Middle Course Landforms (Meanders)

Meander Formation

Meanders are large bends in a river found mainly in the middle course where the river has a moderate gradient and energy. They form due to lateral erosion, which means the river erodes sideways rather than downwards. This sideways erosion causes the river to bend and curve.

The flow of water in a meander is not straight but follows a spiral path called helicoidal flow. This corkscrew-like movement moves water from the outer bend to the inner bend, helping to erode the outer bank and deposit sediment on the inner bank.

On the outer bend, the water flows faster, causing more erosion and forming a steep, often undercut bank called a river cliff. On the inner bend, the water flows slower, allowing sediment to settle and build up a gently sloping area called a slip-off slope.

For example, the River Thames in its middle course near Oxford has many meanders with clearly visible slip-off slopes and river cliffs.

Processes in Meanders

The main processes shaping meanders are erosion and deposition, controlled by the speed of water flow.

• Erosion on the outer bend: The water moves fastest here, increasing the river's energy to erode the bank. Two key types of erosion occur:
• Hydraulic action: The force of water hitting the riverbank causes air to be trapped in cracks, breaking the rock apart.
• Abrasion: Sediment and rocks carried by the river scrape and wear away the bank.
• Deposition on the inner bend: Water flows slower here, so it loses energy and drops sediment, building up the slip-off slope.

These processes cause the meander to become more pronounced over time, with the outer bend eroding and the inner bend growing.

For instance, in the middle course of the River Severn, erosion on outer bends and deposition on inner bends create distinctive meander shapes.

Features Associated with Meanders

Meanders often lead to the formation of other distinctive landforms:

• Oxbow lakes: Over time, erosion on the outer bends of two adjacent meanders can cause the neck of land between them to become very narrow. During a flood, the river may cut through this neck, creating a new, straighter channel. The old meander loop is then cut off, forming an oxbow lake.
• Meander scars: These are marks left on the floodplain where old meanders once flowed before being cut off. They appear as curved depressions or areas of different vegetation.
• Floodplain development: Meanders erode sideways, widening the valley floor and creating a flat area called a floodplain. This is often fertile land used for farming.

For example, oxbow lakes are common along the middle course of the River Trent, showing how meanders evolve.

Importance of Meanders in River Landscape

Meanders play a vital role in shaping the river landscape and affecting the surrounding environment:

• Influence on river course: Meanders cause the river to follow a winding path, increasing its length and slowing down the flow compared to a straight channel.
• Impact on surrounding land: The erosion of outer banks can threaten farmland or settlements, while deposition on inner banks creates fertile land. Meanders also contribute to the development of wide floodplains.
• Role in sediment transport: Meanders help move sediment downstream by eroding banks and depositing material, maintaining the river’s balance between erosion and deposition.

For example, the meandering River Wye supports rich farmland on its floodplain but also requires careful management to prevent erosion damage.

Learning Example: Calculating the length of a meander

If a straight section of river is 2 km long but the river meanders, increasing its length by 50%, what is the length of the river following the meander?

Length of meander = straight length + 50% of straight length = $2 + 0.5 \times 2 = 3\,\mathrm{km}$

This increase in length means the river flows more slowly compared to a straight channel, affecting erosion and deposition patterns along its course.