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Rivers: Key concepts

Erosion, the Hydrological Cycle, the Drainage Basin - learn about the key concepts related to rivers with our concise summaries here.

The Hydrological cycle and the drainage basin

The hydrological cycle is concerned with the water on the land, above and below ground as well as moisture in the atmosphere. Every part of the earth’s land is in a drainage basin. Therefore every part of the earth’s land is part of the hydrological cycle. Water may be held as groundwater, locked up in ice, stored in oceans or lakes or be in transit between these stores in one of many forms of precipitation.

A drainage basin is simply the area of land drained by a river (and its tributaries). Each drainage basin is separated from another one by a watershed, water either travels one side or the other of this dividing “line”. The source of a river, where it begins, is usually very close to the watershed but not always. Sometimes the source may be an underground spring which comes to the surface anywhere within a drainage basin. In a cold environment the source of the river could be meltwater from a glacier or from snowmelt. As the water flows into a channel, the individual channels join up at a confluence. These are called as tributaries. As a river flows from the upper stage, through the middle to the lower stage the size of the main channel increases as does the volume of water. The velocity of the river increases downstream as a river nears its mouth.

The discharge of a river is the amount of water in cumecs (cubic metres per second) at a certain point, for example its mouth. Discharge may be measured at gauging stations. These play an important part in monitoring the flow of water through a channel which can help forecast flooding. There are a number of factors that affect the discharge of a river which include physical and human ones.

As the amount of precipitation is the most influential factor affecting discharge (see Physical below), and therefore the most important physical contributor to flooding, it is plotted on a graph called a hydrograph. A hydrograph (sometimes referred to a storm hydrograph) shows time (x-axis) against precipitation (y-axis) and discharge (z-axis). A hydrograph is an important tool in managing and lessening the impact of flooding as it enables scientists to view the lag time (the time between peak rainfall and peak discharge). The increase in the discharge shown on the hydrograph is called the rising limb and the decrease is shown as the falling limb. You need to be able to label and interpret a hydrograph or storm hydrograph. The difference between the two is that a storm hydrograph is over a much shorter period of time when there is a greater risk of flooding.


Physical factors affecting the hydrological cycle include the size of the drainage basin itself. This a determining factor of the amount of water that will eventually end up in the main channel. However, in a dry area a large drainage basin can have minimal effect on discharge if there is little or no precipitation (for example rain, snow or fog). The most common form of precipitation is rainfall and this is usually directly linked to discharge but not always. For example, if a drainage basin has a large amount of permeable rock (water can flow through it) and/or vegetation cover then the water can flow underground (as groundwater by a process called throughflow) or be intercepted respectively. Where a drainage basin has more bare rock then water will reach the channel more quickly than one that has mostly soil which will hold the water, or at least slow its flow down against the force of gravity. Water flowing over the surface in a drainage basin is called runoff, water that flows through the soil is called baseflow. It is important to note that both runoff and baseflow contribute to the water in a river channel. If you look at the bank of a river you can witness baseflow seeping out through the soil.

Urbanisation at any scale, usually involving covering natural surfaces with concrete/tarmac or other impermeable surfaces, will enable the water to enter the channel more directly unless there are holding areas where water is temporarily stored (not very common). This is an example of a human factor affecting the drainage basin. More likely is that humans will build a storm channel of some type. These may help divert water away from one area but the route to the main channel is more direct and the lag time is reduced so flooding risk may increase further downstream.

With an ever increasing demand for housing, the land which is covered in impermeable surfaces rises causing increased risk of flooding.

Erosion

Erosion is the removal or material by something such as water. In a river’s profile this is divided up into two main types which are:

o Vertical erosion - this is (usually) greatest nearer to the source of the river and is where the water erodes down into the bedrock.

o Lateral erosion – when the water in the channel erodes the land from side to side rather than down. This occurs in the middle and low stages of the river.

Types of erosion:

o Hydraulic action – this is where material is removed by the sheer force of the water. Material is “washed away”. An example of this may be where water flows against a crack in a river’s bank and compresses air into it causing pressure and further weakening of the immediate area. The crack will grow and the section of the river’s bank will fall into the channel. This is most common where the water is flowing at high velocity. Material can of course be removed from the river’s bed in this way too.

o Corrosion – when water comes into contact with rock, depending on the type, it may dissolve the rock. This is a form of chemical erosion and is obviously relatively slow. An example of this may be if the bed is made of chalk or limestone which is easily dissolved.

o Corrasion – when eroded material is carried along by the force of the flow of the river it may come into contact with the bank or bed and dislodge more material. This is sometimes referred to as “sandpapering”. This occurs when there is a lot of material in the river ( load) and is increased during peak flow or turbulent flow.

o Attrition – this is often confused with corrasion as it is similar. The difference is that material is eroded by bumping and knocking into itself rather than the back of bed.

Transportation

Transportation is the process of moving material from one place to another and a river does this in the following ways:

o Traction – When material rolls along the river’s bed it is said to be transported by traction. When this occurs it causes corrasion and attrition. This tends to be larger particles of material or even boulders. In times of peak flow following a heavy storm, a river can move enormous rocks by this method.

o Saltation – As water flows along the river channel it may have enough energy to lift material off the bed but not enough to keep it off the bottom for more than a second or so. This is known as saltation. A particle falling back down to the bed may dislodge another one contributing to the process. This usually affect smaller pebbles and stones.

o Suspension – smaller particles may be lifted off the river bed by the flow of water and be light enough to stay off the bottom. They are “suspended” in the water and so are said to be transported by suspension.

o Solution – when material is corroded it is dissolved into the water and carried along in a solution. To further your understanding of this idea, the only way that this type of load of a river can be measured is by collecting samples of water and boiling it to leave behind residue which is the precipitated load (this term is not to be confused with rainfall!).

o Hjulstrom Curve – the Hjulstrom Curve is a graph that shows a river’s ability to transport material in relation to its velocity and particle size. It shows the critical erosion velocity and the mean settling curve. These are the minimum velocity that is needed to transport and erode a particle and the velocity at which particles are dropped or settle onto the river bed respectively.

Deposition

Deposition – is not a series of processes but something that occurs when a river loses the energy to carry a particle and ceases to transport it. The particle falls to the river’s bed and is deposited. Material may also be deposited on the banks of a river. This normally happens when a river loses its energy which could be due to lack of flow (no precipitation) or water being removed by humans. When a river nears sea level it will flatten out and slow down, this may cause a river to deposit its load.

Features

The features of a river’s processes can usually be categorised by where along a river’s profile they most commonly occur. In the upper course of a river there are different features to the middle and lower courses which are related to the factors covered so far.

Upper Course:

o V-shaped valleys – these are as they sound! They have steep sides, often with bare rock or very thin soil. The channel shape is shallow and narrow, often with large boulders in it. Bedload tends to be angular and irregular in size. Interlocking spurs are associated with this stage of the river. These are valley sides that protrude from the side and the river wends its way around them. As you look upstream they appear to interlock with each other. The gradient in this stage is much steeper than the middle and lower stages.

o Waterfalls – Fast flowing water may encounter softer rock on its way down the upper course of a river. The water will erode the softer rock away more quickly than the resistant rock above and so a step will form. Eventually this will continue to grow and the resistant rock will overhang the soft rock. As the water flows over the resistant protrusion it will create a plunge pool and may also undercut the resistant rock. Eventually the resistant rock will overhang too much to support itself and collapse into the plunge pool. The process continues again and the waterfall retreats upstream. As it moves upstream a gorge is formed.

o Rapids – Along the course of a river there may be a sudden change in slope that causes the water to flow more quickly (or rapidly!) and this creates turbulence. As turbulence increases so does its erosive power.

o Potholes – if water has to move around an obstacle such as a protrusion of hard rock then a small current may be the result. This can cause uneven erosion on a particular spot. One or more small stones may become trapped in the depression that has formed and they swirl around in the current. As they do this they erode the depression even further. Eventually a pothole may form. When a river’s level decreases potholes may typically have a number of varying size pebbles in them.

Middle course

o Wider, shallower valleys – a decrease in the angle of the river’s course means that the river has time to erode laterally as well as vertically; the river’s gradient is shallower. This widens out the valley floor creating a wider cross-sectional profile with more gently sloping valley sides. The channel shape increases in width and depth and the river’s discharge is higher than the upper stage. The river may flood in the middle section and as a result of this there may be flood-plains and levees.

o Meanders – differences in the rivers bed may cause current irregularities which may in turn lead the river to erode parts of its bank but not others. Over time this can lead to the formation of a bend. The irregularities in the river are called riffles and pools. These are a sequence of shallow and deeper parts of the channel, caused by changing geology or areas of deposited gravel and sand. Eventually the bends in the river will increase and they will bend to such an extent that they resemble a horse show. These bends are now meanders. The flow of water on the outside of the bend is faster and therefore more erosive; continued erosion will undercut the bank which will eventually collapse forming a river-cliff. Conversely, on the inside of the bend the river has less energy and deposits material to form a slip-off slope.

o Ox-bow lakes – when the meanders become so pronounced they will be very close together and may eventually cut through to form a straight channel and leave a loop. The cut through might occur during a flood when discharge and energy is high. The cut off meander is called an ox-bow lake.

Lower course

o Wide flat bottomed valleys – often several kilometres in width, the valley floor in the low stage of a river is flat with very gently sloping sides and a very gradual downstream gradient. The river channel is wider than the middle stage even if just by a few metres.

o Floodplains – when the river reaches bankful level it will overflow its banks and flood. The water from the channel will spread out over the flood plain which is the large flat area either side of the channel. As soon as the water hits the flood plain it loses a lot of its energy and it will deposit material close to the main channel. The heavier material will be deposited first, this may form a levee (a raised section on and just beyond the river bank) with some of the smaller particles being carried further from the channel in suspension. Flood after flood brings layer after layer of this material and beyond the levees the land is fertile and very good for agriculture. As it is flat it is also good for building on, which can increase the impact of flooding.

o Deltas – hitting a large body of water such as the sea of a lake means that the river will lose its energy almost immediately and deposit its load. This will effectively pile up at the river’s mouth. It may collect to such an extent that it forms a feature known as a delta. Deltas vary in size from small ones a few metres across to vast ones such as the Nile Delta which covers 240 km of the Mediterranean coast and is 160 km long.