The long profile or a river illustrates the changes in altitude of the course of the river from its source, along the entire lengh of its channel, to the river mouth. In general, the long profile is smoothly concave, with the gradient being steeper in the upper course and becoming progressively gentler towards the mouth. Irregularities in the gradient frequently occur and may be represented by rapids, waterfalls or lakes. There may also be marked breaks or changes in slope, known as knick points, which generally are the product og rejuvenation. Rejuvenation occurs either when sea level falls or when the land surface rises. Either situation allows the river to revive its erosion activity in a vertical direction. The river adjust to the new base level, at first in its lowest reaches, and then progressively inland.
The valley cross profile is the view of the valley from one side to another. For example, the valley cross profile of a river in a upland area typically has a V-shape, with steep sides and a narrow bottom. Variations in the cross profile can be described and explained as follows:
- in the upper course - a narrow steep-sided valley where rivers occupies all of the valley floor. This is a result of dominant vertical erosion by the river.
- in the middle course - a wider valley with distinct valley bluffs, and a flat floodplain. This is as a result of lateral erosion, which widens the valley floor.
- in the lower course - a very wide, flat floodplain in which the valley sides are difficult to locate. Here there is lack of erosion, and reduced competence of the river, which results in large-scale deposition.
Over a long period of time a river may display an even and progressive decrease in gradient down the valley, creating the typical smooth concave shape which has adjusted to the discharge and the load of the river. The idea of grade was originally put forward by W.M. Davies, who argued that irregularities in the long profile which would reflect changes in the underlying geology are eventually worn away by river erosion to give a smooth graded profile. This may also be referred to aas the profile of dynamic equilibrium, where a balance has been achieved between the processes of erosion and deposition. More recently, it has been accepted that the channel nay still be graded if it exhibits some irregularities in its long profile.
Potential Energy & Kinetic Energy
Some geographers define the graded river as that which has been attained when a river uses up all of its energy in the movement of water and sediment so that no free energy is left to undertake further erosion. In this situation the gradient at each point along the river is sufficient to discharge the water and load but there is little energy available for further erosion. Such a balance between energy and work cannot occur at a particular moment in time but it is suggested as an average position over a long time. Theroetically, river systems should reach an equilibrium when the inputs and outputs are balanced, but changes in the system bring adjustments to the profile as the river attempts to counter the change. In this way it regulates the system. If the volume and load of the river change over the long term, then the river's channel and its long profile will also adjust to the new conditions.
Potential Energy & Kinetic Energy
In relation to rivers, potential or (stored) energy is fixed by the altitude of the source of the stream in relation to base level.
Kinetic energy, or energy due to movement, is generated by the flow of the river which converts potential energy into moving energy. The amount of kinetic energy is determined by the volume of flowing water (discharge), the slope or channel gradient down which it is flowing and its average velocity. An increase in velocity and/or discharge results in an increase in kinetic energy.
All channel processes are dependent on the amount of energy available. This is a delicate balance. If there is excess after transportation of load the river will erode, but if energy is insufficient to move the load deposition will occur. The river channel adjusts in shape and size to accomadate changes in the volume of water and sediment.