Complex sentence: The reservation originally included bottom lands along the Missouri, which had been farmed previously by Mandan and Arikara, and others prior to these tribes being decimated in smallpox and other epidemics in the 18th century.
Split into simpler sentences: The reservation originally included bottom lands along the Missouri, which had been farmed previously by Mandan and Arikara, and others prior to these tribes. The peoples were decimated in smallpox and other epidemics in the 18th century.