Richard Hammond explores the historical, ecological and economic importance of four rivers. He begins by journey along Britain's longest river, the Severn, discovering how it has been a waterway of barriers, division and conflict, but has now become a river of connection. He also sees how its banks were fought over, used as a transport route for hundreds of years, and how it is now being opened back up as a transport route for one of Britain's rarest migratory fish.
