The major flooding threat in Orange County is the Santa Ana River. In 1938, the Santa Ana River flooded parts of Anaheim, Santa Ana, and Garden Grove, reportedly killing more than 50 people. Although the Prado Dam helped to substantially reduce the flood damage, the 1969 storm caused the largest dollar loss in Orange County history. Santa Ana River is owned by the U.S. Army Corps of Engineers, and operated by U.S. Army Corps of Engineers Los Angeles District and Orange County Flood Control District.
Despite the corps extensive efforts at flood control protection, it appears that portions of the County, which would not be inundated by the river overflow during the 100-year event, could be subject to flooding from overflow of storm water drainage facilities that are presently inadequate for carrying the 100-year discharge. East Garden Grove-Wintersburg Channel and Ocean View Channel system is one of the underlying channel systems of the Santa Ana River floodplain. This drainage system does not have the capacity to contain the 100-year flood because the channel banks and levees are overtopped at several locations.
In addition to the Santa Ana River, other areas subject to flooding during severe storms include the area adjacent to Atwood Channel, Brea Creek Channel, Carbon Canyon Channel, Capistrano Beach Storm Channel, El Modena Irvine Channel, Fullerton creek Channel, Hickey Canyon Storm Channel, Houston Storm Channel, Horno Creek Channel, Modjeska Canyon, Silverado Canyon, Niguel Storm Drain, Oso Creek Channel, San Juan Creek Channel, Santiago Creek Channel, and Trabuco Creek Channel.
In the central portion of the County areas adjacent to Santiago Creek and Collins Channel may be inundated. Large portions of the San Diego Creek watershed in the City of Irvine and unincorporated area of the County are also subject to inundation. In the southern part of the County, the flooding is mostly confined to the canyon areas; however, these areas are also of concern since their development is expanding.