Upper Course Landforms (Waterfalls & Gorges)

Formation of Waterfalls

Waterfalls form in the upper course of a river where there are layers of hard and soft rock. The river flows over the hard rock, which is more resistant to erosion, and then over the softer rock beneath it, which erodes more quickly.

The main river erosion processes involved are:

• Hydraulic action: The force of the water hitting the riverbed and banks causes rock to break off.
• Abrasion: Sediment and rocks carried by the river scrape and wear away the softer rock.

Because the soft rock erodes faster, it creates a step in the riverbed. The hard rock above eventually forms an overhang as it is undercut by erosion of the softer rock beneath.

As the overhang becomes unstable, it collapses into the plunge pool below, which is deepened by the swirling action of water and rock fragments. This process repeats, causing the waterfall to retreat upstream over time.

For instance, if a river flows over a band of hard granite followed by softer sandstone, the sandstone will erode faster, creating a waterfall at the boundary.

Characteristics of Waterfalls

Waterfalls have distinctive features:

• Steep vertical drop: The water falls sharply from a height, often tens of metres.
• Plunge pool: A deep basin at the base of the waterfall formed by the erosive action of falling water and swirling rocks.
• Overhang of hard rock: The harder rock layer projects outwards above the softer rock that has been eroded underneath.
• Spray and mist: The falling water creates a misty spray around the base, often visible on calm days.

These features make waterfalls visually striking and important for river landscapes in upland areas.

Formation of Gorges

Gorges are narrow, steep-sided valleys formed by the upstream retreat of waterfalls. As the waterfall erodes the softer rock beneath the hard rock, the waterfall moves backwards, carving out a deep, narrow valley.

The main processes involved are:

• Vertical erosion: The river cuts downwards into the bedrock, deepening the valley.
• Waterfall retreat: Repeated collapse of the overhang causes the waterfall to move upstream, leaving behind a gorge.

The rock type affects gorge formation. Harder rocks resist erosion, so gorges tend to form where there is alternating hard and soft rock. The softer rock erodes away, allowing the waterfall to retreat and the gorge to lengthen.

Examples in the UK

One of the best-known examples of a waterfall and gorge in the UK is High Force on the River Tees in County Durham. It has a vertical drop of about 21 metres and a deep plunge pool below. High Force's height is typical for waterfalls in upland UK rivers.

At High Force, the river flows over a hard layer of Whin Sill (a type of igneous rock) onto softer limestone beneath. This difference in rock hardness causes the waterfall to form.

The waterfall has retreated upstream over thousands of years, carving out a gorge downstream. The gorge is narrow and steep-sided, typical of upper course river landforms.

Human impact on these features is minimal, as the area is mainly rural and protected. This allows natural processes to continue shaping the landscape.

For example, the plunge pool at High Force is constantly deepened by the swirling action of water and rock fragments, which erode the softer rock below the hard cap.