The native oaks along the banks of the lower Dart have suggested a possible etymology, that - like the Derwent - the river takes its name from the Brythonic for oak (Old Gaelic dair, Breton derv, Cornish derow, Welsh derw), but it is also true that the root der- / dar- / dur- means water (Breton dour, Cornish dowr, Wlesh dŵr) and that dairt is Erse for a heifer.
"I know what Mr. Baxter says of Derventio, that it comes from the Welsh Derwent or Dirwyn, all of a piece with his Corguba aforementioned; inventions and boilings-over of a fertile brain. The Welsh tongue never had the word Derwent; therefore his whole building is without foundation. I have traced it to its original British name, but will not attempt the etymology of it. If it is from Derw, oak: why was not every river that ran through a forest called Derwenydd?"
Letter from Mr. Lewis Morris, of Penbryn, Cardiganshire, to Mr. Edward Richard, at Ystrad Meuryg, in that County, The Gentleman's Magazine, vol. 66 (1789)