River Landforms in the UK

Waterfalls and Gorges Waterfalls form when a river flows over a horizontal band of resistant rock (the cap rock) overlying less resistant, softer rock.

• First, differential erosion occurs, where the softer rock erodes much faster than the harder rock.
• Then, this creates an indentation beneath the hard rock, leaving an unsupported overhang.
• At the base, the force of the falling water creates a deep depression called a plunge pool.
• Eventually, gravity causes the overhang to collapse into the plunge pool.
• Finally, this process repeats, causing the waterfall to move upstream in a process called recession. Over time, this retreat leaves behind a steep-sided, narrow valley called a gorge.

Landforms of Erosion and Deposition: Meanders and Ox-bow Lakes

Why do rivers rarely flow in straight lines for long? As a river enters its middle course, it begins to erode sideways rather than downwards, creating sweeping bends.

Meanders Lateral erosion (sideways erosion) widens the river valley and creates bends called meanders.

• The thalweg is the line of maximum velocity and fastest flow within the channel.
• As the river bends, the thalweg swings to the outside bank, causing erosion to form a steep river cliff.
• On the inside of the bend, the water moves much slower. This lack of energy leads to deposition, forming a gentle, sandy slip-off slope.
• This creates an asymmetrical cross-profile that is deep on the outside and shallow on the inside, while a corkscrew-like water movement called helicoidal flow transfers eroded material from the outer bank to the inner bank downstream.

Ox-bow Lakes Over time, meanders migrate laterally and downstream, increasing their curvature.

• First, continuous erosion on adjacent outer bends narrows the land between them, creating a thin meander neck.
• Then, during a period of high discharge or a flood, the river breaches the neck to flow straight through.
• As the floodwaters subside, slower water at the edges causes deposition, eventually sealing off the old loop.
• Finally, the isolated loop is left behind as a crescent-shaped, stagnant ox-bow lake.

Depositional Landforms: Levées, Flood Plains, and Estuaries

Every time a river bursts its banks, it is actually building its own natural flood defences. In the lower course, the gradient is flat, water volume is high, and deposition is the dominant process.

Flood Plains and Levées A flood plain is a wide, flat area on either side of a river, composed of fine, nutrient-rich sediment called alluvium.

• First, the river overflows its banks during a flood event.
• As water spreads over the land, increased friction with the ground and vegetation causes a sudden decrease in velocity.
• Because it loses energy, the river deposits its heaviest, coarsest material first, right next to the channel.
• Finer material travels further across the plain. Over successive floods, the coarse deposits build up into raised natural embankments called levées, which can reach 2 to 8 metres in height.

Estuaries An estuary is where a river meets the sea, containing a mixture of fresh and salty water.

• As the river flows into the sea, its velocity drops significantly due to the wider channel and the opposing force of the incoming tide.
• This sudden drop in energy causes the river to deposit its sediment load.
• A chemical process called flocculation occurs when salt water makes fine clay particles clump together, sink, and settle rapidly.
• Continuous deposition builds up large areas of fine sediment known as mudflats, which are exposed at low tide.

UK River Case Study: The River Tees

Studying a real river system helps map these physical processes from the hills to the coast. The River Tees in north-east England is 137 km long, flowing from its source high in the Pennines to its mouth at the North Sea.

The Upper Course

• The upper drainage basin has very high rainfall (over 2000mm) and impermeable rocks.
• It features interlocking spurs near Cauldron Snout.
• High Force Waterfall is located here, dropping 21 metres. It formed because a hard igneous cap rock called Whin Sill overlies softer limestone.
• The recession of High Force has created a 700-metre-long gorge downstream.

The Middle and Lower Courses

• In the middle course, extensive meanders have developed around towns like Barnard Castle.
• Moving into the lower course, depositional features like ox-bow lakes and levées are prominent near Yarm.
• At the mouth near Middlesbrough, the estuary features vast mudflats, such as Seal Sands.