Landforms and Their Evolution: Geomorphic Processes, Running Water, and Landscape Development

Fundamentals of Landforms and Their Evolution

• Definition of Landform:

  • Small to medium tracts or parcels of the Earth's surface are defined as landforms.
  • Every landform possesses its own physical shape, size, and constituent materials.
  • Landforms are produced by the continuous actions of specific geomorphic processes and agents.

• Definition of Landscape:

  • Several related landforms grouped together make up landscapes, which constitute large tracts of the Earth's surface.

• Characteristics and Lifecycle of Landforms:

  • The actions of most geomorphic processes and agents operate slowly, taking extended periods of time to shape landforms.
  • Every landform has a distinct beginning.
  • Once formed, landforms undergo dynamic modifications in shape, size, and physical nature over time due to persistent geomorphic action.

• Drivers of Landform Modification:

  • Changes in climatic conditions alter geomorphic process intensity or switch the active processes entirely.
  • Vertical or horizontal tectonic movements of landmasses introduce structural changes that modify existing landforms.

• Concept of Evolution:

  • Implies the sequential transformation stages of a segment of the Earth's surface from one landform into another.
  • Includes the developmental history and structural changes of individual landforms over time.
  • Landmasses undergo a lifecycle of development analogous to living organisms, progressing through three major stages: Youth, Maturity, and Old Age.

Mechanics of Running Water and Degradation

• Dominance in Humid Regions:

  • In humid regions receiving heavy rainfall, running water serves as the primary geomorphic agent responsible for land surface degradation.

• Components of Running Water:

  • Overland Flow: Unconfined sheet flow across the general land surface.
  • Linear Flow: Confined stream and river flow within valleys.

• Sequence of Degradation from Overland Flow:

  • Sheet Erosion: Caused by initial unconfined overland flow moving over surface irregularities.
  • Frictional Removal: The sheer friction of the flowing water column removes minor to major surface materials in the direction of flow.
  • Rill Formation: Frictional removal concentrates water flow, eroding small, narrow channels called rills.
  • Gully Formation: Rills progressively deepen, widen, and lengthen into gullies.
  • Valley Networks: Gullies deepen, widen, lengthen, and unite to build an interconnected network of valleys.

• Gradient and Erosion Relationships:

  • Vigorous, youthful rivers flowing over steep gradients produce prominent erosional landforms via intense down-cutting.
  • Continued erosion lowers channel gradients over time, decreasing water velocity and initiating active deposition.
  • Small-scale depositional forms can occur on steep slopes, but large-scale depositional landforms develop primarily over medium to gentle slopes.
  • As stream bed gradients flatten, downward vertical cutting becomes less dominant, while lateral erosion of stream banks increases, ultimately reducing elevated hills and valleys into flat plains.

Evolutionary Stages of Running Water Landscapes

• Youth Stage:

  • Stream Density & Network: Streams are few in number with poor drainage integration, flowing over original land slopes.
  • Valley Structure: Valleys are shallow and V-shaped, featuring no floodplains or extremely narrow floodplains along trunk streams.
  • Inter-stream Areas: Drainage divides are broad, flat, and poorly drained, containing extensive marshes, swamps, and lakes.
  • Channel Features: Meanders may develop over broad upland surfaces and may eventually entrench themselves into the uplands; waterfalls and rapids exist where local hard rock bodies are exposed.

• Mature Stage:

  • Stream Density & Network: Streams are plentiful, exhibiting well-integrated drainage networks.
  • Valley Structure: Valleys remain V-shaped but become significantly deeper; trunk streams broaden to accommodate wider floodplains.
  • Channel Features: Streams flow in meanders confined within the valley sides; waterfalls and rapids disappear entirely.
  • Divide Transformation: Broad, flat inter-stream areas, swamps, and marshes disappear as stream divides narrow into sharp ridges.

• Old Stage:

  • Stream Density & Network: Smaller tributaries are few in number and possess very gentle gradients.
  • Valley & Floodplain Structure: Streams meander freely across vast, low-gradient floodplains, developing natural levees and oxbow lakes.
  • Divide Transformation: Drainage divides become broad and flat again, featuring lakes, swamps, and marshes.
  • Relief State: The vast majority of the landscape is reduced to a level at or slightly above sea level.

• Peneplain and Monadnocks:

  • Peneplain: An almost-plain lowland of faint relief resulting from long-term stream erosion and complete flattening of drainage basin divides.
  • Monadnocks: Isolated, low-relief remnants of resistant rock that stand out above the general surface of a peneplain.

Erosional Landforms Created by Running Water

• Valleys:

  • Development Progression: Valleys originate as small, narrow rills, which expand into long, wide gullies, eventually deepening, widening, and lengthening into full valleys.
  • Morphological Types: Include V-shaped valleys, gorges, and canyons.
  • Gorge Characteristics: A deep valley with extremely steep to vertical straight sides, maintaining almost equal width at both top and bottom. Commonly forms in hard, resistant rocks (e.g., The Valley of Kaveri river near Hogenekal, Dharmapuri district, Tamil Nadu).
  • Canyon Characteristics: A variant of a gorge characterized by steep, step-like side slopes and a valley that is significantly wider at the top than at the bottom. Commonly forms in horizontally bedded sedimentary rocks (e.g., Entrenched meander loop of the Colorado River, USA).

• Potholes and Plunge Pools:

  • Potholes: Circular depressions formed on the rocky beds of hill-streams by stream erosion combined with the abrasive grinding of rotating rock fragments (pebbles and boulders). Continuous enlargement causes adjacent potholes to merge, deepening the stream valley.
  • Plunge Pools: Exceptionally large, deep, and wide potholes formed at the base of waterfalls. Formed by the hydraulic force of falling water and the rotational scouring of trapped boulders.

• Incised or Entrenched Meanders:

  • Very deep and wide meanders carved into solid hard rock surfaces.
  • Rapidly flowing streams on steep slopes concentrate erosion on the channel bottom, limiting lateral erosion compared to streams on gentle slopes.
  • Sinuous and meandering courses originally established over gentle floodplains or delta plains become deeply incised into underlying hard rock when vertical down-cutting is renewed (e.g., Entrenched meander loop of the Colorado River, USA).

• River Terraces:

  • Surfaces representing old valley floor or floodplain levels produced by vertical erosion into existing depositional floodplains.
  • Composition: Can consist of bedrock surfaces free of alluvium, or alluvial terraces composed of stream deposits.
  • Paired Terraces: Terraces occurring at identical elevations on opposite sides of a river valley.

Depositional Landforms Created by Running Water

• Alluvial Fans:

  • Formed when mountain streams discharging high-velocity flows break onto low-gradient foot slope plains.
  • Coarse bed load carried down steep mountain slopes becomes too heavy to transport over gentler gradients, spreading out as a broad, low-to-high cone-shaped deposit.
  • Streams on alluvial fans frequently shift out of their channels, splitting into multiple branching channels termed distributaries.
  • Climatic Variants: Humid regions produce low-angle cones with gentle head-to-toe slopes; arid and semi-arid regions produce high-angle cones with steep slopes (e.g., Alluvial fan deposited by a hill stream on the way to Amarnath, Jammu and Kashmir).

• Deltas:

  • Formed where river loads are deposited directly into a marine body or sea.
  • Occurs when dumped river load is not swept away by oceanic currents, accumulating as a low-angle cone.
  • Sediment Stratification: Unlike alluvial fans, delta deposits are well-sorted and clearly stratified. The coarsest materials settle out first along the shoreline, while fine fractions like silts and clays are carried further into open water.
  • Growth Pattern: As deltas build outward into the sea, distributary channels lengthen and branch further (e.g., Krishna River delta, Andhra Pradesh).

• Floodplains, Natural Levees, and Point Bars:

  • Active Floodplain: The river channel bed composed of ongoing, active stream deposits.
  • Inactive Floodplain: The plain area situated above the river bank level, built up during major floods.
  • Types of Floodplain Deposits: Channel deposits consist of coarse sediments filling abandoned or cut-off channels; flood deposits consist of fine silts and clays spread by overbank floodwaters.
  • Delta Plains: Floodplains located within a deltaic region.
  • Natural Levees: Low, linear, parallel ridges of coarse deposits formed directly along the banks of large rivers, frequently dissected into individual mounds.
  • Point Bars (Meander Bars): Linear sediment deposits found on the concave (inner) side of meanders in large rivers, displaying uniform width and profile with mixed sediment sizes.

Channel Dynamics and Meander Formation

• Nature of Meanders:

  • Loop-like channel patterns developing across broad floodplains and delta plains.
  • A meander is classified as a specific channel pattern rather than a distinct landform.

• Causes of Meander Formation:

  • The tendency of water flowing over extremely gentle gradients to exert lateral pressure against channel banks.
  • Unconsolidated alluvial bank materials containing physical irregularities that redirect water flow laterally.
  • The Coriolis force, which deflects fluid flow in a manner identical to atmospheric wind deflection.

• Erosion and Deposition Dynamics in Meanders:

  • Slow, leisurely flow on low gradients causes minor bank irregularities to grow into pronounced curvatures.
  • Concave Bank (Cut-off Bank): Subject to active erosion and undercutting by stream flow, resulting in a steep scarp profile.
  • Convex Bank (Slip-off Bank): Subject to active sediment deposition, producing a long, gentle slope profile.
  • Oxbow Lakes: Deep meander loops erode through their narrow neck points (inflection points), cutting off the main loop and leaving behind crescent-shaped water bodies called oxbow lakes (e.g., Meandering Burhi Gandak River near Muzaffarpur, Bihar).

Groundwater Dynamics and Landscape Evolution

• Geomorphic Role of Groundwater:
  • Focuses on the active work of groundwater in eroding landmasses and driving overall landform evolution rather than its function as a water resource.
  • Performs chemical weathering, dissolution, and subsurface erosion, altering surface topography and forming underground drainage networks.