The West Fork originates in Montana near Rock Candy Mountain, flows northeast into British Columbia, then southeast back into Montana to join the main Yaak River.

In Montana, the Yaak River and its tributaries mostly lie within Kootenai National Forest.

[5] According to British Columbia's Geographical Names Information System, "Yahk" is a Kootenai word meaning either "arrow" or "bow" and referring either to the Yaak River or the Kootenai River.

The southward curve of the Kootenai River (from Canada into the United States and back into Canada) is said to be a "bow", with the Yaak River possibly being the "arrow" (if the name is from the Kootenai word "a'k").

[6] According to the USGS, variant names of the Yaak River include A'ak, Yaac, Yahk, Yahkh, and Yak.