Complex sentence: Their population was decimated by smallpox in the late 1830s, and due to their reduced numbers, they started to live closer to the Mandan and Hidatsa tribes who lived in the same area.
Split into simpler sentences: Their population was decimated by smallpox in the late 1830s. Due to their reduced numbers, they started to live closer to the Mandan and Hidatsa tribes who lived in the same area.