Depositional river landforms

Overview

How levees are formed, two processes that build a floodplain and bluff line.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a levee, and where would you find one?

   Model answer

   A levee is a natural raised embankment of coarse sediment running along the bank on one or both sides of a river channel, higher than the floodplain behind it. Levees are found in the lower course, where the river regularly overtops its banks onto a wide floodplain and carries a heavy load of sand and silt. They can be a metre or two high and slope gently away from the channel towards the floodplain.

2. 2

   Explain how levees are formed.

   Model answer

   When the river exceeds bankfull discharge and spills over its banks, the escaping water suddenly becomes very shallow and spreads over a wide area, and friction with the floodplain surface and its vegetation causes an immediate loss of velocity. Because competence falls with velocity, the coarsest material the river is carrying - sand and fine gravel - can no longer be held and is dropped straight away, right at the edge of the channel. Finer silts and clays remain in suspension a little longer and are carried further out across the floodplain. Each flood therefore adds a layer of coarse material at the bank top and fine material beyond, and over many years the repeated coarse deposition builds the banks up into raised ridges.

3. 3

   Why can a levee-lined river end up flowing above the level of its own floodplain, and why is that dangerous?

   Model answer

   Levees raise the banks, so the channel can hold more water and floods less often; but the same river is still depositing sediment on its bed between floods, and because it now rarely spills out it drops much of that load in the channel, raising the bed as well. Over centuries both bed and banks rise until the water surface sits higher than the surrounding floodplain. This is dangerous because if a levee is ever breached or overtopped, water escapes onto lower ground and cannot drain back into the channel, so flooding is deeper, more extensive and far longer-lasting than it would otherwise have been.

4. 4

   What is a floodplain?

   Model answer

   A floodplain is the wide, flat area of land on either side of a river in its middle and lower course, built of alluvium, over which the river spreads when it exceeds bankfull discharge. It is typically bounded at its outer edge by steeper slopes called bluffs, which mark the limit of lateral migration of the channel, and its width is governed by the width of the meander belt. Its deposits are fertile, which is why floodplains attract intensive agriculture and settlement despite the flood risk.

5. 5

   Explain the two processes that build a floodplain.

   Model answer

   The first is lateral erosion and migration. As meanders swing from side to side across the valley floor, the outer bends undercut the valley sides and the whole meander belt shifts downvalley, planing the valley floor flat and progressively widening it, while point bar deposits are left behind on the inside of each abandoned bend as a layer of coarse alluvium. The second is vertical accretion by overbank deposition. Each time the river floods, the fine silt and clay held in suspension settles out across the plain as the water becomes still, adding a thin fertile layer. Over thousands of years these two processes together create a wide flat surface underlain by deep alluvium, coarse at depth from the migrating channel and fine near the surface from flooding.

6. 6

   Why is floodplain soil so fertile?

   Model answer

   Because overbank flooding repeatedly deposits fine silt and clay across it. These fine particles have a large surface area and a high capacity to hold both water and plant nutrients, and the material is rich in minerals eroded from across the whole catchment upstream, together with organic matter carried by the flood. Each flood renews the supply, so the fertility is replenished naturally rather than being exhausted by cropping. The land is also flat, easy to cultivate and close to a water supply, which is why floodplain agriculture is so productive.

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