It rises in Alnham in the Cheviot Hills and discharges into the North Sea at Alnmouth on the east coast of England.

The Aln is a relatively small river but has been important through history as one of the boundaries along which English and Scottish troops marching to war had to cross; for that reason, it was at times heavily defended.

For example, the river flows past Learchild Roman Fort and, more significantly, Alnwick Castle which was built for this purpose.

[7] Grey heron, barn owls, kestrels and buzzards can be observed hunting along the banks of the river.

Part of the estuary is designated a Site of Special Scientific Interest,[8] and the river below Lesbury footbridge (the normal tidal limit, except on high springs and in surge events) was made a marine conservation zone in 2013.