The Santa Ana River is the largest river entirely within Southern California in the United States. Its drainage basin spans four counties. It rises in the San Bernardino Mountains and flows past the cities of San Bernardino and Riverside, before cutting through the northern tip of the Santa Ana Mountains and flowing southwest past Santa Ana to drain into the Pacific Ocean. The Santa Ana River is 96 miles (154 km) long, and drains a watershed of 2,650 square miles (6,900 km2).
For its size the Santa Ana drainage basin is quite diverse. It ranges from high peaks of inland mountains in the north and east, to the hot, dry interior and semi-desert basin, to flat coastal plains in the west. Its climates range from dry alpine to chaparral and desert, and the watershed as a whole is very arid. Relatively little water actually flows in the river or most of its tributaries. One of its largest tributaries, the San Jacinto River, rarely reaches the Santa Ana except in extremely wet years. The relative lack of vegetation also makes the river prone to flash flooding. Even so, a wide variety of animal and plant life has always been dependent on the river.
People have lived on the Santa Ana River for at least 9,000 years. There were four distinct indigenous groups in the area, all of which depended heavily on the river for their livelihoods. The river was first crossed by Europeans in 1769, when it received its name from members of the Spanish Portola expedition. Because it is one of the largest water sources in the four-county region, many large ranchos developed alongside the river and one of its major tributaries, Santiago Creek. This period of growth culminated in the establishment of many large cities on the river, including Santa Ana, Riverside and Anaheim, all of which derived their names from the River. In the early 20th century, devastating floods poured down the Santa Ana River, leading to much of the river being channelized and dammed in recent times.
Human habitation on the Santa Ana River dates back 9,000 to 12,000 years ago, close to the early stages of the Holocene period. The first Native Americans to live in the area were nomadic tribes that traveled from place to place, grazing animals on fertile grasslands and gathering fruits and seeds for food. The ancestors of these early people originated from the Shoshone and Uto-Aztecan people of the northwestern United States. Eventually, the human population of the watershed reached a peak of about 15,000. About 8,000 years ago, the climate experienced a change becoming more arid and the originally nomadic tribes began to stay in individual places longer, becoming semi-nomadic. However, they did not establish agriculture, nor did they raise animals or live in villages. Like many Native American tribes in California, acorns were a staple food of many of the inland valley people. People closer to the ocean often fished and hunted small animals, often from tide pools and coastal stream areas, for food.
Several major premodern Native American groups eventually gained control of lands along the river: the Yuhaviatam or Yuharetum people in the upper basin, the Payomkowishum in the southeastern basin, the Cahuilla in the desert areas of the watershed, and the Tongva people in the lower basin. The Yuhaviatam generally lived in the mountain headwaters of the Santa Ana River and its tributaries rimming the present-day Inland Empire basin, in present-day San Bernardino County, as well as in the foothills of the San Bernardino Mountains. The Tongva lived on the flat coastal plains of present-day Orange County south of the Santa Ana Mountains. They were also the larger of the two groups, controlling all the coastal lands from the San Gabriel Mountains in the north to Aliso Creek in the south, including all of the Los Angeles Basin.