River landscapes: Middle Course

• River landscapes are divided into stages: upper, middle, and lower course.
• The gradient is much less steep in the middle course compared to the upper course.
• The volume of water increases as more tributaries join the main river.
• Lateral erosion becomes more important than vertical erosion in the middle course.
• Significant features of the middle course include river beaches (slip-off slopes), meanders, and river cliffs.

Meanders and Erosion

• Meanders grow larger, with the current being fastest and most powerful on their outer bends.
• Erosion happens quickly where banks on the outside of meanders are undercut.
• Undercutting leads to bank collapse and retreat over time.
• Erosion on the outside bend can make very steep river cliffs if the meander reaches the valley's edge.
• Water is deeper on the outside of the meander and shallower on the inside.
• Faster water has more energy to erode.
• Rivers erode outer bends through corrasion, corrosion, and hydraulic action.
• The river deposits material on the inside of the bend due to slower water movement, forming a river beach/slip-off slope.
• Continuous erosion on the outer bank and deposition on the inner bank forms a meander.
• Meanders change shape and move downstream over time.

Oxbow Lakes

• These form as the river meanders across the valley, eroding laterally.
• Faster flow on outside bends causes erosion.
• Slower flow on inside bends leads to deposition.
• Continuous erosion and deposition narrows the meander neck, and flooding can cause the river to cut through.
• The river then continues on a straighter path.
• The old meander is abandoned.
• New deposition seals off the ends, forming a cut-off that then becomes an oxbow lake, which eventually dries up.

Levees

• These form where the river is on its floodplain, and there is a very low gradient.
• The river is mainly depositing silt and sand.
• During floods, silt is deposited on the river banks rather than across the floodplain.
• Frequent flooding builds the levees up.
• When the river flows normally, it deposits material in the river bed.
• This raises the river level above the floodplain.
• The built-up levees protect the floodplain from flooding.

Braiding

• Braiding occurs when a river divides into two or more channels across a distance.
• Channels are separated by islands of sediment.
• Braiding happens when a river carries a very large load, especially sand and gravel, compared to its velocity.
• Discharge changes rapidly from season to season.
• The river deposits so much sediment that the channel becomes choked.
• The river is forced to split, finding its way through deposits.
• Banks formed from sands and gravels are unstable.
• The channel becomes very wide relative to its depth, and the channels and sediment islands constantly change.

Deltas

• Deltas are formed by sediment deposition at the mouth of a river where it enters a sea or a lake.
• Not all rivers end in a delta.
• Large rivers in their lower course have the energy to transport a great deal of material in suspension.
• The river's speed reduces as it enters the sea, causing deposition.
• Sand is deposited first, and lighter silts and clays are carried further out.
• Layers of sediment build up on the sea floor.
• This creates a large platform of river sediment, a delta.
• When a river flows into a delta, it flows over its own deposits, causing the river to braid.
• Each channel in a delta is called a distributary.
• Deltas need rivers with large sediment loads, and coastal currents/waves must not be too strong.
• Strong currents/waves would remove sediment faster than the river can deposit it, spreading the sediment over a wider area.
• Deltas have three main types: fan-shaped (arcuate), bird’s foot (digitate) and estuarine (cuspate)
  • Fan-shaped deltas are triangular with a rounded margin.
  • Bird’s-foot deltas have distributaries flanked by sediment extending out to sea.
  • Estuarine deltas forms as islands in the river's mouth.