An actively flowing river may carve a gorge if it flows through a plateau which is made up of layers of resistant rock alternating with lays of less resistant rock.
If the region in which the plateau is found is arid or semi-arid there will be little weathering of the valley sides resulting in a narrow and deep gorge.
When the gorge is large it is sometimes referred to as a canyon for example the Fish River Canyon in Namibia and the Grand Canyon in the United States.
The later was formed in part by the process of river rejuvenation.
Gorges can be formed due to vertical erosion in areas of vertical uplift.
They can also result from the collapse of underground caves in limestone regions.
Vertical erosion into resistant rock can also result into the formation of a gorge as the valley walls on both sides of the river remain intact due to minimal weathering. For example the Lupata gorge was developed when the river incised into resistant rhyolite rock.
A gorge can also result from the upward migration of a waterfall for example the gorges at Victoria Falls.
Vertical erosion on a once buried hard rock layer by an existing stream in cases of superimposed drainage.
Down-cutting of the predator or victor stream in cases of river capture for example the Pungwe Gorge.
Formation of a gorge due to a retreating waterfall. Image credit buzzle.com
The diagram above shows a gorge being formed as a result of a waterfall migrating upstream
Note this is just one of many ways in which a waterfall can be formed.
To learn about more landforms formed as a result of river action click here.