Waterfalls

What?“a cascade of water falling from a height, formed when a river or stream flows over a precipice or steep incline”

Upper courseCaused by erosionVary in size

A waterfall is a steep drop along the course of a river. It has a ledge with water cascading into a characteristic plunge pool at the base. The rocks at the top of the waterfall are often hard and resistant, forming a cap rock, and softer rocks below are undercut. The waterfall may lie within a gorge and are often cause of gorges.Waterfalls often form when a band of resistant rock lies over softer, less resistant rocks. 

How?There is hard rock overlaying soft rock along the river bed.

How?Over time the river bed is eroded by hydraulic action, solution (usually on softer rock such as chalk and limestone) and abrasion.

The soft rock is less resistant to erosion than hard rock and therefore erodes at a faster rate causing a step along the river bed.

How?

Hydraulic action continues to erode the step and creates a notch over which the water flows.

Hydraulic action also erodes the base of the waterfall and creates a plunge pool which grows deeper and extends to behind the water.

The notch protrudes further and the soft rock is undercut from the force of the water.

How?

As the notch is undercut,the hard rock can no longer support itself so collapses and falls into the plunge pool.

How?Rocks and boulders enter the plunge pool because of the collapsed notch. Abrasion and hydraulic action are now working together to erode the plunge pool (it gets deeper) and notch. (These can sometimes cause potholes.)

Splash back from the fall hitting the lunge pool can erode rock from the face behind the waterfall. This contributes to the debris in the plunge pool.

How?Erosion continues and the process repeats it’s self. The plunge pool erodes and the notch is undercut, it will collapse and hence restart the process. This causes the water fall to retreat upstream.

How?

As the waterfall retreats upstream it forms a steep sided gorge which the river runs through.

Case studyVictoria FallsZambzi RiverBorder of Zimbabwe and Zambia

Not the highest or widest. It is twice the height of Niagra Falls and twice the width of Horshoe Falls.THE LARGEST SHEET OF FALLING WATER IN THE WORLD.HEIGHT: 1,708 metres WIDTH: 108 metres

REMEMBER Zs

Case studyVictoria FallsSix main gorges:First Gorge (the one the river falls into at Victoria Falls)Second GorgeThird Gorge (contains Victoria Falls Power Station)Fourth GorgeFifth GorgeSongwe Gorge

Case studyVictoria FallsMade from:

Basalt Sandstone

Case studyVictoria Falls