MITIHANI POPOTE EXAMINATION SERIES TOPIC: RIVER PROCESSES AND LANDFORMS

RIVER PROCESSES AND LANDFORMS

1. Explain the features of river erosion (features which are formed at the first/torrent stage of the river)

   In the youthful (torrent) stage, rivers predominantly erode vertically, creating distinctive features:

   • V-shaped valleys: Formed by vertical erosion with minimal lateral erosion, creating steep-sided valleys
   • Gorges: Deep, narrow valleys with near-vertical sides formed in resistant rock
   • Waterfalls: Sudden vertical drops where resistant rock overlies softer rock
   • Rapids: Sections of turbulent water flow caused by uneven river bed gradient
   • Potholes: Circular depressions drilled into bedrock by swirling pebbles
   • Interlocking spurs: Projecting ridges that alternate from either side of the valley as the river winds around resistant rock

   [Diagram would show a cross-section of a V-shaped valley with interlocking spurs]

   Erosional processes at this stage:

   • Hydraulic action: Force of water dislodges particles
   • Abrasion/Corrasion: Rocks scrape against the channel
   • Attrition: Rocks collide and break into smaller pieces
   • Solution/Corrosion: Chemical dissolution of soluble rocks

2. Explain the geological structures of waterfalls formation

   Waterfalls form through specific geological conditions and processes:

   Key geological structures:

   • Differential erosion: A resistant cap rock (e.g., basalt, sandstone) overlies softer rock (e.g., shale, limestone)
   • Faulting: Vertical displacement creates a sudden drop in river course
   • Glacial hanging valleys: Tributary valleys left "hanging" above main glacial valley
   • Knickpoints: Sharp changes in gradient along the river's long profile

   Formation process:

   1. Softer rock beneath cap rock erodes faster through hydraulic action and abrasion
   2. Undercutting creates an overhang of resistant rock
   3. Overhang eventually collapses, causing the waterfall to retreat upstream
   4. Process repeats, forming a steep-sided gorge downstream

   [Diagram would show cross-section of waterfall with cap rock, plunge pool, and undercutting]

   Examples:

   • Niagara Falls (resistant dolostone over softer shale)
   • Victoria Falls (basalt over sandstone)
   • Angel Falls (fault-controlled)

3. Explain the features of river deposition and factors which influence river deposition

   Features formed by river deposition:

   • Floodplains: Flat areas adjacent to river channels built up by alluvial deposits
   • Levees: Natural embankments along river banks formed during floods
   • Alluvial fans: Fan-shaped deposits at base of mountains where gradient decreases
   • Deltas: Deposits at river mouths where sediment load exceeds removal capacity
   • Point bars: Deposits on inner bends of meanders
   • Braided channels: Networks of channels separated by islands of deposited sediment

   Factors influencing deposition:

   Factor	Effect on Deposition
   Decreased velocity	Reduces transport energy, causing sediment to settle
   Reduced gradient	Decreases stream energy and transport capacity
   Increased sediment load	Exceeds river's transport capacity
   Channel widening	Reduces depth and velocity
   Obstructions	Creates localized areas of reduced flow
   River mouth conditions	Tidal/wave action affects delta formation

4. Explain the causes and landforms of river rejuvenation

   Causes of rejuvenation:

   • Uplift of land: Increases potential energy and gradient
   • Fall in sea level: Lowers base level, increasing river's erosive power
   • Increased discharge: From climate change or river capture
   • Isostatic rebound: Land rises after glacial melting

   Resulting landforms:

   • Knickpoints: Sharp breaks in long profile marking old base level
   • River terraces: Former floodplains left elevated after renewed downcutting
   • Incised meanders: Meanders cut deeply into bedrock
   • Valley-in-valley forms: New V-shaped valley within older, wider valley
   • Wind gaps: Abandoned water gaps in former drainage routes

   [Diagram would show a cross-section of river terraces and incised meanders]

   Types of rejuvenation:

   • Dynamic: Caused by tectonic uplift
   • Static: Caused by base level fall
   • Eustatic: Global sea level changes

5. Explain the conditions and evidences of River capture

   Conditions favoring river capture (stream piracy):

   • Adjacent drainage systems with significant elevation differences
   • Presence of easily erodible rock or weak zones between systems
   • One river has steeper gradient/higher erosive power
   • Headward erosion actively extending one valley toward another

   Evidence of river capture:

   • Elbow of capture: Sharp bend in the pirate river's course
   • Wind gap: Dry valley marking former course of beheaded stream
   • Misfit stream: Small stream in large valley (former main channel)
   • Change in sediment: Sudden change in sediment characteristics downstream
   • Barbed drainage: Tributaries joining at acute angles upstream
   • Knickpoint: Abrupt change in gradient at capture point

   [Diagram would show before/after capture with elbow, wind gap, and misfit stream]

   Example: The River Wey in England captured the headwaters of the River Blackwater

6. Attempt the classification of deltas

   Deltas are classified based on dominant processes and morphology:

   Classification Basis	Types	Characteristics
   Dominant Process	River-dominated	Elongate shape (bird's foot), e.g., Mississippi Delta
   	Wave-dominated	Smooth, arcuate shape, e.g., Nile Delta
   	Tide-dominated	Estuarine with tidal channels, e.g., Ganges-Brahmaputra
   Morphology	Arcuate	Fan-shaped with smooth curves
   	Bird's foot	Distributaries extend like toes
   	Cuspate	Tooth-shaped, pointed seaward
   	Estuarine	Formed within drowned river valleys

   Additional classifications:

   • Gilbert deltas: Coarse sediments in lakes (steep foreset beds)
   • Tidal deltas: Formed by tidal currents rather than rivers
   • Inland deltas: Form in deserts/arid regions (e.g., Okavango)

7. Explain the types of accordant and discordant drainage systems

   Accordant drainage systems (consequent) follow the geological structure:

   • Consequent streams: Flow down primary slope of landform
   • Subsequent streams: Develop along weak zones (faults, soft rock)
   • Resequent streams: Flow same direction as original slope but at lower level
   • Obsequent streams: Flow opposite to consequent drainage
   • Insequent streams: Irregular patterns with no structural control

   Discordant drainage systems cut across geological structures:

   • Antecedent drainage: Maintains course despite uplift (e.g., Indus through Himalayas)
   • Superimposed drainage: Develops on overlying rock then cuts into underlying structure

   [Diagram would show examples of accordant and discordant drainage patterns]

   Comparison:

   Feature	Accordant	Discordant
   Relationship to structure	Conforms to structure	Cross-cuts structure
   Formation time	Develops with landscape	Older than current structure
   Common patterns	Trellis, parallel	Radial, rectangular

8. Explain the obstacles/irregularities to the attainment of graded river profile

   A graded profile (equilibrium between erosion and deposition) faces these obstacles:

   • Lithological variations: Resistant rock outcrops create local steepening
   • Tectonic activity: Uplift or faulting disrupts equilibrium
   • Base level changes: Sea level fluctuations alter downstream conditions
   • Climate change: Alters discharge and sediment load
   • Human interventions: Dams, channelization, mining disrupt natural balance
   • Glacial impacts: Moraines or altered drainage from past glaciation
   • Knickpoints: Inherited from previous erosion cycles
   • Tributary inputs: Sudden additions of water/sediment

   Effects of these obstacles:

   • Creation of rapids and waterfalls
   • Formation of lakes or alluvial deposits
   • Development of terraces
   • Changes in channel pattern (meandering to braided)

9. Explain the factors for the formation of the long and cross profile of the river

   Long profile factors (elevation changes from source to mouth):

   • Base level: Ultimate limit of erosion (usually sea level)
   • Rock resistance: Hard rocks create steeper segments
   • Tectonic history: Uplift events create convexities
   • Climate: Affects discharge and erosion rates
   • Sediment load: More load requires steeper gradient for transport
   • Glacial history: Overdeepened valleys in formerly glaciated areas

   Cross profile factors (valley shape at a point along course):

   • Stage of development: Youth (V-shaped) to mature (floodplain)
   • Dominant process: Vertical vs. lateral erosion balance
   • Lithology: Resistant rocks form steeper valley sides
   • Mass wasting: Contributes to valley widening
   • Climate: Weathering processes affect slope angles
   • Vegetation: Stabilizes slopes affecting form

   [Diagram would show typical long profile (concave) and cross profiles at different stages]

   Relationship between profiles:

   • Upper course: Steep long profile → V-shaped cross profile
   • Middle course: Moderate gradient → wider valley with floodplain
   • Lower course: Gentle gradient → broad floodplain with levees