River profiles

Rivers can be described as having two distinct profiles: long profile and cross-valley profile. As geographers, we need to be able to describe and explain the form and structure of both.

Long profile

This is the change in altitude and gradient from the source to the mouth of a river.

Essentially, a river erodes its way down through the landscape (vertical erosion) until it reaches base level (usually the sea but sometimes an inland lake). The long profile is often portrayed as a smooth concave shape although in reality there are sometimes sharp drops in gradient due to differential geology or rejuvenation (how a river responds to changes in base level caused by sea level change). The profile may exhibit a stepped profile where there are breaks in slope where waterfalls may be present.

It is thought that in the long-term, rivers seek to establish an equilibrium between energy, discharge and channel processes to produce a graded profile. Some define the graded river as one in which inputs and outputs are balanced in the long term. At any given point along a river’s profile, the slope angle may be sufficient to discharge water and load but not to cause additional erosion to occur. In this way, the river is in equilibrium.

However, a river will respond to changes in energy available – perhaps through an increase in discharge caused by higher seasonal precipitation – and the amount of erosion and deposition which occurs as a result will be adjusted to the new input conditions

Cross-valley profile

The cross-valley profile is the cross-section through the river valley at any given point along the long profile.

It includes the top of one side of the valley, across the valley floor which contains the river, up to the top of the other side of the valley. It changes with distance downstream from a steep-sided, narrow valley near the source, to a wider, deeper valley towards the mouth.

The explanation for this change in profile requires an understanding of the role of potential and kinetic energy.

At the source, the river is at its maximum height above sea level and therefore has the maximum amount of gravitational potential energy. This gives the river the impetus to erode vertically down towards base level and is what creates the narrow valley typically found in the upper course. As distance from the source increases kinetic energy is generated by the movement of the water, enabling the river to erode further. The amount of kinetic energy generated by the river is determined by the discharge, slope angle and velocity of the water.

Further along the long profile, the cross-valley profile becomes widened and deepened by a combination of lateral and vertical erosion. Rivers in the middle and lower course are characterised by an increase in discharge and this gives the water the (kinetic) energy to erode the riverbed and banks further. There is an excess of energy available due to more efficient usage (channel bed roughness is reduced so less energy is absorbed overcoming friction) and this is used by the river to erode laterally, producing a much wider valley bottom with side slopes which are well separated. In addition, with the decrease in gradient associated with the lower reaches of a river comes an increase in deposition, and this leads to the formation of wide floodplains and levees either side of the channel

Further influences on cross-valley profiles

The precise shape of a river valley is not just a function of energy and erosion. There are other factors which determine the shape and angle of the slopes which make up the valley sides.

Vegetation

The roots of plants bind the soil together and reduce the chances of mass movement. Similarly, a canopy layer provided by shrubs or trees protects the bedrock or soil layer from hydrological processes such as infiltration which may cause accelerated rates of sub-aerial processes (weathering and mass movement).

The removal of vegetation (possibly through deforestation) removes the stabilising root systems and protective canopy layer and leaves valley sides vulnerable to weathering and mass movement.

Climate

Seasonal changes in weather affect the extent to which the valley sides are subject to sub-aerial processes. Cool, wet winters in temperate climates can lead to an increase in the rate of slope movement through the penetration of precipitation into joints and bedding planes. Valley sides in upland areas can also experience higher rates of mechanical weathering, particularly freeze-thaw weathering (frost shattering) and frost heave with the larger diurnal (day to night) temperature ranges experienced at higher altitudes.

Warm, dry summers can also have a profound effect on rates of sub-aerial processes. The rate of chemical weathering (particularly carbonation and solution) increases in warmer temperatures, as does plant growth. This can result in higher rates of biological weathering as plant roots grow into cracks and joints in rock strata, causing the breakdown of rock either through mechanical processes or through chelation (the release of organic acids as plants decompose).

Geology

Rock type can affect the angle of the valley sides. Hard, impermeable rocks like granite are less likely to be undercut by the river at the base or broken up by weathering and tend to form near-vertical valley sides. Conversely, softer, permeable rocks like sandstone form gently-sloping valley sides since they are more vulnerable to weathering.

Rock structure is arguably a more significant factor in explaining slope profile. Physically strong rocks like granite and carboniferous limestone are prone to mass movement because of their vertical jointing. The joints act as a line of weakness from which a rock can pull away since it determines how much water can seep into the rock. Well saturated rocks are prone to slip.

Aspect

Adret slopes (slopes facing the sun: south-facing slopes in the northern hemisphere) receive higher rates of insolation (energy from the sun) and so are characterised by accelerated rates of chemical weathering and exfoliation (expanding and contracting) due to a larger diurnal temperature range.

Ubac slopes (slopes facing away from the sun: north facing in the northern hemisphere) tend to receive less sunlight and experience lower average temperatures (particularly at high latitudes) and so are prone to higher rates of frost shattering and mass movement.

Geological history

Some cross-valley profiles can be explained entirely by an event or series of events which occurred in the geological past. For example, the U-shaped valleys in the Lake District were deepened during the Pleistocene glaciation which ended approximately 11,500 years ago. They have been subject to fluvial and sub-aerial processes since then, but the shape of the valley is explained by glacial erosion rather than post-glacial processes.

Similarly, if base level falls as a result of sea level change, a river can become rejuvenated. This will cause a river to experience a renewal in gravitational potential energy which leads it to start eroding vertically once again (assuming a graded profile). This can lead to the formation of landforms such as knick point waterfalls and incised meanders.