Changing Channel Characteristics AS Geography

The channel cross profile (or section) is the view of the river bed and banks from one side to the other side at any one point on its course. As a river flows from its source to its mouth, a number of typical changes takes place in the channel morphology. In the upper course, the channel is narrow and uneven, because of the presence of deposited 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 lower course, the river widens and deepens 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 shape 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 rive, there is more friction. Friction results in energy loss, and consquently, the velocity of the river is slowed. As channels become larger and smoother, in the middle and lower course of the river, they tend to be more efficient. The wetted perimeter is proportionally smaller than the volume of water flowing in the channel. Therefore, there is less friction to reduce velocity. Although the turbulent flow of mountain streams might appear faster than that of the gently meandering downstream channel, average velocity is actually slower. This is because so much energy is expended overcoming friction on the uneven channel bed in the upper course, whereas in the lower course there is little to disrupt water flow.

Channel shape is described by the hydraulic radius. This is calculated using the formula:

hydraulic radius = cross-sectional area of the channel / wetted perimeter

A high hydraulic radius means that the river is efficient. This is because the moving water loses proportionally less energy in overcoming friction than when the ratio between the 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. For example, at bankfull, a channel that is 10m wide and 2m deep has a hyraulic radis of 20/14 = 1.43. A channel that is 20m wide and 4m deep has a hyraulic radius of 80/28 = 2.86.