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 , 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
Great Flood of 1862
Great Flood of 1862
Santa Ana River