Introduction to Levee and Floodplain Formation
Rivers are dynamic systems that constantly shape our landscape through erosion, transportation and deposition. Two key landforms created by these processes are levees and floodplains, which form in the middle and lower courses of rivers. These features are crucial for understanding river behaviour, flood management and human settlement patterns.
Key Definitions:
- Levee: A raised ridge of sediment that forms along the banks of a river, created by repeated flooding and deposition.
- Floodplain: The flat area of land adjacent to a river that experiences periodic flooding and is formed by the deposition of sediment during flood events.
You met the long profile and the middle and lower courses in The River Journey: Long Profile, Source, Tributaries, Confluence and Mouth, and the way rivers drop their load in River Transportation and Deposition. Levees and floodplains are what that deposition builds when a slowing river spills over its banks.
Levee Formation
Levees are natural embankments that form alongside river channels. They develop through a series of flooding events and are most prominent in the middle and lower courses of rivers. Here's how they form:
The Process of Levee Formation
💧 Step 1: Flooding
When a river floods, water spills over the banks and onto the surrounding land. The river's velocity suddenly decreases as it leaves the confined channel.
🌑 Step 2: Deposition
As the water slows down, it loses energy and can no longer transport all its sediment. The largest and heaviest particles are deposited first, right at the edge of the channel.
📝 Step 3: Build-up
With each flood, more sediment is deposited on top of previous deposits. Over time, this builds up to form raised banks or ridges along the sides of the river - these are levees.
The coarsest material (sand and small pebbles) is deposited closest to the river channel, while finer silts and clays are carried further onto the floodplain. This sorting of material creates a distinctive profile where levees slope gently away from the river. As levees grow higher, they can actually contain small floods within the channel, with only larger floods spilling over.
Floodplain Formation
Floodplains are the flat areas of land adjacent to rivers that are subject to periodic flooding. They form in the middle and lower courses of rivers where the gradient is gentler and the channel is less confined.
How Floodplains Develop
Floodplains develop through a combination of lateral erosion and deposition:
🔃 Lateral Erosion
As rivers flow, they erode their banks laterally (sideways), especially on the outside of meanders where flow is fastest. This lateral erosion gradually widens the valley floor, creating space for the floodplain to develop. Over time, the river may carve out a wide, flat valley.
🌞 Flood Deposition
When rivers flood, they deposit layers of sediment across the valley floor. Fine silts and clays are carried furthest from the channel and settle on the floodplain. Each flood adds a new layer of alluvium (river-deposited material), gradually building up the floodplain surface.
Floodplains often contain distinctive features formed by river processes:
- Meander scars: Curved depressions left when a meander is cut off and abandoned
- Oxbow lakes: Curved lakes formed when a meander is cut off from the main river
- Backswamps: Low-lying, marshy areas in the floodplain furthest from the river
- Braided channels: Multiple smaller channels that split and rejoin across the floodplain
Case Study Focus: The Mississippi River
The Mississippi River in the USA has one of the world's most extensive floodplain and levee systems. Its floodplain covers over 90,000 square kilometres across several states. Natural levees along the Mississippi can reach heights of 4-5 metres, though many have been artificially reinforced.
In 1927, catastrophic flooding occurred when levees failed in 145 places, flooding an area of 70,000 square kilometres and displacing 700,000 people. This disaster led to the comprehensive Mississippi River and Tributaries Project, which included strengthening levees, creating floodways and building control structures.
Today, the Mississippi's levee system stretches for over 3,500 kilometres, protecting communities and farmland. However, some scientists argue that constraining the river with levees prevents natural floodplain processes and may increase flood risks downstream.
The Importance of Levees and Floodplains
A flat, fertile floodplain beside a river - built up by sediment left behind in floods
🌿 Ecological Importance
Floodplains are among the most biologically productive ecosystems on Earth. Regular flooding brings nutrients that support diverse plant and animal communities. Wetlands in floodplains filter water, recharge groundwater and provide habitat for fish, birds and other wildlife. Levees create distinct ecological zones with different soil conditions and vegetation types.
🏠 Human Significance
Throughout history, humans have settled near rivers, using floodplains for agriculture due to their flat terrain and fertile soils. Natural levees often provided slightly elevated, well-drained sites for settlements. Today, many major cities are located on floodplains, protected by both natural and artificial levee systems. However, development on floodplains increases flood risk and can disrupt natural river processes.
Human Modification of Levees and Floodplains
People often raise natural levees or build artificial ones that are higher and steeper, and many floodplains are now built on. These flood management choices, including the River Thames defences, are covered in Managing River Flooding: Hard, Soft and Sustainable Strategies.
Why This Matters
Climate change is increasing extreme rainfall, which puts more pressure on levees and floodplains. Many regions are now moving towards making room for rivers and restoring floodplains, which you will study in Managing River Flooding: Hard, Soft and Sustainable Strategies.
Understanding levee and floodplain formation is essential for sustainable river management, flood protection and preserving these valuable ecosystems.
