High Force Reservoir LO: To describe the formation of a waterfall and gorge using High Force as an exemplar

High Force is the tallest waterfall in the UK Waterfalls happen where there are layers of hard and soft rock. In the case of the River Tees, water flows over hard (resistant) rock ie whinstone. The softer, limestone rock beneath is vertically eroded more quickly to leave a small step in the channel. The erosional processes are abrasion, corrosion and hydraulic action. Over time, the small step in the softer limestone rock continues to erode to form a vertical drop between the two layers of Whinstone rock. A waterfall has now been formed. Due to the height that the water now falls, strong currents and high pressure erode the river bed at the base of the waterfall, creating a plunge pool. Material is swirled around and the plunge pool becomes deeper and larger. An example is High Force (21.5 metre drop).

Transportation methods TRACTION SALTATION SUSPENSION SOLUTION Particles of rock are dissolved in the water and carried along without being seen. Large boulders and rocks are ‘rolled’ along the bed of the river. Smaller pebbles and stones are ‘bounced’ along the bed of the river. Smaller particles of silt and sand are held within the water and transported along in the flow of water. Match up the transportation methods with the correct description and draw a diagram to show it.

Erosion processes ABRASION SOLUTION ATTRITION HYDRAULIC EROSION The sheer force of the water removed material from the bed and banks of the river. Material is rubbing against the bed and banks of the river wearing them away. Some rock such as calcium carbonate dissolve in the water if it is slightly acidic. The load itself being carried, bang into each other and break up and become smaller and smaller.

A WATERFALL

How is a waterfall formed? What do you notice about the diagram?

Many waterfalls form when rivers meet a band of softer less resistant rock after flowing over a relatively hard resistant rock. The softer rock is worn away more quickly, and the harder rock undercut. The overhead hard rock forms an overhang, which will eventually collapse, to form a deep plunge pool. This process is repeated causing the waterfall to retreat upstream creating a gorge in its wake.

Model answer Waterfalls occur where there are alternate layers of hard and soft rock in the landscape. In the case of the River Tees, water flows over hard (resistant) rock ie whinstone. The softer, limestone rock beneath is vertically eroded more quickly to leave a small step in the channel. The erosional processes are abrasion, corrosion and hydraulic action. Over time, the small step in the softer limestone rock continues to erode to form a vertical drop between the two layers of Whinstone rock. A waterfall has now been formed. Due to the height that the water now falls, strong currents and high pressure erode the river bed at the base of the waterfall, creating a plunge pool. Material is swirled around and the plunge pool becomes deeper and larger. An example is High Force (71 feet/21.5 metre drop).