Landforms and Landscape Processes » River middle and lower course features

What you'll learn this session

Study time: 30 minutes

The key features of middle and lower course river landscapes
How meanders, oxbow lakes and floodplains form
The processes of erosion, transportation and deposition in rivers
Case studies of significant river landforms
How human activities affect river landscapes

Middle and Lower Course River Features

As rivers flow from their source to their mouth, they create different landforms along their journey. The middle and lower courses of a river have distinctive features shaped by the changing balance between erosion, transportation and deposition processes.

Key Definitions:

Middle course: The section of a river where it flows over gentler slopes and begins to erode laterally (sideways) as well as downwards.
Lower course: The final section of a river where the land is flatter and deposition becomes the dominant process.
Lateral erosion: The sideways wearing away of the riverbanks, particularly on the outside of bends.
Deposition: The process where a river drops the material it has been carrying when it loses energy.

♥ River Processes

Rivers shape the landscape through three main processes:

Erosion: The wearing away of the riverbed and banks through hydraulic action, abrasion, attrition and solution.
Transportation: The movement of eroded material downstream through traction, saltation, suspension and solution.
Deposition: The dropping of material when a river loses energy, typically when it slows down.

♦ River Energy

A river's ability to erode and transport material depends on its energy:

In the middle course, the river has moderate energy.
In the lower course, the river has less energy due to the gentler gradient.
When energy decreases, deposition occurs.
The balance between erosion and deposition creates distinctive landforms.

Meanders

Meanders are winding bends that form in the middle and lower courses of a river. They develop through a combination of erosion and deposition processes.

How Meanders Form

Meanders develop through this sequence:

The river flows fastest along the outside of a slight bend (the thalweg).
This faster flow causes more erosion on the outside of the bend, creating a river cliff or bluff.
The slower flow on the inside of the bend allows material to be deposited, forming a slip-off slope or point bar.
Over time, the outside bend erodes further while deposition continues on the inside, making the meander more pronounced.
This process continues, causing the meander to migrate across the valley floor.

→ River Cliff

The steep outer bank of a meander formed by erosion. The water flows fastest here, causing lateral erosion through hydraulic action and abrasion. This creates a near-vertical bank that can be several metres high.

← Slip-off Slope

The gently sloping inner bank of a meander where deposition occurs. As the river slows down on the inside of the bend, it deposits sediment, creating a gradual slope of sand and gravel that may be exposed during low flow.

Oxbow Lakes

Oxbow lakes are curved lakes that form when a meander is cut off from the main river channel. They represent the next stage in the evolution of a meander.

Formation of Oxbow Lakes

Oxbow lakes form through this process:

As meanders become more pronounced, the neck (the narrow piece of land between the bends) gets narrower.
During floods, the river may flow across this narrow neck rather than around the whole meander.
Over time, this new, straighter route becomes the main channel as it offers a more efficient path.
The original meander gets sealed off by deposition at both ends, creating a curved lake separate from the river.
Eventually, the oxbow lake may dry up completely, leaving just a meander scar in the landscape.

Case Study Focus: The Mississippi River

The Mississippi River in the USA provides excellent examples of meanders and oxbow lakes. The river's path has changed dramatically over time, with many oxbow lakes forming along its course. One famous example is Lake Chicot in Arkansas, which at 21 miles long is the largest oxbow lake in North America. It formed around 600 years ago when the Mississippi changed course. The area around the Mississippi has over 100 oxbow lakes, showing how dynamic river systems can be over time.

Floodplains and Levees

The lower course of a river is characterised by wide, flat areas called floodplains, which are formed by repeated flooding and deposition.

Floodplain Formation

Floodplains develop through these processes:

When a river floods, it spills onto the surrounding land.
As the water spreads out, it slows down and deposits the sediment it's carrying.
The heaviest material is deposited first, near the river channel.
Over many floods, layers of sediment build up, creating a flat, fertile plain.
The floodplain widens as the river continues to meander across the valley floor.

Natural Levees

Natural levees are raised banks that form alongside river channels in the lower course:

During floods, the heaviest material is deposited closest to the river channel.
This creates slightly raised banks (levees) on either side of the river.
These natural embankments can grow higher with each flood.
Levees can actually increase flood risk by containing the river until it reaches a much higher level.

⊕ Fertile Soil

Floodplains have some of the most fertile soil in the world due to the regular deposition of nutrient-rich silt during floods. This is why many early civilisations developed in river valleys.

⊕ Flat Land

The flat terrain of floodplains makes them attractive for human settlement, agriculture and transport networks, despite the flood risk they pose.

⊕ Flood Storage

Floodplains act as natural flood storage areas, allowing floodwater to spread out and reducing the impact downstream. This natural function is often compromised by development.

Deltas

Deltas form where rivers meet slower-moving or still water bodies like seas or lakes. They are complex depositional landforms that occur at the mouth of rivers.

Delta Formation

Deltas form through these processes:

As a river approaches its mouth, it slows down dramatically.
This causes the river to deposit the sediment it's carrying.
Over time, this sediment builds up, creating new land that extends into the sea or lake.
The river often splits into multiple channels called distributaries as it flows across the delta.
Different types of deltas form depending on the balance between river processes, waves and tides.

Case Study Focus: The Nile Delta

The Nile Delta in Egypt is one of the world's largest river deltas, covering about 240 km of Mediterranean coastline. It has been formed by thousands of years of deposition at the mouth of the Nile River. The delta is roughly triangular in shape (like the Greek letter delta, Δ) and is intensively farmed due to its fertile soil. About half of Egypt's population lives on or near the delta. However, the delta is now threatened by rising sea levels, subsidence (sinking land) and reduced sediment supply due to the Aswan Dam upstream, which traps sediment that would naturally replenish the delta.

Human Impacts on River Landscapes

Human activities can significantly alter river processes and landforms in the middle and lower courses:

⊕ River Management

Humans manage rivers through:

Channelisation: Straightening and deepening rivers to improve flow.
Artificial levees: Building embankments to prevent flooding.
Dams and reservoirs: Controlling flow and storing water.
Dredging: Removing sediment to deepen channels.

These interventions can disrupt natural processes and affect landform development.

⊕ Sustainable Approaches

More sustainable river management includes:

River restoration: Returning straightened rivers to their natural meandering state.
Managed retreat: Allowing flooding in certain areas to protect others.
Floodplain zoning: Restricting development in flood-prone areas.
Natural flood management: Using vegetation and landforms to slow water flow.

Summary of Middle and Lower Course Features

The middle and lower courses of rivers create distinctive landscapes through the interplay of erosion, transportation and deposition:

Middle course: Characterised by meanders, with lateral erosion becoming more important than vertical erosion.
Lower course: Dominated by depositional features including floodplains, levees and deltas.
Key landforms: Meanders, oxbow lakes, floodplains, levees and deltas all form through predictable processes.
Human interaction: These landscapes are heavily influenced by human activities, which can both disrupt and work with natural processes.

Understanding these processes and landforms is essential for sustainable management of river environments and for reducing flood risk to communities living along rivers.