The Derwent is a river in Tasmania, Australia. It was named after the River Derwent, Cumbria

River Derwent, Cumbria

The Derwent is a river in the Lake District of the county of Cumbria in the north of England. The name Derwent is derived from a Celtic word for "oak trees"....

 by British Commodore John Hayes

John Hayes (explorer)

Sir John Hayes was an 18th century explorer for the British East India Company, who explored eastern Tasmania and named the Derwent River in 1793.-References:John Hayes is one of the unsung heroes of animation....

 who explored it in 1793. The name is Brythonic

Brythonic languages

The Brythonic or Brittonic languages form one of the two branches of the Insular Celtic language family, the other being Goidelic. The name Brythonic was derived by Welsh Celticist John Rhys from the Welsh word Brython, meaning an indigenous Briton as opposed to an Anglo-Saxon or Gael...

 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.

The banks of the Derwent were once covered by forests and occupied by Aborigines.