Photo: Santa Ana River, 1961, before concrete lining and channelization.
July 28, 1769 - The first Europeans cross the Santa Ana River near Hotuuknga (a village in present day Anaheim). Juan Crespí, diarist for Gaspar de Portolà's Spanish expedition notes “The bed of the river is well grown with sycamores, alders, willows, and other trees...It is evident from the sand on its banks that in the rainy season it must have great floods which would prevent crossing it...”
Yes, the county was devastated by floods and described, by the US Army Corp of Engineers, as the greatest flood threat west of the Mississippi. We can thank Prado Dam, Seven Oaks Dam and flood control measures for our protection from a 100 year flood. Could we see areas of Orange County flooded again with the upcoming El Nino? Hopefully not but time will tell.
Source: http://www.kcet.org/updaily/socal_focus/history/la-as-subject/how-orange-county-tamed-the-santa-ana-river.html