Archaeological finds near the southern end of the San Joaquin Valley suggested that humans first arrived in the region as early as 12,000 but no later than 5,000 years ago. The two major ethnic groups were the Miwok people, who inhabited the northern end of the San Joaquin Valley and the Sacramento-San Joaquin Delta region, and the Yokuts tribes, scattered around the more arid southern portion of the basin. During these pre-European times, the San Joaquin River flowed through rich grasslands and sprawling marshes, flooding every few years and transforming much of the valley into lakes. At the southern end of the valley lay the vast Tulare Lake, formerly the largest freshwater lake in the western United States, which connected with the San Joaquin through a series of marshes and sloughs. The rich vegetation and wildlife surrounding these bodies of water made the San Joaquin Valley a favored home as well as a stopping-off place for other nomadic peoples. The native people, mostly hunter-gatherers, lived off this land of abundance; during the 18th century, the population of the San Joaquin Valley was estimated at more than 69,000, representing one of the greatest concentrations of native people anywhere in North America.