Complex sentence: The two valleys have abundant level and gently-sloping land with fair to good soil development and abundant groundwater supplies, however malaria made them nearly uninhabitable except to the Tharu ethnic group who had evolved resistance.
Split into simpler sentences: The two valleys have abundant level and gently-sloping land with fair to good soil development, and abundant groundwater supplies. Malaria made them nearly uninhabitable except to the Tharu ethnic group that had evolved resistance.