The banks are an important part of the life of the river. They provide breeding sites for mammals and birds that feed in or near water, and a habitat for adult insects, such as dragonflies, emerging from the river. The basic composition of the bank may be layers of mud, shingle, sand or boulders. The water erodes the banks as it flows along and they can become very unstable unless there is plenty of vegetation to hold them together. Trees, such as willow and alder, stabilize muddy and sandy banks with their network of roots. Reeds and grasses are good stabilizers too.