A barrier jutting out from the bank of a river and designed to prevent bank erosion. It also keeps water in the centre of the river, and so makes the river more navigable, at times of low flow.

The Rhine in Germany. The groins are the low banks that stick out into the water on the far bank.

Groins are used not just to protect banks, but also to push more water into the centre of a river. This has two effects: it means that the river flows faster and so scours more deeply in the centre. This is good for navigation.

By keeping water in the centre of the channel, the depth of water is also kept higher during drought. This makes a river navigable for longer.