Covered over in 1750 but still running, underground from Hampstead to Westminster. One of its sources is at Shepherd's Well. Diamond Geezer has tracked the entire course. LondonMyLondon shows it, briefly, uncovered in 2013. In 2008 we read that a property developer, James Bowdidge, was keen to resurrect the Tyburn. Requiring the demolition of many buildings including Westminster City Council offices this is unlikely to happen. Undeterred Bowdidge has created the Tyburn Angling Society, in expectation of eventually having somewhere to fish. See his pdf presentation.
2019: Great page by MOLA about the historic geography of the river.