Complex sentence: Some legends of the Kami and Cahuilla tribes probably refer to Lake Cahuilla, often stating that the lake bed tended to be dry but occasionally flooded; then the tribes would have to relocate to the mountains.
Split into simpler sentences: Some legends of the Kami and Cahuilla tribes probably refer to Lake Cahuilla. They state that the lake bed tended to be dry but also occasionally flooded; during which times the tribes would have to relocate to the mountains.