6.2 Floodplains and terraces

Floodplains and terraces are key features in river systems. Floodplains are flat areas next to rivers that flood regularly, while terraces are old floodplains now above flood level. Both are made of river-deposited sediments and reflect the river's history.

These landforms shape river valleys and impact flooding patterns. Floodplains form through river migration and sediment deposition. Terraces develop when rivers cut down, leaving old floodplains high and dry. Understanding these features helps us grasp river dynamics and landscape evolution.

Floodplains and Terraces in Fluvial Systems

Defining Floodplains and Terraces

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• Floodplains consist of flat, low-lying areas adjacent to rivers periodically inundated during high flow events
• Terraces represent abandoned floodplains no longer regularly flooded due to changes in river dynamics or regional uplift
• Alluvial sediments deposited by the river during flood events compose both floodplains and terraces
• River valley components reflect the geomorphic history and current dynamics of the fluvial system
• Factors influencing floodplain and terrace characteristics include river discharge, sediment load, and tectonic activity

Composition and Characteristics

• Sedimentary deposits in floodplains and terraces range from fine-grained silts and clays to coarser sands and gravels
• Floodplain topography often features natural levees, backswamps, and oxbow lakes
• Terrace surfaces may exhibit remnant river features such as paleochannels or scroll bars
• Vegetation patterns on floodplains and terraces often reflect differences in soil moisture and flooding frequency
• Soil development on terraces varies with age, with older terraces showing more advanced pedogenesis (soil formation)

Formation of Floodplains and Terraces

Floodplain Formation Processes

• Lateral accretion deposits point bars on the inside of meander bends as the river migrates across its valley
• Vertical accretion results from suspended sediment deposition on the floodplain surface during overbank flooding
• Channel avulsion creates new floodplain areas when the river abandons its current course and establishes a new channel
• Floodplain aggradation occurs when sediment deposition exceeds erosion over time
• Overbank sedimentation rates vary spatially, often decreasing with distance from the main channel

Terrace Formation Mechanisms

• River incision into its floodplain leaves the former floodplain surface elevated above the new river level
• Base level changes (lowering of sea level) trigger terrace formation by causing river incision
• Climate fluctuations alter river discharge and sediment load, potentially leading to terrace development
• Tectonic uplift raises the landscape relative to the river, promoting incision and terrace formation
• Terrace preservation depends on the balance between erosion and deposition within the river system
• Multiple terrace levels may form through repeated cycles of incision and stability

Types of Floodplains and Terraces

Floodplain Classifications

• Active floodplains experience regular flooding events (annual to decadal timescales)
• Inactive floodplains rarely flood due to channel incision or artificial flood control measures
• Braided river floodplains feature multiple interconnected channels separated by temporary sediment bars
• Anastomosing river floodplains consist of multiple stable channels with vegetated islands
• Meandering river floodplains exhibit a single sinuous channel with point bars and cutoff features

Terrace Categories

• Paired terraces occur on both sides of the river valley at similar elevations
• Unpaired terraces form on only one side of the valley or at different elevations on each side
• Strath terraces represent erosional features cut into bedrock with a thin veneer of alluvial sediments
• Fill terraces compose entirely of alluvial sediments deposited during aggradational phases
• Climatic terraces form in response to climate-driven changes in river dynamics (glacial-interglacial cycles)
• Tectonic terraces result from uplift or subsidence affecting the river's longitudinal profile

Significance of Floodplains and Terraces

Paleoenvironmental Reconstruction

• Floodplains and terraces serve as archives of past river behavior, preserving sedimentological evidence
• Spatial distribution and elevation of terraces reconstruct long-term river incision rates and patterns
• Sediment characteristics within floodplains and terraces provide information on past flow regimes (flood magnitudes)
• Organic material preserved in floodplain deposits allows for radiocarbon dating and paleoclimate reconstruction
• Pollen records in floodplain sediments offer insights into past vegetation changes and climate shifts

Applications in Geomorphology and Hydrology

• Terrace sequences infer regional tectonic activity, climate change, and sea-level fluctuations over geological timescales
• Study of floodplains and terraces contributes to understanding landscape evolution and sediment budgets
• Analysis of floodplain and terrace morphology proves crucial for flood risk assessment (flood extent mapping)
• Land-use planning utilizes floodplain and terrace data to determine suitable areas for development
• River restoration projects incorporate knowledge of historical floodplain and terrace configurations