The Cowlitz people were originally two distinct tribes: the Lower Cowlitz and the Upper Cowlitz. Only the Lower Cowlitz spoke Cowlitz; the Upper Cowlitz, a Sahaptin tribe, spoke a dialect of Yakama.
Cowlitz is most similar to Lower Chehalis, another Tsamosan language, although it does contains some oddities, such as the word for one, utsus (in contrast to the Lower Chehalis paw).