River Landforms

Waterfalls and Gorges: The Power of Differential Erosion

• Waterfalls form where a river flows over a layer of resistant rock that sits directly on top of a layer of less resistant rock.
• The softer rock erodes much faster than the hard rock above it—a process known as differential erosion.
• Hydraulic action and abrasion undercut the soft rock, leaving the hard rock above protruding as an unsupported overhang.
• Swirling water and abrasive debris at the base of the waterfall carve out a deep basin called a plunge pool.
• Eventually, gravity pulls the unsupported overhang down, causing it to collapse into the plunge pool.
• This cycle repeats continuously, causing the waterfall to retreat upstream. As it moves back, it carves out a steep-sided, narrow valley known as a gorge.
• Case Study: High Force on the River Tees is a 21-metre waterfall where hard Whinstone overlies softer Limestone and Shale.

Meanders: The Interaction of Erosion and Deposition

• In the middle and lower courses, the river valley flattens out, and lateral erosion becomes the dominant force.
• The thalweg is the line of maximum velocity and force in the river. Centrifugal force pushes the thalweg to the outside bend of a meander.
• Outer Bend: The water is deeper, meaning less friction and higher energy. Hydraulic action and abrasion undercut the bank, causing it to collapse and form a steep river cliff.
• Inner Bend: The water here travels a shorter distance and is much shallower. High friction causes a significant loss of energy, forcing the river to undergo deposition (dropping its load).
• This deposition builds a gentle, crescent-shaped slope of sand and gravel known as a point bar (or slip-off slope).
• A secondary corkscrew-like water movement, called helicoidal flow, erodes the outer bank and transports the sediment across the channel to the inner bank.
• This continuous interaction causes the meander to migrate sideways, widening the flood plain.

Oxbow Lakes: When Meanders Break Free

• Continuous lateral erosion on the outer banks of two adjacent meander loops causes them to migrate toward each other.
• This eventually forms a very narrow strip of land between the bends known as a swan neck meander.
• The final breakthrough almost always occurs during a period of high discharge (a flood). The river uses this excess energy to cut straight through the narrow neck, taking the shortest, most efficient route.
• The new, straight channel becomes the main thalweg, causing the velocity to drop significantly at the entrance and exit of the old meander loop.
• This lack of energy causes massive deposition, which eventually seals off the loop entirely.
• The isolated, crescent-shaped body of stagnant water is an oxbow lake. Over time, it will fill with sediment and vegetation to become a meander scar.

Flood Plains and Levees: The Role of Deposition

• When a river exceeds its capacity during a period of high discharge, it bursts its banks and spreads over the flat valley floor.
• As water spreads out, the wetted perimeter (the length of the boundary between the water and the land) increases significantly.
• This causes a massive increase in friction, resulting in a rapid loss of velocity and energy.
• Unable to transport its full load, the river drops the coarsest, heaviest material (sand and gravel) first, immediately next to the riverbanks.
• Over time, repeated floods build these coarse deposits into naturally raised embankments called levees.
• Finer, lighter particles like silt and clay (collectively known as alluvium) are carried further across the land before settling.
• Repeated flooding deposits layers of alluvium, building up a wide, flat, and highly fertile area known as a flood plain.