11.5 Waterfalls and gorges

Question 1

Describe the characteristics of a waterfall and gorge

Answer 1

• Found in the upper course
• Vertical drop in the channel
• Resistant rock either side of a narrow channel
• Immediately downstream of a waterfall
• Turbulent fast flowing water
• Boulders on the river

Question 2

What is a waterfall and when is it formed

Answer 2

A sudden drop along the river course. It forms when there are horizontal bands of resistant rocks positioned over exposed, less resistant rock

Question 3

How does a waterfall form

Answer 3

1. The soft rock is eroded quicker than the hard rock and this creates a step
2. As erosion continues, the hard rock is undercut forming an overhang
3. Abrasion and hydraulic action erode the soft rock to create a plunge pool
4. This gets bigger, increasing the size of the overhang until the hard rock is no longer supported and it collapses
5. This process continues and the waterfall retreats upresam
6. A steep-sided valley is left where the waterfall once was. This is called a gorge

Question 4

What are the characteristics of a gorge

Answer 4

Narrow, steep sided valley immediately downstream of a waterfall

Question 5

When does the gorge grow longer

Answer 5

Every time the overhanging cap rock breaks off, the waterfall retreats and the gorge grows longer

Question 6

What is the knickpoint

Answer 6

where there’s a sudden change in the slope of the channel bed, such as a waterfall or a series of rapids