High Quality Content by WIKIPEDIA articles! The Derwent is a river in Tasmania, Australia. It was named after the River Derwent, Cumbria by British Commodore John Hayes who explored it in 1793. The name is Brythonic Celtic for "valley thick with oaks". John Hays placed the name "Derwent River" only in the upper part of the river. Matthew Flinders placed the name on all of the river.