Unit 5: Fluvial and glacial processes

I. Orientation

Geomorphology examines how external processes sculpt Earth’s surface. Fluvial systems are governed mainly by flowing water, while glacial systems are governed by moving ice; both involve erosion, transport, deposition, weathering, and adjustment to slope, discharge, climate, and sediment supply. Their landforms record the balance between stream or ice energy and the resistance of bedrock, sediment, and vegetation.

• Governing principle: Landscapes develop through the interaction of process, material, energy, and time.
• Process zones: Erosion removes material, transportation moves it, and deposition stores it where energy declines.
• Energy controls: Water velocity, discharge, channel gradient, ice thickness, and glacier slope influence geomorphic work.
• Sediment competence: Competence is the largest particle a flow can move; capacity is the total sediment quantity it can carry.
• Threshold conditions: Movement begins when fluid or ice shear stress exceeds the resisting force of sediment, bedrock, or frozen substrate.
• Base level: Sea level is the ultimate base level for many rivers; local lakes, resistant rock, and dams act as temporary or local base levels.
• Time scale: Floods and glacier surges can alter landforms rapidly, whereas valley incision and glacial erosion commonly require thousands to millions of years.
• System perspective: A drainage basin or glacier is an open system receiving energy and sediment and transferring them downslope.

II. Fluvial processes and landforms — water-shaped landscapes

A. Orientation

A fluvial system consists of precipitation-fed channels, floodplains, groundwater stores, sediment, and the drainage basin supplying them. River form reflects discharge, gradient, channel roughness, sediment load, bank resistance, and human intervention.

B. Fluvial processes and landforms

Fluvial processes explain how running water erodes, transports, and deposits sediment to produce characteristic channel and valley forms.

• Flow and discharge: Discharge is the volume of water passing a cross-section per unit time.
  • The relationship is:
    
    TEXT

        Q = A × V

    
    where Q is discharge in cubic metres per second (m³/s), A is cross-sectional area in square metres (m²), and V is mean velocity in metres per second (m/s). A channel 20 m² in area flowing at 2 m/s carries 40 m³/s.
• Velocity patterns: Velocity is generally greatest near the channel centre and surface, but friction reduces it along the bed, banks, and channel margins. Helicoidal flow helps erode outer bends and deposit sediment on inner bends.
• Hydraulic action: The force of water entering cracks and joints can loosen bedrock, especially during floods. Turbulence and air compression are important in waterfalls and confined channels.
• Abrasion and attrition: Abrasion occurs when transported clasts scrape the bed and banks, producing potholes and channel widening. Attrition makes sediment smaller and more rounded as particles collide.
• Solution: Soluble minerals such as limestone and gypsum dissolve into the water. Solution load may remain invisible but can form karst drainage and contribute substantially to total river load.
• Transportation mechanisms: Traction rolls large boulders along the bed; saltation moves sand and small pebbles in short hops; suspension carries fine silt and clay; solution carries dissolved ions.
• Erosion and channel profile: In the upper course, steep gradients and high potential energy encourage vertical erosion, creating V-shaped valleys, interlocking spurs, rapids, and waterfalls. A waterfall retreats through plunge-pool erosion and collapse, leaving a gorge.
• Meanders: In a sinuous channel, helicoidal flow concentrates erosion on the outer cut bank and deposition on the inner slip-off slope. Continued lateral migration enlarges the meander loop.
• Oxbow lakes: During a flood, a river may cut through a narrow meander neck. Deposition seals the abandoned loop, producing an oxbow lake that may later infill into a meander scar.
• Floodplains: Repeated overbank floods deposit fine alluvium across the valley floor. Natural levees form when coarse sediment is dropped beside the channel as floodwater loses velocity; backswamps receive finer clay farther from the channel.
• Upper- and lower-course contrasts: Steep, narrow valleys are typical of youthful or upland rivers, whereas lower-course rivers commonly have wide floodplains, meanders, levees, and deltas because deposition and lateral migration dominate.
• Deltas and alluvial fans: A delta forms where a river enters standing water and loses competence, depositing distributary sediments. An alluvial fan forms where a confined stream exits a steep valley onto a flatter plain and rapidly loses energy.

C. Applications and limitations

Fluvial landforms are useful indicators of flood risk, sediment budgets, and environmental change, but they are not controlled by water alone.

• Flood risk: Flood magnitude depends on rainfall intensity, catchment size, antecedent soil moisture, vegetation, urban surfaces, and channel capacity. Impermeable roads increase rapid surface runoff.
• Human modification: Dams trap sediment and alter discharge seasonality; embankments restrict floodplain inundation but can raise water levels during extreme floods; channelization increases local conveyance while often reducing habitat.
• Interpretive limitation: A meander, terrace, or gravel bar may reflect several events and controls. River terraces, for example, can result from uplift, climate-driven discharge change, falling base level, or changes in sediment supply.
• Management principle: Restoring floodplain connectivity and allowing room for channel migration can reduce hazard more sustainably than relying only on hard engineering.

III. Glacial processes and landforms — ice-shaped landscapes

A. Orientation

A glacier is a persistent mass of snow and ice that flows downslope under gravity. It forms where annual accumulation exceeds ablation over long periods. Glaciers occur as valley glaciers confined by mountains and ice sheets or ice caps that spread across broad regions.

• Mass balance: Accumulation includes snowfall, avalanching, and refreezing; ablation includes melting, sublimation, and calving. Positive balance causes advance, while negative balance causes retreat.
• Equilibrium line: The equilibrium line separates the accumulation zone from the ablation zone, where annual gain equals annual loss.
• Glacier flow: Ice moves by internal deformation and basal sliding. Flow is faster in the central upper part of a valley glacier and slower at the bed and margins because of friction.
• Thermal condition: Warm-based glaciers can slide over their beds and erode effectively; cold-based glaciers are frozen to the substrate and are less erosive.
• Glacial load: Moving ice can carry clay-sized flour, cobbles, and enormous boulders, while meltwater reorganizes sediment into sorted deposits.

B. Fluvial processes and landforms

Glacial processes include plucking, abrasion, transport, deposition, and meltwater action. They produce erosional forms that are usually broader and more angular than river forms.

• Plucking: Meltwater enters bedrock joints, freezes, and helps loosen blocks. Basal ice then removes them, creating steep rock steps and contributing to glacial quarrying.
• Abrasion: Rock fragments frozen into basal ice grind against the bed, polishing and striating bedrock. Striations indicate the direction of ice movement.
• Glacial transport: Till is carried within, beneath, or on the ice and is deposited unsorted. Meltwater transports sediment in channels and deposits it in sorted layers.
• U-shaped valleys: A valley glacier widens and deepens a former V-shaped river valley through abrasion and plucking. The resulting trough has a broad floor and steep sides.
• Hanging valleys: Smaller tributary glaciers erode less deeply than the main glacier, leaving a tributary valley perched above the main trough. Waterfalls commonly develop at its mouth.
• Cirques, arêtes, and horns: A cirque is a bowl-shaped hollow formed by ice accumulation and rotational erosion. Headward erosion between cirques creates a sharp arête; erosion surrounding several cirques may leave a pyramidal horn, such as the Matterhorn.
• Truncated spurs: Interlocking spurs projecting into a river valley are cut off by the greater erosive power of a glacier, producing steep triangular faces along a U-shaped trough.
• Moraines: Lateral moraines accumulate along glacier sides, medial moraines form where tributary glaciers join, and terminal moraines mark the furthest advance. Ground moraine is till deposited beneath or behind retreating ice.
• Drumlins: These streamlined hills of till have a blunt stoss end facing the former ice source and a tapered lee end. Their alignment can reveal ice-flow direction, although their exact formation may involve several subglacial processes.
• Erratics: A glacial erratic is a boulder transported away from its source area and deposited on unrelated bedrock. Its lithology can help reconstruct former ice-flow pathways.
• Meltwater landforms: Eskers are sinuous ridges of sand and gravel deposited by subglacial channels. Kames are irregular stratified deposits, while outwash plains contain sorted sand and gravel beyond a glacier margin.
• Fjords: When a glacial trough is flooded by the sea, it becomes a fjord. Its deep basin and steep sides reflect substantial glacial overdeepening below present sea level.

C. Applications and limitations

Glacial landforms reconstruct past climate and ice extent, but present-day glacier behaviour also creates environmental and hazard concerns.

• Reconstructing ice movement: Striation orientation, drumlin alignment, moraine position, and erratic composition together indicate former flow direction and glacier limits. A terminal moraine records a stillstand or readvance, not necessarily the maximum age of the glacier.
• Climate indicators: Retreating glacier fronts often indicate sustained negative mass balance, but short-term advance can occur through increased snowfall or glacier surges even during regional warming.
• Hazards: Rapid melting can produce glacial lake outburst floods; unstable moraine dams may fail suddenly. Calving ice and avalanches threaten settlements and transport routes.
• Water resources: Seasonal meltwater supports downstream irrigation, hydropower, and ecosystems, but long-term ice loss may eventually reduce dry-season supply.
• Interpretive limitation: Similar ridges may be moraines, eskers, or fluvial deposits. Landform identification requires sediment texture, sorting, orientation, relationship to topography, and evidence of ice contact rather than shape alone.
• Landscape legacy: Many temperate landscapes retain glacial deposits and valleys long after ice disappears. Modern rivers may rework these sediments, producing mixed fluvial–glacial landforms.