Found often by rivers. Bark is dark grey and fissured, while the young branches are smooth and slightly green in colour. The twigs are sometimes sticky to the touch. Leaves are oval, almost circular, with shallow irregular teeth. Has catkins are that are dark yellow-brown and stubby. Flowers are found on the same tree, much smaller than catkins, red and cone-like. The seeds have small wings.
The wood becomes hard like stone when wet, so used for the leg posts of crannogs (and indeed, most of Venice is sitting on them).
 
From Trees for Life