People have inhabited the San Joaquin Valley for more than 8,000 years, and it was long one of the major population centers of pre-Columbian California. Starting in the late 18th century, successive waves of explorers then settlers, mainly Spanish and American, emigrated to the San Joaquin basin, first exploiting then driving out the indigenous tribes. The newcomers quickly appropriated the rich natural and hydrologic resources of the watershed for use in farms and cities, but found themselves plagued by flood and drought. Because of the uniform topography of the San Joaquin Valley, floods once transformed much of the lower river into a huge inland sea.