NCERT Summary: Land Forms - 2 | Geography for UPSC CSE PDF Download

Summary of Fluvial Landforms

Fluvial landforms, those shaped by the action of rivers, are a fundamental aspect of geomorphology. These landforms result from two primary processes: erosion and deposition. Erosional landforms, such as valleys, gullies, entrenched meanders, river terraces, and plunge pools, are created by the wearing away of soil and rock due to flowing water. On the other hand, depositional landforms, including deltas, alluvial fans, floodplains, and meanders, are formed when rivers deposit sediments in flat areas. These landforms are often rich in nutrients, providing fertile ground for agriculture. Studying fluvial landforms is essential for understanding and managing modern issues like soil infertility, land degradation, and water-related problems.

Background of Fluvial Landforms

The evolution of fluvial landforms is closely tied to Earth's geological history. The formation of river systems dates back to the Precambrian eon, more than 4 billion years ago. As rivers began to shape the Earth's surface, processes like erosion and deposition started to create various landforms. Over time, geological activities such as tectonic movements, volcanic eruptions, and plate interactions have influenced the development of these features. In recent history, human activities have further impacted fluvial landforms. Construction of dams, river channelization, urban development, and agricultural practices have altered natural river systems, leading to significant changes in fluvial landscapes. Thus, the history of fluvial landforms is a dynamic interplay of natural processes and human intervention, evolving continuously over geological time scales.

Introduction to Fluvial Landforms

Fluvial landforms are the result of rivers interacting with the landscape, shaping it through both depositional and erosional processes. These landforms are crucial in understanding how rivers modify their surroundings and contribute to various ecological and geographical features. Depositional landforms are formed by the accumulation of sediments carried by rivers, while erosional landforms result from the removal and wear of land material. The study of these landforms reveals much about the geological history of an area and the ongoing processes that shape our environment.

Formation Processes of Fluvial Landforms

• Erosion: Rivers erode the landscape through various mechanisms, creating features such as valleys, canyons, and gorges. Erosion occurs vertically as rivers cut into their beds, and horizontally as they widen their channels. This process involves hydraulic action (the physical force of water), abrasion (the wearing away of surfaces by sediment), and corrosion (the chemical dissolution of rocks).

• Transportation: Once eroded, sediments are transported downstream by the river. This transportation occurs through different methods: traction (where sediments roll or slide along the riverbed), saltation (where sediments bounce along the bed), and suspension (where sediments are carried within the water column).

• Deposition: As the velocity of river water decreases, sediments are deposited. Deposition occurs in areas where the flow energy is reduced, such as inside meander bends, river mouths, or where the river enters larger bodies of water. This process creates various landforms, including floodplains, alluvial fans, and deltas.

• Channel Evolution: River channels undergo continuous changes due to erosion, sediment transport, and deposition. Factors such as variations in river discharge, sediment supply, tectonic activities, and human interventions (e.g., dam construction and river engineering) influence the morphology and behavior of river channels, leading to the formation of new landforms.

Depositional Landforms

• Deltas: Deltas form where rivers deposit sediments at their mouth as they slow down upon entering a larger water body. This results in a network of distributary channels that spread across a flat delta plain. Key components of deltas include distributary channels (branches of the river), the delta plain (the flat land between channels), and the delta front (the edge where sediment meets the sea). Prominent deltas in India include the Ganga-Brahmaputra Delta (Sunderbans), the Godavari Delta, and the Krishna Delta. Deltas are crucial for coastal protection, providing fertile agricultural lands and important habitats for numerous species.

• Floodplains: Floodplains are flat areas adjacent to rivers where sediment is deposited during floods. They are characterized by two main components: the floodway (the primary stream) and the flood fringe (the outer areas less frequently inundated). Examples of floodplains in India include the Indo-Gangetic Plains and the Yamuna Floodplains. Floodplains are vital for supporting diverse ecosystems, regulating river flow, and facilitating groundwater recharge.

• Alluvial Fans: These landforms are created when rivers deposit sediments from steep mountain regions onto flatter areas, forming a fan-shaped structure. Alluvial fans are dynamic environments influenced by flooding and sediment transport. Notable examples in India include the Ganga and Sutlej Alluvial Fans. Alluvial fans support varied plant and animal life and are important for groundwater replenishment.

• Meanders: Meanders are curves or bends in a river formed through erosion of the outer banks and sediment deposition on the inner banks. Over time, meanders can form ox-bow lakes when the river cuts off a loop, creating a crescent-shaped lake. Examples of ox-bow lakes in India include Chilika Lake and Kanwar Lake. Meanders play a significant role in nutrient cycling and provide diverse habitats.

Erosional Landforms

• Valleys: Valleys are shaped primarily by river erosion, creating V-shaped depressions with steep sides. While valleys can also be formed by glacial or tectonic activities, fluvial erosion is the most common process. Examples of fluvial valleys in India include the Kashmir and Bhagirathi Valleys. Valleys offer fertile lands, recreational opportunities, and habitats for various species.

• River Terraces: Formed by alternating periods of erosion and deposition, river terraces are elevated remnants of former floodplains. As rivers cut deeper into their beds, they leave behind terraces along the valley sides. Notable examples include the Ganga and Yamuna River Terraces. River terraces provide insights into historical river dynamics and are suitable for terrace farming.

• Plunge Pools: These are depressions formed at the base of waterfalls due to the intense hydraulic pressure of falling water. Examples in India include Dudhsagar Falls and Jog Falls. Plunge pools are significant for supporting specialized aquatic life and can attract tourists due to their dramatic landscapes.

• Gullies/Rills: These narrow, deep channels are created by the concentrated flow of water eroding soil and rock. Examples include the Chambal Ravines. Gullies contribute to land degradation and can pose risks to infrastructure.

• Entrenched Meanders: These meanders are deeply incised into the landscape, forming pronounced, sinuous bends with steep walls. Examples in India include the meanders of the Narmada and Tapi Rivers. Entrenched meanders provide insights into the erosional history of river systems and support unique ecosystems.

Conclusion

Fluvial landforms, shaped by the dynamic interplay of erosion and deposition, create a wide range of landscapes from meandering rivers to steep valleys. These processes not only alter the physical environment but also influence nutrient distribution and ecological health. Understanding these landforms is crucial for effective water resource management, hazard mitigation, and the preservation of ecological integrity in riverine environments.

Outwash Plains

Outwash plains are broad, flat areas formed by glacio-fluvial deposits located at the base of glacial mountains or beyond the limits of continental ice sheets. These plains are characterized by the accumulation of sediments transported by meltwater from glaciers, resulting in an array of deposits including gravel, silt, sand, and clay. The formation of outwash plains involves the following processes:

• Sediment Transport: As glaciers advance, meltwater streams carry debris and sediments away from the ice front. This sediment-laden water flows into low-lying areas, depositing materials as it slows down.

• Formation of Alluvial Fans: When meltwater emerges from the glacier, it spreads out and deposits sediment in the form of broad, fan-shaped accumulations known as alluvial fans. These fans can merge to form extensive outwash plains.

• Sediment Composition: Outwash plains are composed of various sediments ranging from coarse gravel to fine clay. The texture and composition of the deposits depend on the nature of the glacial material and the energy of the meltwater.

• Landscape Features: Outwash plains often feature gentle slopes and broad, flat surfaces. They are typically located beyond the terminal moraine of a glacier and can cover large areas.

These plains are significant for their role in sedimentary processes and their contribution to understanding past glacial activity.

Drumlins

Drumlins are elongated, oval-shaped ridges formed by glacial processes, primarily composed of glacial till—a mixture of clay, silt, sand, and gravel. Key characteristics and formation processes of drumlins include:

• Shape and Orientation: Drumlins have a streamlined, tear-shaped appearance with a blunt, steep end (stoss end) and a tapered, gently sloping end (tail). The long axis of drumlins is parallel to the direction of glacial movement.

• Formation: Drumlins are formed beneath a glacier where debris is deposited through fissures in the ice. The stoss end of the drumlin is shaped by the moving ice, which pushes and molds the sediment, while the tail end is streamlined by the glacier's movement.

• Size: Drumlins can vary in size but typically measure up to 1 kilometer in length and 30 meters in height. Their size and shape provide clues about the direction and intensity of past glacial flows.

• Significance: Drumlins help geologists reconstruct the history of glacial movements and ice sheet dynamics. They are commonly found in regions previously covered by glaciers.

Waves and Currents

Coastal processes, driven primarily by waves and currents, are highly dynamic and can cause rapid changes to coastal landscapes. Key aspects of coastal processes include:

• Wave Action: Waves play a crucial role in shaping coastlines. The energy of breaking waves erodes shorelines, churns sediments on the sea floor, and influences coastal landforms.
• Storm and Tsunami Waves: Storm waves and tsunami waves possess much greater energy compared to normal breaking waves, leading to more significant and often destructive changes along the coast.
• Configuration of Land and Sea Floor: The interaction between waves and the shape of the coastline and sea floor determines the nature of coastal erosion and deposition.
• Coast Type: Coastal landforms are influenced by whether the coast is advancing (emerging) or retreating (submerging) relative to sea level.

High Rocky Coasts

High rocky coasts are characterized by steep, rugged terrain where erosion processes dominate. Features and processes include:

• Indented Coastline: Fjords and drowned river valleys create highly irregular and indented coastlines. The steep drop-offs from the land into the sea contribute to dramatic coastal scenery.
• Erosion Features: Waves aggressively erode cliff faces, leading to the formation of cliffs and wave-cut platforms. Over time, continuous wave action smooths out irregularities along the shore.
• Wave-Cut Platforms and Terraces: As cliffs retreat, they leave behind wave-cut platforms. With further sediment accumulation and wave action, wave-built terraces can develop in front of these platforms.
• Depositional Features: Materials eroded from cliffs can form beaches, bars, barrier bars, and spits. Bars are submerged ridges of sand or shingle, while barrier bars that rise above water can form spits. Lagoons may form behind barrier bars and spits, which can eventually become filled with sediment to form coastal plains.

Low Sedimentary Coasts

Low sedimentary coasts feature smooth, gently sloping terrain where depositional processes dominate. Key features and processes include:

• Smooth Coastline: Rivers build up coastal plains and deltas, resulting in a relatively smooth coastline with occasional lagoons and tidal creeks.
• Depositional Features: As waves break over these gentle slopes, they churn sediments and form bars, barrier bars, spits, and lagoons. Over time, lagoons may become swamps and coastal plains.
• Sediment Supply: The maintenance of depositional features depends on a continuous supply of sediment. Large rivers delivering significant sediment loads contribute to delta formation along these coasts.
• Impact of Storms and Tsunamis: Storm and tsunami waves can cause substantial changes, even in sediment-rich environments, by dramatically altering coastal features.

Understanding these processes and landforms is crucial for managing coastal environments, predicting changes, and mitigating potential hazards related to coastal erosion and sedimentation.