The features formed in the upper course of the river are due to erosion. The first one is a V-shaped valley, which is steep sided, and narrow. As the river erodes downwards, soil and loose rock on the sides of the valley are moved downhill by slope wash. The river winds its way around interlocking spurs of hard rock. This is another feature.
A bit further down, a waterfall usually occurs. It is a steep drop. It has a high head of water and a plunge pool at the bottom. The waterfall is formed by the undercut of softer rock and the harder more resistant rock on top cannot support the weight of the water so it collapses. Gradually moving backwards. This forms a gorge of recession.
The last feature is the rapids. These are formed because there are layers of hard and soft rock going vertically down into the ground. The sharp boulders cut into the softer rock making the ground uneven and therefore the water flows more rapidly over the bumps.
As the river flows downstream from the upper course to the middle course, the gradient over which it flows becomes less steep and the river is not as high above as its base level. The river also becomes wider and deeper. The river continues to erode vertically, but lateral or sideways erosion becomes more important.
The middle course is where the most transportation takes place. There are again 4 types of transportation. Traction, which is when large boulders, simply roll along the riverbed. Saltation, which is when smaller pebbles, are bounced along the bed, picked up and dropped as river flow changes. Suspension, which is when the finer sand and silt particles are carried along suspended in the flow of water. The last one is solution, which is when calcium carbonate materials dissolve in water and can be carried along in the river flow.
The transportation takes place in the lower course as well just like erosion does. Most of the transported material gets deposited in the lower course.
There aren’t as many features in the middle course as there are in the lower course but there are some.
When the river emerges from its upland area the river begins to meander. A meander is simply a bend in the rivers course. On the outside of a meander the water is deeper and the current flows faster. The force of the water erodes and undercuts the outside bend by corrasion, forming a steep bank called a river cliff, which is another feature. On the inside bend the water is less forceful and the current is less strong which encourages deposition. Sand and small pebbles are deposited creating a gentle slip-off-slope.
The reason the valley becomes wider is because lateral erosion occurs on the outer bend of the meander, widening the valley floor and eroding away at the ends of the interlocking spurs. A flatter valley floor is created. Meanders occur further down in the lower course as well.
Meanwhile all the pebbles that were jagged and large near the source they are now rounder, smoother and smaller, and are constantly being eroded.
The lower course of the river is the final stage where the river widens up as far as it can go and joins the sea. This part is where it is a whole lot wider, deeper, and the channel gradient is nearly flat, compared to when it was steep at the beginning. The velocity of the water is much quicker because there is less friction. Most of the erosion has been done. But it still occurs. The flow of current is travelling so much faster than the above to stages.
Here is where most of the features are formed. The first one is connected with meanders. When the outer bend is eroded and the inner bend is deposited on an ox-bow lake is formed. The old meander is cut off and there is a straight channel. When the ox-bow lake dries up it is called a meander scar.
Close to the estuary, which is where the river meets the sea, flood plains can often occur. A flood plain is a very flat area of land that lies next to the river. When floods occur the water spills over its banks and floods the plain. This land is very good for farming because there are lots of nutrients in the soil.
One of the final features is where all the transported material gets deposited. This is in the mouth and it is called a delta. As more and more material gets deposited, a flat area of silt starts to build up. Sometimes in a delta the rain creates a small lagoon in the middle of it.
Overall as a river travels downstream, the width increases, the depth increases, the channel gradient decreases, the valley sides obviously decrease and the pebbles, rocks and boulders decrease in size, become smoother and rounder.
