The upper course
Different river processes lead to different landforms.
In areas where vertical erosion is dominant – waterfalls and gorges are commonly found
In areas where lateral erosion and deposition become more important – meanders and oxbow lakes develop
In areas where deposition is the most significant process – floodplains and levees become a key aspect of the landscape.
Waterfalls
Picture
Formation of a waterfall:
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.
Lots of useful weblinks
Video on how waterfalls are formed –
http://www.bbc.co.uk/learningzone/clips/waterfalls-geology-and-formation/4314.html
Video on waterfalls, plunge pool and potholes –
http://www.bbc.co.uk/learningzone/clips/waterfalls-plunge-pools-and-potholes/404.html
How are gorges formed?
A gorge is a steep-sided valley in the upper course of the river. It is usually v-shaped.
http://www.cleo.net.uk/resources/displayframe.php?src=307/consultants_resources%2F_files%2Fgorge.swf
Video on the formation of gorges –
http://www.bbc.co.uk/learningzone/clips/waterfalls-and-gorges-erosion-and-deposition/3239.html