The middle course of a river has more energy and volume then in the upper course. The gradient is more gentle and lateral (sideways) erosion has widened the channel. The river channel has also become deeper. Meanders are typical landforms found in this stage of the river.
Meanders and Oxbow Lakes
A meander is a winding curve or bend in a river. They are typical of the middle and lower course of a river. This is because vertical erosion is replaced by a sideways form of erosion called LATERAL erosion, plus deposition within the floodplain.
The video below shows a meander on High Cup Gill, Cumbria.
The image below shows a series of meanders. Notice the deposition on the inside of the meanders (pale material) and the river cliffs or bluffs (indicated by dark shadows) on the outside of the meanders.
An upland river meander
Again, the image below shows a series of meanders. The river shown in the photograph is swollen due to recent rainfall. Once again we can see deposition on the inside curves of the meander.
Below is a simple summary of the formation of an oxbow lake.
Erosion, transportation and deposition are all processes that create the characteristic features of meanders shown in the images above. There are several stages involved in the creation of meanders and oxbow lakes. These are discussed below.
In low flow conditions straight river channels have bars of sediment on their beds. Flowing water weavers around these bars of sediment. This creates deeper pathways where most of the water flows called pools and shallow areas where less water flows called riffles. This causes the river flow to swing from side to side.
The aerial photograph below shows bars of sediment exposed due to the low level of water in the channel. Notice how the flow of water weaves around the sediment bars.
Where the river swings towards the bank lateral (sidewayes) erosion causes undercutting. On the opposite side of the channel where the velocity (speed of the flow of water) is lower material is deposited. Therefore the river does not get any wider. The image below shows evidence of undercutting on the outer bank and deposition on the inner bank of the meander.
Continued erosion along the outer bank, as the result of hydraulic action and abrasion, creates a river cliff or bluff. A point bar forms on the inner bank. This is a gently sloping deposit of sand, gravel and pebbles. The image below shows a point bar.
Material deposited on the point bar on the inside of the meander
Meanders are perpetuated through a process called helicoidal flow. As the surface flow of water hits the outer bank it corkscrews, flows along the river bed then deposits eroded material on the inner bank.
Eventually the neck of the meander will be breached by the river creating an ox-bow lake. The aerial photograph below shows a meander in the River Derwent, North Yorkshire which will soon be breached.
The photograph below shows how close the nexk of the meander is to being breached. Once this occurs an ox-bow lake will form.
Meander on the River Derwent
Formation of an oxbow lake