Diagrams showing formation of a waterfall answer
A waterfall is a vertical fall of water usually found along the upper
course of a river. It is formed by the processes of erosion.
Processes.
Hydraulic Action: This is when the power of moving water erodes the
banks and bed of a river.
Corrasion: This is when the load of the river scrapes and gouges the river
bank and bed.
Solution: This is when mild acids such as carbonic acid dissolve rocks
like limestone and chalk very slowly over a long period of time.
Formation:
As a river flows downslope in the upper stage, it may flow over areas of
hard resistant rock followed by an area of softer less resistant rock. Over
time the less resistant rock will be eroded quicker than the harder rock by
the processes of hydraulic action and corrasion. See diagram 1. Over
time, the softer rock is eroded more and a vertical fall of water known as
a waterfall is formed. This is shown on diagram 2.
As the fall of water becomes more vertical and higher, the force of
hydraulic action is increased hugely at the base of the waterfall and this
erodes the base of the water fall to form a deep base known as a plunge
pool. See diagram 3.
As this process continues, water splashes back from the plunge pool
against the back of the waterfall. Solution may then dissolve the base of
the waterfall from this splashing. When this happens, a process called
undercutting takes place as seen in diagram 3.
Eventually, if enough undercutting takes place, the water fall collapses
and the whole cycle begins again. A newer waterfall is now formed
further back upriver. This is known as headward erosion. If this cycle
continues, a valley may form on either side of the waterfall down river of
it. This high sided valley is known as a gorge.
Examples:
Examples of waterfalls in Ireland include the River Lee in Cork near
Dripsey, Torc Waterfall in Killarney Co. Kerry and Powerscourt waterfall
in Co. Wicklow.