Formation of waterfalls and gorges in the upper course of a river

In the upper course of a river, where the gradient is steep, distinctive landforms such as waterfalls and gorges are created through specific erosion processes.

How waterfalls develop over time

• Differential erosion - Waterfalls begin to form when a river flows over an area with hard rock upstream and softer rock downstream. The softer rock erodes more quickly.
• Creation of a step - As the softer rock wears away faster than the hard rock, a noticeable 'step' emerges in the river's path.
• Formation of a steep drop - Over time, the continuous flow of water over this step deepens the erosion of the softer rock, resulting in a waterfall.
• Undercutting and collapse - The hard rock at the edge of the waterfall is gradually undercut by erosion at its base. Without support, sections of this hard rock eventually collapse.
• Development of a plunge pool - At the base of the waterfall, fallen rocks are swirled around by the water, abrading the softer rock further and creating a deep plunge pool.
• Retreat of the waterfall - As undercutting and collapse continue, the waterfall moves upstream over time.
• Formation of a gorge - The upstream retreat of the waterfall leaves behind a narrow, steep-sided gorge.

Development of interlocking spurs in river valleys

Another characteristic feature of the upper course of a river is the presence of interlocking spurs.

Process of interlocking spur formation

• Dominance of vertical erosion - In the upper course, the river's energy is primarily directed downwards, cutting vertically into the landscape rather than eroding laterally (sideways).
• Creation of V-shaped valleys - This vertical erosion results in steep-sided, V-shaped valleys, a hallmark of the upper course.
• Winding around obstacles - Due to the river's inability to erode laterally, it must navigate around protruding high hillsides or ridges that block its direct path.
• Interlocking pattern of hillsides - As the river winds through the landscape, the hillsides on either side appear to interlock with one another when viewed from above, much like the teeth of a zipper. These are known as interlocking spurs.

Key erosion processes shaping river landscapes

Erosion is the primary force at work in the upper course of a river, sculpting the land through various mechanisms.

Main types of erosion in rivers

• Hydraulic action - The force of moving water wears away rock and soil.
• Abrasion - Sediment and pebbles carried by the river act like sandpaper, grinding against the riverbed and banks.
• Undercutting - Erosion at the base of a rock layer removes supporting material.
• Collapse - When rock or soil is undercut and loses support, it falls into the river.
• Retreat - The upstream movement of features like waterfalls as erosion continuously undercuts and collapses rock.
• Vertical erosion - The downward cutting of the river into its bed, deepening the channel.
• Lateral erosion - The sideways wearing away of river banks, which is less significant in the upper course compared to vertical erosion.