Middle Course of a River - Processes and Features

In the middle course of a river the gradient decreases (it flattens out) and the discharge increases. This is due to the fact that many more tributaries have joined the main channel, leading to an increased volume of water, as well as the fact that the high level of gravitational potential energy which was found in the upper course of the river has been converted to kinetic energy.

This enables the river to erode laterally as well as vertically, thus widening the river channel. The river also starts to deposit load due to variations in energy in the channel cross-section and longitudinally (caused by differences in geology, gradient, land use and weather conditions). River energy fluctuations along its course leads to a dynamic environment whereby the form of the river can change season by season.

Many landforms are formed in the middle course as a direct consequence of the relative energy balance changing, resulting in both erosion and deposition. The most obvious example of this is a meander.

Meanders

Meanders are the characteristic bends in a river that are found all along the length of a river but particularly in the middle course. They can vary in size, but generally have deposits of different sized sediment on the inside of the bend called a slip-off slope or point bar, and a concave section on the opposite bank of the river on the outer bend called a river cliff or bluff.

The initial cause of meandering rivers is related to the way in which different types of flow within a river:

• laminar flow – horizontal movement in a plane.
• turbulent flow – horizontal and vertical eddies in the water sometimes producing mini-whirlpools
• helicoidal flow – corkscrew movement of water which is thought to transport eroded material downstream from one meander bend to the next

These, combined with the natural sinuosity (the movement of a river in an S shape across the landscape) of water leads to different rates of erosion and deposition. It is thought that the frictional drag exerted on the water by the riverbed and banks causes lower layers of water to move more slowly and in a sinuous manner. They interfere with upper layers of water and cause them to move in a more exaggerated pattern of sinuosity. Inevitably, once sinuous flow is established it causes the river to collide with the banks and results in erosion.

The increase in discharge typically found in the middle course leads to an increase in the rate of erosion (both vertical and lateral) and the formation of pools in the channel. The rate of formation of pools increases in times of higher discharge and leads to more efficient movement of water. In addition, erosion of the channel causes there to be an increase in the availability of material being transported. This material is often deposited further along the channel in less efficient sections with a lower velocity, to form riffles. Riffles reduce the efficiency of channel flow still further by increasing the wetted perimeter (the length of channel in contact with water) and consequently increases friction. This reduces the velocity of the water and leads to further deposition. Coarse particles in riffles are more angular and generate greater flow resistance than the fine particles in pools so more energy is expended as water flows over them (the water is shallower). This, in turn, encourages further deposition. Pools have a shallower sediment gradient so less energy is expended by the deeper water flowing over them, further increasing channel efficiency.

Pool and riffle sequences cause changes in the efficiency of a river channel which, in turn, causes changes to the way in which the water moves within the channel. The swing of flow caused by riffles forces the line of greatest velocity (the thalweg) to the outside of the channel where it starts to erode the bank by hydraulic action and abrasion.

It is thought that once meander development is established, helicoidal flow plays an important role. Some believe that faster flow on the outside bend of a meander causes water to ‘pile up’ and so to increased water pressure, while the slower velocity on the inside bend caused by friction with riffles leads to a decrease in water pressure. Since water always moves from high to low pressure a cross-channel flow of water becomes established whereby water (and sediment) is drawn downwards from the outside bend towards the inside bend in order to equalise pressure. Others believe that this cross-channel helicoidal flow is not strong enough to transport sediment as well as water and it is generally agreed that most sediment is transported from the river cliff where it is eroded upstream and deposited on the next slip-off slope downstream rather than directly opposite.

Over time, the sinuosity of the meander increases as pools lengthen until new sets of pools and riffles form in the straight sections between meanders. The whole channel migrates laterally (becomes wider) and becomes more sinuous, acquiring the typical meander shape.

Over time meanders can eventually develop into oxbow lakes. As the meander increases in sinuosity it can create a narrow neck of land between successive loops. This neck of land can be eroded through in times of high discharge (floods, for example) as a river current takes the most direct path, leaving the meander as a cut-off. As the new straighter channel becomes established the original meander resides as an oxbow lake. The lake is further isolated by deposition at the neck and it will slowly become infilled with sediment washed off the surrounding floodplain. The water in the lake will also slowly evaporate and it will become infilled with reeds and marsh vegetation. This can happen in less than 10 years.