Although some sources define the river as originating out of Nueltin Lake,[2][3] according to the Canadian Geographical Names Database the river begins at Snyder Lake in northwestern Manitoba.

[2][3] Its drainage basin covers an area of 64,399.6 square kilometres (24,864.8 sq mi).

[5] The river's name in Chipewyan is Łuazedes (pronounced thlu-assee-des), meaning "little fish river",[6][7] in reference to the plentiful grayling in its waters.

[8] It is known to the Inuit as the "big river" and used by them to travel inland where they trap arctic foxes and hunt caribou.

[2] The Thlewiaza was first mapped in 1912 by Ernest Oberholtzer and Billy Magee, an Ojibwe trapper.