Meanders and flood plains

In the middle and lower courses, a river winds in large bends called meanders:

1. On the outside of a bend, the water is deeper and flows fastest. It erodes the bank by lateral erosion, undercutting it into a steep river cliff.
2. On the inside of the bend, the water is shallow and slow, so it deposits sand and gravel, building a gentle slip-off slope.
3. As the outside erodes and the inside builds up, the bend gets bigger, and the meander moves slowly across the valley.

1. Erosion on the outside of two bends brings them closer, so the neck of land between them gets narrower.
2. During a flood, the river cuts through the neck, taking the shortest, fastest route.
3. The river now flows along the new, straighter channel.
4. Deposition seals off the ends of the old bend.
5. The cut-off bend becomes a horseshoe-shaped ox-bow lake, which slowly dries up and fills with plants.

A flood plain is the wide, flat land beside a river in its lower course:

• Meanders move across the valley, eroding its sides and widening it.
• When the river floods, it spreads over the land, slows down and deposits fine silt, building up a flat, fertile plain.

Levées are raised banks along the river:

1. When the river floods, it slows fastest at the edge of the channel.
2. The heaviest material is dropped first, right beside the river; the finer silt is carried further.
3. Flood after flood, the coarse material builds up into banks: levées.

An estuary is the wide, tidal mouth of a river, where fresh water from the river meets salt water from the sea.

• As the tide comes in, the river's flow is held back; the water slows, and fine silt and mud are deposited.
• At low tide, large areas of mudflats are left uncovered.
• Where the mudflats are covered less often, salt-tolerant plants grow, and salt marsh forms.

Estuaries are important habitats: birds feed on the worms and shellfish in the mud, and fish breed in the sheltered water.