page_title: Santa Ana River
section_title: Floods, droughts and legacy
hierarchical_section_title: Santa Ana River / History / Floods, droughts and legacy
caption_attribution_description: English: Map of the 100-year floodplain of the Santa Ana river with flood control structures labelled
context_page_description: The Santa Ana River is the largest river entirely within Southern California in the United States. 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 96 miles long, and its drainage basin is 2,650 square miles in size.
  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. Although it includes areas of alpine and highland forest, the majority of the watershed consists of arid desert and chaparral environments. 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. The San Jacinto River, which drains the southern half of the watershed, rarely reaches the Santa Ana except in extremely wet years.
context_section_description: Notwithstanding the increased prosperity in the 1860s, this decade was also the scene of a series of natural disasters. In the Great Flood of 1862, heavy rains dropped by a series of winter storms caused the Santa Ana to burst its banks, flooding thousands of acres of land and killing 20 to 40 people in the greatest flood it had experienced in recorded history. The levees along the river burst in many places, flooding part of the Inland Empire into a continuous body of water several miles wide stretching from the mouth of Santa Ana Canyon to where the river cuts through the Santa Ana Mountains. Downstream in Orange County, the river overwhelmed nearly all the existing floodworks and transformed the coastal plain into a transient inland sea. The flow, now calculated as a 1,000-year flood, peaked at roughly 9,000 cubic meters per second (320,000 cu ft/s), over half the average flow of the Mississippi River. Even after the flood, detrimental conditions continued in the region. For the two years following the flood, an intense drought caused the deaths of tens of thousands of head of livestock. Despite all of the hardships experienced in the three years, after conditions finally returned to normal, the Santa Ana River watershed again became a prospering agricultural region. The cities of Santa Ana and Riverside were established in 1869 and 1870, respectively.
  1934 and 1938 saw a further pair of devastating floods that in part brought an end to the area's citrus industry. In the Los Angeles flood of 1938, the Santa Ana again burst its banks and flooded Anaheim and Orange in up to 4 feet (1.2 m) of water, stripping away thousands of acres of rich topsoil and destroying many of the citrus groves. Almost 60 people were killed in the disaster and about 68,400 acres (277 km²) of land were flooded, despite the fact that the flow in the river was only one-third of that of the 1862 flood. With the extreme damage from the floods, the U.S. Army Corps of Engineers made the decision to dam and concrete the river beginning in the 1940s, and declared it as the greatest flood hazard in the U.S. west of the Mississippi River. Prado Dam, built in 1941, was designed to capture floodwaters from the Inland Empire about 30 miles (48 km) upstream from the river's mouth. The dam's impoundment, Prado Flood Control Basin, was designed to handle a 70-year flood.
  With the increased flood protection afforded by the Prado Dam, major industrial development migrating south from the Los Angeles Basin, and the Southern California housing boom in the 1950s and 1960s, the Santa Ana River watershed began its third and final transition—from agricultural to urban. The population of the Santa Ana River basin increased dramatically, but brought with it the threat of greater damage from floods, somewhat compromising the protection afforded by Prado Dam. Because housing and urban areas encroached on the river's historic floodplain—an area once occupied by farms—and the river became confined to a narrow channel—a flood similar to the ones surrounding the turn of the 20th century would cause much more damage. The construction of roads and buildings also heightened the runoff that would flow into the river during rainfall, a process known as urban runoff. In fact, the river flooded again in 1969, and while much of the runoff from the Inland Empire was captured behind Prado Dam—probably saving Orange County from an even greater flood—Santiago Creek, a large tributary flowing from the Santa Ana Mountains, eroded its banks until it swept away portions of residential communities in the cities of Tustin and Orange.
  In 1964, the Santa Ana River Mainstem Project, which involved concreting the lower 30.4 miles (48.9 km) of the river, was first proposed. Construction work began in 1989, and today, through much of Orange County, the river's channel is essentially an enormous box culvert. The second dam, Seven Oaks Dam, was completed in 1999. This dam captures flood runoff from Santa Ana Canyon before it can enter the Inland Empire. The dam was designed to withstand a 350-year flo
caption: Army Corps of Engineers map of the 100-year floodplain of the Santa Ana River
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