For many decades the semiarid Chaco ecosystems of Argentina have experienced anthropogenic disturbances, such as fire and cattle grazing, that have shaped the present-day vegetation in concert with climatic and edaphic conditions. For example, the dynamic relationship between grassland and woodland may be altered by the frequency and intensity of fire and cattle grazing. We analyzed woody vegetation among sectors of two ancient watercourses with similar geomorphological histories; one undisturbed for some time (Copo National Park) and the other with a long history of cattle grazing and fire (Copo Provincial Reserve). The undisturbed site had lower total species richness, diversity, basal area, density of both adults and seedlings, and cover of woody vegetation than the disturbed site, and doubled the number of branches per adult shrub or tree. Species composition of woody plants was notably different between the two sites. Considering the similar geomorphological history of the two ancient watercourses, we concluded that much of the contrast in their woody vegetation is likely due to land use history, especially their very different fire regimes.