Channel cross profiles
The channel cross profile (or section) is a view of the river bed and banks from one side to the other at any one point on it course. As a river flows from its source to its mouth, a number of typical changes take place in the channel morhology.
The channel is narrow and uneven, because of the presence of deposted boulders. Where both banks are being eroded channels tend to be broadly rectangular in shape.
As the river enters its middle course and starts to meander, the channel becomes asymmetrical on the river bends but mainly smooth and symmetrical on the straight stretches.
In the loweer course, the river widens and deppens further, but banks of deposition and eyots (islands of deposition) can disrupt the shape of the channel cross-section, leading to a braided channel. Sometimes embankments called levees can be seen on either side of the channel. Levees can also be man-made.
The Bradshaw model
The Bradshaw model is a theoretical model that helps describe the changes we would expect as a river travels from its source regions in its uppercourse with increasing distance downstream towards its mouth. It is depicted below as a series of triangles, and I have added a couple of extra variables that might be worth investigating in the field. If the triangle gets wider towards the downstream section it means that on average that variable increases with increasing distance downstream. If the triangle decreases in size it means that on average the variable decreases with increasing distance towards the mouth.
The shapes of the channel influences the velocity of the river. In the upper course, where the channel is narrow and uneven due to the presence of large boulders, there is a large wetted perimeter. The wetted perimeter is the total length of the river bed and banks in cross section that are in contact with the water in the channel.
River levels only rise after heavy rain or snowmelt and in the upper course the river is relatively shallow. When there is a large wetted perimeter in relation to the amount of water in the river, there is more friction. Friction results in energy loss and, consequently the velocity of the river is slowed. As channels beocme larger and smoother, in the middle and lower course of the river they tend to be more efficient. The wetted perimeter is proportionately smallers than the volume of water flowing in the channel. Therefore there is less friction to reduce velocity. Although theturbulent flow of mountain streams migh appear faster than that of the genlty meandering downstream channel, average velocity is actually slower. This is because so much energy is expnded overcomig friction on the uneven channel bed in the upper course, whereas in the lower course there is little to disrupt water flow:
Hydraulic radius = cross-sectional area of the channel / wetted perimeter
A high hydraulic radius means that te river is efficient. This is because the moving water lose proportionatley less energy in ovrcoming friction than when the ratio between cross-sectional area and the wetted perimeter is low. Larger channels tend to be more efficient; area increases to a greater degree than wetted perimeter.