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Unlock This CourseFlood plains are some of the most important landforms created by rivers. These flat areas of land next to rivers are formed over thousands of years through a combination of erosion and deposition. Understanding how flood plains form helps us understand why rivers flood and why these areas are so valuable for farming and settlement.
Flood plains are created in the middle and lower courses of rivers, where the gradient becomes gentler and the river has more energy for lateral (sideways) erosion rather than vertical (downward) erosion.
Key Definitions:
In the upper course, rivers have steep gradients and high energy for vertical erosion, cutting deep V-shaped valleys. As rivers reach their middle and lower courses, the gradient becomes gentler. The river still has energy, but now uses it mainly for lateral erosion and transportation of sediment.
Flood plain formation is a gradual process that happens over thousands of years. It involves several key processes working together to create these important landforms.
As rivers develop meanders in their middle course, they begin to erode laterally (sideways) rather than vertically (downwards). The fastest flowing water on the outside of meander bends erodes the river banks through hydraulic action and abrasion. Over time, this lateral erosion widens the valley floor, creating space for the flood plain to develop.
Fast flowing water erodes the bank through hydraulic action. The river channel becomes deeper here, forming a river cliff.
Slower flowing water deposits sediment, forming a slip-off slope. This creates a shallow area called a point bar.
Over time, meanders migrate downstream and across the valley floor, widening it further.
When rivers flood, they overflow their banks and spread across the surrounding land. The flood water carries lots of sediment (alluvium) picked up from erosion upstream. As the water spreads out over the flat land, it slows down and loses energy. This causes it to deposit the sediment it's carrying.
The heaviest sediment (sand and gravel) is deposited first, close to the river channel. This builds up over time to form natural embankments called levées. The finer sediment (silt and clay) is carried further from the channel before being deposited across the wider flood plain.
The fine sediment deposited during floods is rich in nutrients. This makes flood plain soils some of the most fertile in the world, which is why many of the world's major agricultural areas are located on flood plains. The River Nile's flood plain in Egypt has been farmed for over 5,000 years because of these fertile soils.
As meanders continue to migrate across the valley floor through lateral erosion, they sometimes become so curved that they almost form a complete loop. During a flood, the river may cut through the narrow neck of land, creating a new, straighter channel. The old meander bend becomes cut off, forming an ox-bow lake.
This process of meander migration and cut-off leaves behind a patchwork of different aged deposits across the flood plain. Some areas may have been deposited recently, while others may be much older.
Levées: Natural embankments formed by coarse sediment deposition close to the channel during floods.
Back swamps: Areas of poor drainage behind levées where fine sediment and water collect.
Ox-bow lakes: Curved lakes formed when meanders are cut off from the main channel.
Bluff lines: The edge of the flood plain where it meets higher ground or valley sides.
Flood plains are extremely valuable to humans for several reasons, which explains why many of the world's major cities and agricultural areas are located on them.
The fertile alluvial soils deposited during floods make flood plains ideal for farming. The flat land is easy to plough and irrigate and the regular flooding traditionally provided natural fertiliser. Many of the world's major food-producing regions are located on flood plains.
The flat land of flood plains makes them ideal for building settlements and transport networks. Rivers provide water supply, transport routes and power for industry. However, this puts human activities at risk from flooding.
The Mississippi River has created one of the world's largest flood plains, stretching over 2,300 miles from Minnesota to Louisiana. The flood plain varies in width from 20 miles to over 100 miles. Over thousands of years, the river has deposited sediment up to 100 feet deep in some areas. This has created some of the most fertile agricultural land in the USA, supporting major crops like corn, soybeans and cotton. However, human activities like building levées and draining wetlands have increased flood risk in recent decades.
While flood plains provide many benefits, they also present challenges for human activities. The natural flooding that creates and maintains flood plains can cause significant damage to human settlements and activities.
Traditional flood plain management worked with natural processes. Farmers understood that fields would flood occasionally and planned accordingly. Many settlements were built on slightly higher ground within the flood plain to avoid the worst flooding.
Today, many flood plains are protected by artificial flood defences like embankments, flood barriers and drainage systems. While these protect human activities, they can also disrupt natural processes and sometimes make flooding worse downstream.
Flood Risk: Natural flooding can damage property and disrupt human activities.
Development Pressure: Flat, accessible land attracts development, increasing flood risk.
Environmental Impact: Flood defences can disrupt natural ecosystems and sediment flow.
Climate Change: Changing rainfall patterns may increase flood frequency and intensity.
Modern approaches to flood plain management try to balance human needs with natural processes. This includes:
The River Thames flood plain supports over 1.5 million people in London and the Thames Valley. The Thames Barrier, completed in 1984, protects London from tidal flooding. However, the Environment Agency also manages flood risk through a combination of flood storage areas, improved flood warnings and sustainable development planning. The Jubilee River, completed in 2002, is an artificial channel that diverts flood water around Maidenhead and Windsor, reducing flood risk while maintaining the natural flood plain ecosystem.
Flood plains are dynamic landforms created by the interaction of river erosion and deposition over thousands of years. They represent some of the most valuable land on Earth for human activities, but also present significant challenges for flood management. Understanding how flood plains form helps us make better decisions about how to use and manage these important landscapes sustainably.
The key to successful flood plain management is working with natural processes rather than against them, recognising that flooding is a natural and necessary part of how these landforms function and maintain their fertility and ecological value.