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Rivers, Drainage, and Coastal Processes

River Systems and Processes​

1. River Classification and Characteristics​

River Definition:

  • Flowing body of water in channel
  • Source: Spring, glacier, lake
  • Mouth: Discharge point (ocean, lake)
  • Discharge: Volume of water flowing per unit time
  • Varies seasonally and with precipitation

Stream Order (Strahler System):

  • 1st order: Smallest tributaries
  • 2nd order: Union of 2 first-order streams
  • Higher orders: Larger rivers
  • Classification aids in analysis

Classification Systems:

  • Size: Discharge volume
  • Gradient: Slope of channel
  • Velocity: Speed of flow
  • Pattern: Course shape (straight, meandering, braided)

2. Fluvial Erosion Processes​

Erosion Mechanisms:

  • Hydraulic action: Water pressure on banks/bed
  • Abrasion: Sediment particles erode surface
  • Attrition: Sediment particles wear down each other
  • Solution: Chemical dissolution (limestone especially)
  • Transport: Material removed by river

Erosion Patterns:

  • Vertical erosion: Deepens valley (youthful streams)
  • Lateral erosion: Widens valley
  • Headward erosion: Upstream growth
  • Bank erosion: Side channel widening
  • Bed erosion: Channel deepening

3. Sediment Transport and Deposition​

Transport Mechanisms:

  • Traction: Largest particles rolling/sliding (bed)
  • Saltation: Bouncing particles (bed)
  • Suspension: Smaller particles suspended (waters)
  • Solution: Dissolved material (invisible)

Deposition:

  • Occurs when energy decreases
  • Coarse sediments deposit first (heaviest)
  • Fine sediments (clay, silt) deposit last
  • Creates sorted deposits

Channel Features:

  • Pools: Deep, low-velocity areas
  • Riffles: Shallow, high-velocity areas
  • Alternate bars: Sediment deposits
  • Meanders: Curves in river path

Fluvial Landforms​

1. Channel Geometry and Forms​

Straight Channels:

  • Uniform gradient
  • Rare in nature
  • Slight sinuosity
  • Relatively stable

Meandering Rivers:

  • Winding channel path
  • Lateral and vertical erosion
  • Sediment deposition on inside of curve
  • Meander migration downstream
  • Oxbow lake formation when neck breaks

Braided Rivers:

  • Multiple channels separated by islands
  • High sediment load (glacial rivers often)
  • Shifting channels
  • Unstable banks
  • Areas with flash flooding

2. Valley Development (Stage Theory)​

Youth Stage:

  • Steep gradient
  • Vertical erosion dominant
  • V-shaped valley
  • Waterfalls, rapids
  • Limited floodplain

Maturity Stage:

  • Moderate gradient
  • Lateral erosion begins
  • Meanders develop
  • Wider floodplain
  • Mixed erosion and deposition

Old Age Stage:

  • Very gentle gradient
  • Lateral erosion dominant
  • Extensive meandering
  • Wide floodplain
  • Sediment deposition main process
  • Delta or alluvial fan formation possible

3. River Valley Features​

Floodplains:

  • Area flooded during high flow
  • Sediment accumulation (alluvium)
  • Rich soils from flooding
  • Human settlement (despite risk)
  • Agricultural productivity

Terraces:

  • Abandoned floodplain level
  • Result of river downcutting
  • Multiple levels possible (terrace sequence)
  • Dating soil ages
  • Settlement areas (higher elevation, safer)

Gorges and Canyons:

  • Steep-sided valleys
  • High relief
  • Head-ward erosion
  • Found in mountains
  • Grand Canyon example

Deltas and Estuaries​

1. Delta Formation​

Definition:

  • Depositional landform at river mouth
  • Sediment accumulation (river discharge exceeds marine energy)
  • Multiple distributary channels
  • Complex topography with lakes/lagoons

Delta Types:

  • Arcuate/bird's foot: Mississippi style (fine sediments)
  • Cuspate: Cape-like projection (sand/gravel)
  • Elongated: River-dominated (strong current)
  • Fan-shaped: Sediment spreads (tide-dominated)

Processes:

  • Vegetation growth: Stabilizes sediment
  • Channel migration: Shifts over time
  • Crevasse splays: Sediment spreads from channel breaks
  • Marine reworking: Waves and tides modify shape

2. Estuaries​

Definition:

  • Transition zone between river/ocean
  • Fresh and salt water mix
  • Tidal influence
  • Different from deltas (no sediment accumulation)

Estuary Types:

  • Drowned river valley: Flooded by sea level rise
  • Bar-built: Barrier bars separate from ocean
  • Fjord: Glacially carved (very deep)
  • Wedge-shaped (salinity stratification)

Ecological Importance:

  • Nursery areas for fish
  • High productivity
  • Unique salinity-adapted species
  • Vulnerable to pollution

Coastal Processes and Features​

1. Coastal Erosion Processes​

Wave Action:

  • Hydraulic pressure: Water forces rock away
  • Abrasion: Rocks/sediment wear surfaces
  • Attrition: Particles wear each other
  • Weathering: Chemical alteration

Cliff Retreat:

  • Vertical erosion removes base (undercutting)
  • Upper cliff becomes unstable
  • Mass movement (falls, slumps)
  • Gradual or catastrophic
  • Rates: mm/year to meters/decade

Coastal Sediment Loss:

  • Erosion removes sediment
  • Transport to deeper water
  • Limited replenishment (dams block)
  • Vulnerable beaches shrink
  • Human structures accelerate loss

2. Coastal Erosional Features​

Caves, Arches, Stacks:

  • Caves: Eroded openings in cliff
  • Arches: Caves break through rock
  • Stacks: Isolated rock pillars (arch collapse)
  • Evolution from caves to stacks to stumps

Platforms and Terraces:

  • Wave-cut platform: Gently sloping base (low tide exposure)
  • Beach platform: High tide mark
  • Raised beaches: Previous sea levels (tectonics, isostatic)
  • Marine terraces: Stepped landforms

Headlands and Bays:

  • Differential erosion: Resistant rock remains as headlands
  • Non-resistant areas erode to bays
  • Bay beaches: Sediment accumulation
  • Headland cliffs: Erosion continues

3. Coastal Depositional Features​

Beaches:

  • Sandy/pebbly accumulations
  • Between low and high tide marks
  • Dynamic equilibrium (seasonal variation)
  • Beach profiles: Summer vs. winter changes
  • Human use and conflict

Spits:

  • Narrow ridges extending into water
  • Formed by longshore drift deposition
  • Hooked at end (wave refraction)
  • Change shape with seasons
  • Hook forms as wave refraction changes

Bars and Lagoons:

  • Bars: Submerged/emerged linear deposits
  • Completely blocks bay: Lagoon forms
  • Freshwater from rivers: Creates stratification
  • Brackish environment: Unique ecology
  • Barrier islands: Protect coast

Salt Marshes:

  • Low-lying coastal areas
  • Intertidal vegetation (salt tolerant)
  • Tidal creeks and channels
  • Sediment accumulation
  • Wildlife habitat

4. Longshore Drift and Sediment Movement​

Wave Refraction:

  • Waves bend as they approach shore
  • Angle of approach important
  • Creates diagonal wave pattern
  • Drives sediment movement

Longshore Current:

  • Water current parallel to shore
  • Carries sediment along coast
  • Weak or strong depending on waves
  • Creates sediment transport

Sediment Cells:

  • Stretches of relatively closed systems
  • Limited alongshore movement
  • Sediment balance within cell
  • Interruptions: Headlands, barriers

Coastal Hazards and Management​

1. Coastal Hazards​

Wave Erosion:

  • Ongoing baseline process
  • Accelerated in places (dam effects, sea level rise)
  • Rapid loss in some regions
  • Buildings threatened

Storm Surge:

  • Extra water pushed ashore by storm wind
  • Associated with tropical cyclones
  • Severe flooding inland
  • Damage to infrastructure

Tsunamis:

  • Earthquake or submarine landslide generated
  • Long-wavelength waves
  • Speed ~800 km/h in deep water
  • Decimating waves on shore
  • Coastal inundation

Subsidence:

  • Sinking of land surface
  • Human extraction: Oil, gas, groundwater
  • Tectonic: Subduction zones
  • Exacerbates coastal problems
  • Venice example

2. Coastal Management​

Hard Engineering:

  • Sea walls: Block wave energy
  • Groynes: Stop longshore drift
  • Breakwaters: Dissipate wave energy
  • Artificial reefs: Wave reduction
  • Expense and environmental impacts

Soft Engineering:

  • Beach nourishment: Sand replenishment
  • Dune restoration: Rebuilds natural barrier
  • Managed realignment: Allows retreat
  • Vegetation planting: Stabilizes substrate
  • More sustainable, ongoing costs

Integrated Coastal Zone Management:

  • Considers all uses and interests
  • Long-term planning
  • Recognizes natural processes
  • Balances development and conservation
  • Challenge: Coordination across boundaries

Summary​

Rivers, drainage, and coasts include:

  • River Processes: Erosion, transport, deposition mechanisms
  • Fluvial Landforms: Meanders, valleys, floodplains, terraces
  • Deltas and Estuaries: Depositional zones, ecological value
  • Coastal Processes: Wave action, cliff erosion, sediment movement
  • Coastal Features: Stacks, beaches, spits, marshes
  • Hazards and Management: Erosion, flooding, mitigation strategies

Understanding fluvial and coastal processes is essential for environmental management and hazard mitigation.