AS Theme 1 – Hydrology & Fluvial Geomorphology: River Channel Processes & Landforms

Essential & Further Reading

  • Core texts students are expected to consult
    • Waugh, D., Geography: An Integrated Approach (3rd ed.) – Chapter 3, pp 68–86
    • Bowen & Pallister, AS Level Geography, pp 18–31
    • Bishop & Prosser, Landform Systems (2nd ed.) – Ch 2, pp 19–37 & Ch 3, pp 38–56
  • Supplementary texts (deepen or cross-check understanding)
    • Ross, Morgan & Heelas, Essential AS Geography – Sect 7, pp 240–252
    • Nagle, Advanced Geography – Ch 4, pp 78–104
    • Clowes & Comfort, Process and Landform – Ch 4, pp 92–96, 104–107, 119–147
    • Strahler & Strahler, Introducing Physical Geography (4th ed.) – Ch 16, pp 546–573
    • Nagle & Spencer, Oxford Revision Guide – pp 31–35
  • Online portals (excellent for diagrams, data, case studies)
    • NASA Earth Observatory – especially Study section (planetary-scale process explanations)
    • Salem State University stream-process pages
    • S-Cool A-level “River Processes & Management” hub

River Channel Processes & Landforms – Big Picture

  • Rivers = open-channel flows performing 3 inseparable roles:
    • Erode the channel
    • Transport eroded material
    • Create erosional & depositional landforms
  • Any drainage basin can be simplified into three overlapping zones (after S.A. Schumm):
    • Sediment production (dominant erosion, upland headwaters)
    • Sediment transfer (mixed processes, middle reaches)
    • Sediment deposition (dominant deposition, lowland/coastal reaches)

River Energy: Sources, Calculation & General Rules

  • Two energy forms operate simultaneously
    • Potential energy → product of water weight & altitude (gravitational)
    • Kinetic energy → energy of motion as water accelerates downslope
  • Simplified energy statement
    E \propto Q + V
    where Q = volume/discharge, V = velocity
  • Practical rules (memorise relationships)
    • Increased energy / water input
      • steeper gradient
      • wider & deeper channel
      • bigger capacity & higher efficiency (higher hydraulic radius)
      • faster velocity ⇒ peak erosion & transport
    • Decreased energy / dwindling input
      • gradient flattens
      • velocity drops
      • erosion stops, transport slows
      • deposition begins
      • channel becomes narrower & shallower

Long-profile Course Comparison

  • Gradient
    • Upper: very steep
    • Middle: moderating
    • Lower: almost flat
  • Velocity
    • Upper: high (low friction)
    • Middle: moderate
    • Lower: low (greater friction despite higher discharge)
  • Dominant processes
    • Upper: vertical & headward erosion (abrasion, attrition, hydraulic action, solution)
    • Middle: still abrasion/attrition, lateral erosion initiates
    • Lower: lateral erosion on outer meander bends; overall erosion wanes; deposition surges
  • Typical sediment calibre & transport mode
    • Upper: large boulders as intermittent bedload; sand/silt in suspension possible
    • Middle: cobbles by traction, more sand/silt suspension
    • Lower: sand→clay spectrum, traction + suspension dominate, high dissolved load
  • Deposition points
    • Upper: only the largest boulders
    • Middle: coarse boulders/cobbles
    • Lower: abundant sand/gravel on bars & floodplains
  • Iconic landforms
    • Upper: waterfalls, plunge pools, inter-locking spurs, gorges, rapids, potholes
    • Middle: rapids, small meanders, embryonic floodplain
    • Lower: mature meanders, large floodplain, pools & riffles, braiding

Erosion Mechanisms (remember “A A C H”)

  • Abrasion (corrasion) – river’s load grinds/sandpapers bed & banks
  • Attrition – load particles hit each other, becoming smaller, rounder
  • Corrosion (solution) – chemical weathering of soluble rocks (e.g. limestone) by slightly acidic water
  • Hydraulic action – sheer force of water + air compression in cracks dislodges material

Sediment Transport Processes & Load Types

  • Load categories
    • Bedload – coarse (boulders → pebbles)
    • Suspended/wash load – fine silts, clays; medium sands
    • Solution load – dissolved ions (Ca$^{2+}$, HCO$_3^-$ etc.)
  • Four transport modes (mnemonic “T S S S” – Traction, Saltation, Suspension, Solution)
    • Traction – rolling & sliding along bed; needs high V; lift/eddy forces assist
    • Saltation – bouncing motion of sand/gravels; intermittent contact
    • Suspension – fine particles held aloft by turbulent eddies; can travel huge distances
    • Solution – invisible ionic transport; independent of velocity, controlled by lithology & pH
  • Competence = largest particle size a river can move; Capacity = total mass/volume it can carry
  • Governing factors for dissolved/suspended percentages
    • Climate (temperature, rainfall regime)
    • Vegetation (root interception, organic acids)
    • Geology (solubility, erodibility, permeability)
    • Relief & slope (controls potential & kinetic energy)
    • Human activity (mining, deforestation, construction)

Deposition & The Hjulstrom Curve

  • Deposition triggers
    • Low discharge (dry season)
    • Velocity reduction where river enters sea/lake
    • Inside meander shallows
    • Sudden load spike (landslide debris)
    • Floodwaters spilling onto floodplain
  • Hjulstrom graph: plots grain diameter (mm, x-axis) vs velocity (cm s$^{-1}$, y-axis)
    • Critical erosion curve – minimum V to lift grain
    • Settling curve – V below which grain deposits
    • Area between = transport window once grain is mobilised
  • Key interpretations
    • Sands (~1\,\text{mm}) require least energy to start moving (~20 cm s$^{-1}$) because they are non-cohesive
    • Clays (<0.004\,\text{mm}) need surprisingly high V to erode due to electrostatic cohesion yet stay suspended at near-zero V
    • Pebbles/cobbles need progressively higher V for erosion; boulders require >300 cm s$^{-1}$
    • Very small drops in V can switch coarse sediments from transport → deposition (narrow gap between curves)
  • Limitations
    • Assumes smooth, uniform channels
    • Ignores natural velocity variability
    • Less representative for gravel-bed rivers where little sediment is in motion

Velocity & Discharge Fundamentals

  • Discharge definition
    Q = V \times A (where A = cross-sectional area) measured in \text{m}^3\,\text{s}^{-1}
  • Velocity definition – distance/time; usually \text{m}\,\text{s}^{-1}
  • Three prime controls on velocity
    1. Channel shape (expressed via Hydraulic Radius)
      • R = \dfrac{A}{P} where P = wetted perimeter
      • Larger R ⇒ less relative contact ⇒ less friction ⇒ higher V
      • Ideal efficient profile ≈ semicircle
    2. Bed & bank roughness
      • Coarse, protruding clasts raise resistance; cohesive muds smoother
    3. Channel slope/gradient
      • Steeper = greater component of gravity downstream
  • Comparative efficiency example
    • Stream A (small wetted perimeter) – high R, low friction, higher V
    • Stream B (broad, shallow) – low R, high friction, lower V

Flow Patterns

  • Laminar Flow
    • Parallel sheets, little vertical mixing; rare in natural rivers – normally dismissed
  • Turbulent Flow
    • Chaotic eddies both vertical & horizontal
    • Intensity rises with velocity once friction is surpassed
    • Critical for suspension competence
  • Helicoidal Flow
    • Corkscrew spiral along meander thalweg
    • Erodes outer bend, deposits inner bend point-bar (riffle-pool maintenance)

Channel Planform Types

  • Straight
    • Rare, short, often structurally controlled
    • Single thread, low sinuosity
  • Meandering
    • Single sinuous thread; most energy-efficient geometry
    • Maintained by outer-bank erosion & point-bar deposition
  • Braided
    • Multiple shifting threads around mid-channel bars
    • Form under high load/variable discharge (e.g. pro-glacial, semi-arid)**
    • Bars build during falling stage; channel threads rework during floods

Major Fluvial Landforms (selected syllabus items)

  • Waterfalls
    • Form where resistant caprock overlies weaker strata or at plateau edges
    • Upstream retreat via undercutting ➔ plunge-pool deepening ➔ hard-rock collapse
  • Gorges
    • Deep, narrow valleys created by rapid vertical incision
    • Triggers: glacial meltwater, tectonic uplift (antecedent rivers), waterfall retreat, cave roof collapse (karst)
  • Alluvial Fans
    • Fan-shaped deposits where steep, confined streams debouch onto plain
    • Velocity drop ⇒ sediment sorting; fine fans (<1°) vs coarse steep fans (≤15°)
    • Size depends on rock resistance, tectonics (Death Valley asymmetry example)
  • River Terraces
    • Abandoned former floodplains perched above current channel
    • Produced by river rejuvenation (base-level fall, uplift, discharge increase, load fall)
    • Paired vs unpaired terraces; oldest = highest, most degraded
  • (Additional homework landforms to note separately: riffle-pool sequences, point bars, floodplains, levees, deltas, bluffs)

Key Terminology Recap

  • Capacity vs Competence
  • Critical erosion velocity vs Settling velocity
  • Hydraulic Radius, Wetted Perimeter
  • Thalweg (line of maximum velocity)
  • Rejuvenation, Antecedent drainage
  • Bedload vs Wash load vs Solution load

Ethical & Practical Connections

  • Human modifications (mining, clear-cutting, dams) alter load & discharge → affect erosion/ deposition patterns ➔ management challenges (flood risk, habitat change)
  • Understanding Hjulstrom thresholds guides engineering (e.g. designing stable channel velocities to minimise unwanted siltation or scour)

Study Guidance & Tasks

  • Complete typed notes (with diagrams) on:
    • Four erosion processes
    • Four transport processes
  • Sub-topic 1.3.5 homework – prepare detailed notes for:
    1. Riffles & Pools
    2. Point Bars
    3. Floodplains
    4. Levees
    5. Deltas
    6. Bluffs
      (Deadline: 4 days)
  • Practice Drill: Use provided Hjulstrom curves to fill blanks (e.g. erosion at 10 cm s$^{-1}$, cohesive clay threshold etc.) and answer Nov 2004 past-paper questions on erosion/transport relationships.