In the upper course of a river, water flows quickly through a narrow channel with a steep gradient; as it does so it cuts downwards. This vertical erosion results in a number of distinctive landforms including the steep sloping v-shaped valley through which the river flows in its upper course.
So how does a v-shaped valley form?
1. Vertical erosion (in the form of abrasion, hydraulic action and solution) in the river channel results in the formation of a steep sided valley
2. Over time the sides of this valley are weakened by weathering processes and continued vertical erosion at the base of the valley
3. Gradually mass movement of materials occurs down the valley sides, gradually creating the distinctive v-shape.
4. This material is then gradually transported away by the river when there is enough energy to do so.
As the river flows through the valley it is forced to swing from side to side around more resistant rock outcrops (spurs). As there is little energy for lateral erosion, the river continues to cut down vertically flowing between spurs of higher land creating interlocking spurs.
Key Term Check
- V-shaped Valley- a valley which resembles a 'v' in cross section. These valleys have steep sloping sides and narrow bottoms.
- Interlocking Spur - spurs are ridges of more resistant rock around which a river is forced to wind as it passes downstream in the upper course. Interlocking spurs form where the river is forced to swing from side to side around these more resistant ridges.
- Load - collective term for the material carried by a river
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 below) 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 Tanzanian rivers, 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.
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.
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