Channel characteristics such as cross profile, wetted perimeter, hydraulic radius, roughness and efficiency change downstream. Describe and explain how channel characteristics change downstream. The channels characteristics such as the cross profile, wetted perimeter, hydraulic radius, roughness and efficiency, change along the course of the river.
The rivers width is a lot wider downstream than it is upstream and is also a lot deeper which would be shown in the cross profile of the river. The cross profile of a river is a slice through the river this then shows exactly how wide and deep the river is and how rough the river bed is depending on if there is boulders or dips. Upstream it is narrow and shallow and further downstream this begins to change to being wide and deep due to the energy from the water causing lateral and vertical erosion. This then has an immediate effect on the wetted perimeter, length of a river channel in contact with the water at a cross section, as the wider and deeper the river channel the more surface area in contact with the water therefore resulting in a larger wetted perimeter. Further upstream nearer the source of the river the wetted perimeter is likely to be small as there is less surface in contact with the water as it is shallow and narrow whereas in the lower course closer to the mouth of the river the wetted perimeter should be a larger number as the river bed and banks will be larger therefore more surface area in contact with the water. The relationship between these two characteristics is shown through the hydraulic radius, the cross sectional area divided by the wetted perimeter. The hydraulic radius links to the efficiency of a river as the higher the hydraulic radius the more efficiency. Downstream the river is more efficient and has a higher hydraulic radius as, as the hydraulic radius increases so does the rate of both lateral and vertical erosion due to the increase of velocity of the river channel. There is also less friction due to the smaller wetted perimeter helping the increase of velocity, leaving more energy for erosion.
The roughness of the river decreases as it goes down the channel due to the increase in erosion downstream. In the upper course it is rougher due to the rocks not being eroded, unlike downstream where they have been eroded because of there being more water. An increase in roughness will result in a decrease in velocity as the water from the channel has to overcome the obstacles in the way, this will then consequent in a rise in friction leading to a decrease in efficiency.
Upstream will have a higher velocity due to the steepness, due to vertical erosion, of the channel as it will have a considerable amount of kinetic energy from this, this therefore means that the river does not deposit much material in the upper course as there is too much energy for the river to deposit anything. There is more gravitational potential energy upstream than downstream, there is not enough energy in the river to keep transporting the rivers load therefore it deposits it in the lower course which also adds to lateral erosion.