Describe and explain the formation of an oxbow lake. (6 marks)

Oxbow lakes are most commonly found in the middle course of the river, where the river has more energy and higher discharge. A meander is first formed due to a balance of erosion and deposition. Helicoidal flow within a bend in the river causes the river's velocity to be higher on the outside bend. This encourages lateral erosion, forming a steep cliff face. Slower velocity on the inside bend causes deposition to occur, resulting in a gentle slip off slope. Over time, continued erosion and deposition makes the meander's bend more pronounced. The increased depth of the river as a result of erosion speeds up this process even further due to the decreased levels of friction, which increases velocity and therefore hydraulic erosion. The neck of the meander will gradually narrow, and, in the event of a flood, the heightened discharge will cut off the meander in an attempt to find the fastest route downstream. Deposition will occur between the meander and the channel due to the lack of velocity and an oxbow lake will form. Over time, evaporation and the growth of vegetation over the area will form an oxbow lake scar.