The Formation Of Meanders • • • • • • • • Meanders are curves found in the middle and lower course of a river caused by erosion and deposition and the change in speed of water causes the meanders. They form when alternating pools (deep areas of water) and riffles (shallow water) develop at equal spaces along a river. The river channel are deeper in pools so they are more efficient so has greater energy and erosive power. Energy is lost as the river travels over a riffle as there’s friction. The distance between the riffles and pools mean the rivers flow is uneven and maximum flow is concentrated on one side of the river. Turbulence increases in and around pools as the water speeds up so the water begins to twist and coil. This can cause a corkscrew like current in the water which spiral from bank to bank between pools. The flow causes more erosion and deepening of the pools and the eroded material is then deposited on the inside of the next bend and the river will lose energy. Combination of the erosion and deposition helps expand the size of the meander. Eventually this can lead to the formation of an oxbow lake. River flow movement Pool Deposition on inside Riffle Erosion on outside The meanders are found in the lower part of the course as all rivers flow down a slope towards the sea and will take the easiest route available. However, the geology of the land means a river will rarely flow in a straight path so this is why it will meander. The rates of deposition and lateral erosion are highest at this part of the river and also contribute to meanders occurring due to little potential energy but lots of kinetic energy allowing it to flow faster forming a meander. Inside bend Lateral erosion is dominant As the meanders have migrated downstream, meander size has increased Slip off slope created on inside bend Inside bend has shallow water and slower flow Outside bend (river cliff can be created) Corkscrew like currents Lateral erosion by abrasion and hydraulic action forms river cliff Deep water and faster flow on outside bend The formation of an Oxbow Lake On the inside of the loop, the river travels more slowly leading to deposition of silt, while water on the outside edges tends to flow faster, eroding the outside banks. Over time the loop of the meander widens until the neck vanishes altogether as the gap has been narrowed by erosion. •In the lower course of the river meanders can become so pronounced that they can form ox-bow lakes. •In the lower course the lateral erosion cuts into the neck of the meander, narrowing it considerably. •Eventually the force of the river breaks through the neck, and as this is the easiest way for the water to go, the old meander is left without any significant amount of water flowing through it. •Quickly the river deposits material along the side of its new course, which completely block off the old meander, creating an ox-bow lake. Eventually the bend becomes isolated from the river's path and an ox-bow lake is formed. The oxbow lakes occur in the lower course as this is where meanders occur and the river is able to take a quicker route by discarding the meanders curve. They are more dominant in the lower course than the middle as meanders are larger so there is more reason to reduce the amount of energy used by finding a quicker route.