Friday, November 10, 2006

Upper Course of the River: Waterfalls

An other feature found in the upper course of a river, where vertical erosion is dominant, is a waterfall. The highest waterfall in the world is the Angel Falls in Venezuela (see picture right) which have a drop of 979m. Other particularly famous examples include Niagara Falls (North America), the Victoria Falls (on the Zambia / Zimbabwe border) and the Iguazu Falls (South America).

Although much smaller in scale, there are many waterfalls in the upper course of UK rivers (e.g. Thornton Falls, Yorkshire - above), but how do they form?

The formation of Waterfalls

1.Waterfalls are found in the upper course of a river. They usually occur where a band of hard rock lies next to soft rock. They may often start as rapids.

2. As the river passes over the hard rock, the soft rock below is eroded (worn away) more quickly than the hard rock leaving the hard rock elevated above the stream bed below.

3. The 'step' in the river bed continues to develop as the river flows over the hard rock step (Cap Rock) as a vertical drop.

4. The drop gets steeper as the river erodes the soft rock beneath by processes such as abrasion and hydraulic action. A plunge pool forms at the base of the waterfall.

5. This erosion gradually undercuts the hard rock and the plunge pool gets bigger due to further hydraulic action and abrasion.Eventually the hard cap rock is unsupported and collapses. The rocks that fall into the plunge pool will continue to enlarge it by abrasion as they are swirled around. A steep sided valley known as a gorge is left behind and as the process continues the waterfall gradually retreats upstream.

Visualising Waterfall Formation:

The labelled diagram of a cross section through a waterfall below, shows the formation process (click on diagram for a larger version).
There are also number of excellent animations on the internet which can help you visualise how a waterfall forms. Try out the following:

1. A good step by step animation of waterfall formation showing all the main stages involved (Wycombe High School)
2. This simple but excellent animation showing an aerial view of waterfall formation clearly shows the development of a gorge as the waterfall retreats upstream! (Cleonet)
3. An animation of waterfall formation from a different 3-dimensional perspective. Look carefully at how the plunge pool is enlarged during the formation process. (Classzone)

Key Term Check:

  • Cap Rock - layer of hard resistant rock forming the 'step' over which the 'falls' occur in a waterfall.

  • Waterfall - a cascade of water over a hard rock step in the upper course of a river

  • Plunge Pool - a deep pool beneath

  • Gorge - a steep sided valley left behind as a waterfall retreats upstream

  • Abrasion - where rocks and boulders scrape away at the river bed and banks

  • Hydraulic Action - where the force of water compresses air in cracks resulting in mini-explosions as the increased pressure in the cracks is then released.

No comments: