Siouan languages
Siouan languages can be grouped into the Western Siouan languages and Catawban languages. The Western Siouan languages can be divided into Missouri River languages (such as Crow and Hidatsa), Mandan, Mississippi River languages (such as Dakotan, Chiwere-Winnebago, and Dhegihan languages), and Ohio Valley Siouan branches. The Catawban languages consist only of Catawban and Woccon.
Siwu language
Siwu has seven oral and five nasal vowels. Several languages of the region have 8- or 9-vowel systems with some form of vowel harmony. In Siwu, noun class prefixes do not harmonize, but root-internally there are some constraints on vowel co-occurrences, attesting to the earlier presence of a system of cross-height vowel harmony (Ford 1973). Siwu is a tonal language with three level tones on the surface: High, Mid, Low. Functional load of tone is high in the lexicon (minimal pairs) as well as in the grammar (tenses marked by tone).