An overhang of hard, resistant rock forms as the spray of the falling water erodes softer, less resistant rock found on the back wall of the waterfall (solution). Repeated undercutting of the overhang eventually causes it to collapse into the plunge pool under the force of its own weight (gravity). The energy of the material falling from the overhang causes further erosion of the plunge pool (abrasion) (Fig. 8.14).
Repeated undercutting and collapse results in the waterfall retreating upstream in a process called headward erosion. The volume of water in the river channel is reduced as the waterfall retreats upstream, reducing the rate of erosion. Below the waterfall, a narrow valley with steep sides called a gorge remains as evidence of the retreating waterfall. A gorge 11 km in length has been created as Niagara Falls on the Canada/USA border has retreated.
Waterfall retreats upstream
Earlier position of falls
Resistant rock
Position of waterfall after retreat
resistant rock
Less
(easily eroded)
Undercutting
Fallen rock Plunge pool
Fig. 8.14(a) Formation of a waterfall.
Original position of waterfall
Fig. 8.14(b) Formation of a waterfall: retreat.
Fig. 8.14(c) Angel Falls on the Carrao River in Venezuela. At 979 m, this is the highest waterfall in the world.
Fig. 8.14(d) Waterfalls will be labelled on an OS map using the word ‘Waterfall’ (or ‘Eas’ if the map is from a Gaeltacht area).
