Complex sentence: The Skidi's main habitations were along the Platte River and some early explorers referred to this waterway as the Panimaha River, since this was before some Skidi migrated south and that name came to be associated with a different group.
Split into simpler sentences: The Skidi's main habitations were along the Platte River. Some early explorers referred to this waterway as the Panimaha River, since this was before some of the Skidi migrated south; that name came to be associated with a different group.