The river downstream of the dam will have less sediment, so will flow faster. This means that erosion rates could increase. Yarm: Like many of the towns along the course of the river, humans have introduced flood prevention schemes. Concrete walls along the river bank were build to get the water through the town quicker. This can increase erosion levels. To counteract this gabions baskets with stones in were introduced to reduce erosion.
Embankments were raised so the river can hold more water. Further downstream, flooding is more likely. Teesside: The river Tees enters the North Sea through its estuary. In this area the river has been greatly modified by humans. A large meander the Mandale Loop was cut off, shortening the river by 4km, meaning the river flows faster to the sea.
The river has been dredged to keep it clear of sediment and to ensure large ships can access the heavy industry and oil refineries in the area. This means that the river will flow faster. The dredged material was also used as part of the land reclamation. This is to prevent flooding, particularly at high tide and in times of storms. These more resistant rocks become the capstones to waterfalls.
The number and thickness of these stronger rock units in a vertical sequence of rocks controls how many water falls there are and how much vertical drop there is on each waterfall. The diagram above shows the formation of a waterfall. Soft rock is undercut 1. This leaves a layer of hard rock which overhangs the layer of soft rock 2. The water flows over the overhang and creates a plunge pool in the soft rock below 3. Eventually the overhang will collapse due to the erosion of the soft rock beneath it.
The waterfall then retreats up stream 4. This creates a steep, gorge-like valley. Along its way there are many tributaries. They join the river at a confluence and so the river gets wider and deeper downstream. Changes in the river downstream are also a result of changes in the processes at work — erosion, transport and deposition. The upper course of the River Tees In the upper course the river flows over hard, impermeable rocks.
The valley has steep sides forming a V-shape. The river channel is shallow and rocky and the river is turbulent and clear. In the upper course there is the famous High Force waterfall Figure 2 and gorge as well as rapids and potholes. Erosion downwards is the main work of the river in the upper course.
At High Force, the waterfall and its gorge of recession have been formed over millions of years. On Figure 2 notice the high head of water, the deep plunge pool at the base and how the rock type changes.
At the top of the waterfall there is a very hard rock called Whinsill, which is the cap rock. Below this lie softer rocks such as limestone, sandstone and shale. At the edge of the plunge pool are large boulders and rocks. At High Force the hard Whinsill lies on top of softer limestone and sandstone.
The river flows over the harder rocks and erodes the softer rocks below more quickly. Over a long time the softer rock is undercut and the hard cap rock overhangs. Eventually the hard rock collapses under the force of gravity. The great power of the water erodes the plunge pool.
The rocks resulting from the collapse of the cap rock and from the erosion of the plunge pool are moved to the edge.
As the cap rock collapses the waterfall retreats upstream. This is repeated again and again causing a steep- sided gorge of recession to be formed. Gradually chips of rock are also broken off the top of the waterfall and the waterfall reduces in height. Here small pebbles have become trapped in hollows in the rock. The pebbles grind and scrape at the bedrock, eroding the hollow by corrasion. At Low Force there are also rapids formed by smaller outcrops of Whinsill.
The less resistant limestone has been eroded away leaving the rapids. The middle course of the River Tees As the River Tees flows downstream the gradient becomes less steep. The river begins to erode sideways lateral erosion rather than downwards and the river begins to deposit sand and gravel. The lateral erosion means the river gets wider, the river valley gets wider and meanders begin to form.
Meanders A meander is a bend in the river Figure 4. On the outside bend of the river the water is deeper and flows more quickly. The force of the water hydraulic power and corrasion erode the outside bend to form a river cliff. On the inside bend the water is shallower and flows more slowly.
Here there is deposition of sand and gravel forming a slip-off slope. High Force waterfall on the River Tees. Related Topics Use the images below to explore related GeoTopics. Landforms in the lower course of a river. Topic Home. River basins. Please Support Internet Geography If you've found the resources on this site useful please consider making a secure donation via PayPal to support the development of the site. Top Posts and Pages Home. Log In.
0コメント