Its upper valley is about 95 kilometres in length, entering Lillooet Lake about 15 km downstream from Pemberton on the eastern outskirts of the Mount Currie reserve of the Lil'wat branch of the St'at'imc people.

Below the 30 km (18.6 mi) length of Lillooet Lake, it resumes again just north of the native community and ghost town of Skookumchuck Hot Springs, which is known in the St'at'imcets language as Skatin.

Until the 1910s, the name Lillooet River also applied to what is now the Alouette River in Maple Ridge; the neighbourhood that grew up on its south branch became known as South Lillooet, but to avoid confusion the new postmaster was requested to come up with a name, choosing Yennadon after his family manor on the Devonshire Moors.

[6] The river name was changed formally on March 31, 1915 with "Alouette" chosen because of its resemblance to the sound of "Lillooet".

[5] The Lillooet River was dammed with breccia from a Plinian style eruption of the Mount Meager massif 2,400 years ago.