In the period immediately before and during European colonization, there were four distinct indigenous groups in the area. Humans have lived on the Santa Ana River for at least 9,000 years. A wide variety of animal and plant communities depend on the riparian zones and remnant wetlands along the Santa Ana River. The San Jacinto River, which drains the southern half of the watershed, rarely reaches the Santa Ana except in extremely wet years. Due to low regional rainfall, the river carries only a small flow except during the brief winter season, when it is prone to massive flash floods. Although it includes areas of alpine and highland forest, the majority of the watershed consists of arid desert and chaparral environments. The Santa Ana drainage basin has a diversity of terrain, ranging from high peaks of inland mountains in the north and east, to the hot, dry interior and semi-desert basins of the Inland Empire, to the flat coastal plain of Orange County. The Santa Ana River is 96 miles (154 km) long, and its drainage basin is 2,650 square miles (6,900 km 2) in size. It rises in the San Bernardino Mountains and flows for most of its length through San Bernardino and Riverside Counties, before cutting through the northern Santa Ana Mountains via Santa Ana Canyon and flowing southwest through urban Orange County to drain into the Pacific Ocean. The Santa Ana River is the largest river entirely within Southern California in the United States.