosion occurs mainly through two processes:

• Plucking: Meltwater enters cracks in the bedrock, freezes, and bonds rock fragments to the glacier. As the glacier moves, these fragments are pulled from the bed and valley sides.
• Abrasion: Rock fragments embedded in the ice scrape and polish the bedrock as the glacier moves. This can produce striations, grooves, and smooth rock surfaces.

Erosion is strongest where the glacier is thick, rapidly moving, and contains abundant debris. Freeze-thaw weathering and frost shattering weaken valley sides and supply additional material to the glacier.

Alpine glaciers and continental ice sheets differ in scale, setting, and influence on the landscape.

• Alpine or valley glaciers: Occur in mountainous areas and flow through pre-existing valleys. They are confined by valley sides and commonly produce cirques, arêtes, horns, U-shaped valleys, and hanging valleys.
• Continental ice sheets: Cover extensive areas of land, often thousands of square kilometres, and are not restricted to individual valleys. They flow outward from central accumulation zones and can bury entire landscapes beneath ice.

Alpine glaciers mainly modify mountain valleys, whereas ice sheets produce widespread erosion and deposition, including drumlins, till plains, eskers, and moraines.

These are characteristic landforms of glacial erosion in mountainous regions.

• Cirque or corrie: A hollow forms where snow accumulates in a sheltered depression. Repeated freeze-thaw action, plucking, and abrasion deepen and widen the hollow.
• Tarn: After the glacier melts, water may collect in the overdeepened cirque basin to form a small lake.
• Arête: A sharp, narrow ridge develops when glaciers erode on opposite sides of a mountain ridge.
• Horn: A pyramidal peak forms when three or more cirques erode a mountain from different directions.

These landforms show the strong erosive effect of glaciers in high mountain environments.

A glacier occupies and modifies a former river valley.

• The glacier thickens and moves downslope under gravity.
• Its sides and bed are eroded by plucking and abrasion.
• The glacier widens the valley by eroding the lower valley sides and deepens it by removing bedrock from the floor.
• Spurs that previously interlocked along the river valley are truncated.
• The resulting valley has a broad, flat floor and steep, relatively straight sides, giving it a U-shaped cross-section.

When the ice retreats, tributary valleys may remain above the main valley floor, producing hanging valleys and waterfalls.

A hanging valley is a tributary valley whose floor is higher than the floor of the main glacial valley.

• The main glacier is larger and has greater erosive power than the tributary glacier.
• It deepens its valley more extensively than the tributary glacier.
• After deglaciation, the tributary valley is left perched above the main valley.
• Water flowing from it may form a waterfall.

Truncated spurs are steep, blunt-ended ridges along the sides of a glacial valley. They form when a glacier cuts through the projecting interlocking spurs of an earlier river valley instead of winding around them.

Glacial deposition occurs when ice melts or loses energy and can no longer transport its sediment load. The deposited material is called till when it is unsorted and unstratified.

• Lateral moraine: Debris deposited along the sides of a valley glacier.
• Medial moraine: Debris formed where two glaciers join and their lateral moraines merge.
• Terminal or end moraine: A ridge deposited at the glacier's maximum advance or snout.
• Ground moraine: A widespread layer of till left beneath a retreating glacier.
• Recessional moraine: A ridge formed during a temporary halt in glacier retreat.

Glacial deposition creates ridges, till plains, drumlins, and other landforms.

These depositional landforms are commonly associated with melting glaciers.

• Drumlins: Streamlined hills of till formed beneath moving ice. Their long axes generally parallel the direction of former ice movement. The blunt end faces the direction from which the ice came, while the tapered end points down-ice.
• Eskers: Long, winding ridges of stratified sand and gravel deposited by meltwater streams flowing within or beneath a glacier. When the ice melts, the sediment ridge remains.
• Outwash plains: Broad, gently sloping areas of sand and gravel deposited by meltwater beyond the glacier's margin. Coarser material is deposited nearer the ice, while finer sediments are transported farther away.

These features record former ice-flow directions and meltwater activity.

Fluvial and glacial erosion are both important surface processes, but they differ substantially.

• Agent: Fluvial erosion is performed by running water, whereas glacial erosion is performed by moving ice containing rock debris.
• Main processes: Rivers use hydraulic action, abrasion, attrition, and solution. Glaciers mainly use plucking and abrasion.
• Valley form: Rivers commonly produce V-shaped valleys, while glaciers produce broad U-shaped valleys.
• Landforms: Rivers form waterfalls, meanders, floodplains, oxbow lakes, and deltas. Glaciers form cirques, arêtes, horns, hanging valleys, moraines, and drumlins.
• Sediments: River deposits are commonly sorted and rounded by transport. Glacial deposits are often angular, unsorted, and unstratified, although meltwater may produce sorted sediments.

Both systems erode, transport, and deposit material according to their energy and environmental conditions.

The effectiveness of fluvial and glacial processes depends on interacting physical and environmental factors.

Fluvial controls:

• River discharge and velocity determine erosive power and sediment transport.
• Channel gradient affects the potential energy of flowing water.
• Rock type, structure, joints, and permeability influence resistance to erosion.
• Climate and rainfall affect runoff and flood frequency.
• Vegetation stabilizes banks and reduces surface runoff.
• Human activities such as deforestation, dams, and urbanization alter discharge and sediment supply.

Glacial controls:

• Temperature determines accumulation, melting, and the presence of meltwater.
• Snowfall controls the mass balance of the glacier.
• Ice thickness and slope affect pressure and flow speed.
• Basal meltwater can lubricate the bed and increase movement.
• The amount and hardness of debris influence abrasion.
• Bedrock structure controls the shape and resistance of the landscape.

Together, these factors determine whether erosion, transportation, or deposition dominates.