(Thlewechodyeth, or "Great Fish"), a river in Mackenzie and Keewatin districts, Canada, rising in Sussex lake, a small body of water in 108° 20' W. and 64° 25' N., and flowing with a very tortuous course N.E. to an inlet of the Arctic Ocean, passing through several large lake-expansions - Pelly, Garry, MacDougall and Franklin. Like the Coppermine, the only other large river of this part of Canada, it is rendered unnavigable by a succession of rapids and rocks. It was discovered and explored by Sir George Back in 1834. Its total length is 560 m.