The Long Profile

The long profile of a river illustrates the changes in altitude of the course of the river from its source, along the entire length 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. This is known as a graded profile and reflects the fact that water does not have as far to fall as it nears the sea and so has less erosive power (potential energy).

It is an idealised profile but shows how vertical erosion dominates in the initial stages of the river’s course as the downcutting tendency is as yet relatively unaffected by lateral erosion owing to the limited volume of water available. As more water joins the channel from surface flow, throughflow, groundwater flow and tributaries, the river has greater volume and increased velocity, giving it more power to erode laterally.

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, these are known an knickpoints, which are generally the product of rejuvenation. Rejuvenation occurs either when the sea level (in relation to the land) falls or when the land surface rises. Either situation allows the river to revive its erosion activity in a vertical direction. The river adjusts to the new base level, at first in its lowest reaches, and then progressively inland.

Isostatic change in sea level refers to the land rising relative to the sea as a result of crustal movement e.g. in Scotland, the land has ‘bounced back’ after the weight of the ice cap was removed at the end of the last ice age around 10,000 years ago.

Change in the relative height of the land and the sea may also result from a rise or fall in sea level, this is called eustatic change. Sea level may result from many causes, one of the most common being the growth of ice caps during a glaciation. As water is locked up in the ice, less is available to be released into the oceans. The effect of a relative rise of the land means that water has further to fall and thus more erosive power generated from potential energy. Therefore, existing valley floors may be cut out as the river attempts to regrade itself in keeping with the new energy levels of the river. This begins in the channel nearest the sea and then migrates back upstream, the current limit of the regarding being marked by a knickpoint, often marked by a sudden break of slope or waterfall.