GCSE Geography | Landforms in the Middle Course (River Landscapes 5)

Erosional and depositional processes in the middle course of the river lead to the formation of meanders and oxbow lakes.

How do meanders form?

Water flows at different speeds in the river channel. This means more erosion on one side and more deposition on the other, so flow downstream is affected, and the faster water swings to the other side (known as a helicoidal flow). On the side where the fast water is, there is more erosion - abrasion and hydraulic action. This makes the river deeper and cuts into the bank, to make a river cliff.

As the helicoidal flow 'corkscrews' between bends, it hits the top part of the outer bank, but lower part of the inside bend - but because it is shallow here, friction slows down the flow of the water and causes it to deposit material. This makes a shallower area of sediment called a point bar.

The faster flow on the outside bend causes vertical erosion making the river bed deeper on this side, and the slow flow leading to deposition leads to a gentle slope on the point bar - this is called the slip-off slope. You can see this in the cross-section diagram below.

As erosion continues to eat into the outside bends, and deposition continues to build up the inside bends, the meander loops will increase in size and start to move over the floodplain. Rivers with many meander loops are called sinuous rivers, like the one in the photo below.

How do oxbow lakes form?

As meanders migrate their bends will get bigger through lateral erosion, and the meander neck will get tighter, leaving a narrow strip of land separating the river channel. A really tight meander is called a swan’s neck meander.

When there’s a lot of discharge (in a flood or after a storm) the river has more energy, so it erodes a new channel straight across the neck of the meander - which becomes a shortcut which the river now uses because it’s easier, even when the water level goes down. This new channel then becomes the main channel as lateral and vertical erosion, make it wider and deeper. Because the loop of the old channel is no longer receiving water, it will start to become cut off, and as flooding deposits more material onto the new river banks this makes the old loop become even more detached. The old channel becomes an oxbow lake when it deposition seals its two ends to separate it from the river, and only the far end of the meander will remain. Eventually it gets filled in with debris and soil and might have trees in it.