It began with a light drizzle in mid-January, 1969. By late February, more than 20 inches of rain from a series of intense storms had saturated soil from the Santa Ana Mountains to the flatlands of central Orange County. Without warning, a towering wall of mud slammed into the Silverado Canyon fire station, killing five of the more than 60 homeless people who had sought shelter there.
Mass evacuations followed as water spilled over the Villa Park Dam and chocolate-colored rivers of boulders, cruddy foam and debris raged over the banks of normally bone-dry Santiago, Trabuco and San Juan creeks, turning roads into rivers.
When the clouds finally parted in early March, the grim toll for the Deluge of 1969 stood at 11 dead and more than $12 million in damage to public and private property.
Despite the devastation, H. G. Osborne, then chief of the county flood-control district, was encouraged that Prado Dam--perched at the head of Santa Ana Canyon near the Orange-Riverside county border--had held against what was widely considered one of the worst storms ever.
Eight months later, Osborne learned that the Deluge of 1969 was mere child's play in the history of the Santa Ana River. The U. S. Army Corps of Engineers, using new methods to compute the intensity of previous floods in the area, found that the river had brought a torrential flood in 1862 that was six times greater and had altered the river's course.
Suddenly Prado Dam was considered grievously inadequate. Based on these findings, the Santa Ana River was soon viewed as the greatest flood threat in the western United States, capable, in a worst-case scenario, of breaching Prado Dam, killing an estimated 3,000 people and causing $11 billion in damage as it inundated densely populated areas from Anaheim to Newport Beach and west to the Los Angeles County line.
Today, after 15 years of study and planning, a proposed $1.1-billion All-River plan to correct Prado Dam's design deficiency has yet to be approved by Congress, amid quarrels over the plan itself and who would pay how much.
Osborne and other flood-control experts repeat what they have been saying for so long: The question is not \o7 if \f7 a flood of the magnitude of 1862 will recur, but \o7 when.\f7
Experts say that is impossible to predict.
"We more or less hold our breath," said Carl Nelson, head of the public works division of the county's Environmental Management Agency.
The Army Corps of Engineers considers the 1862 flood as roughly equivalent to a "standard project flood"--defined as a calamity with an expected recurrence of once every 200 years on the river, or one chance in 200.
But Dennis Majors, the corps' manager for the All-River project, said it is possible that such a flood could occur more frequently on the river that begins 90 miles from the ocean in the San Bernardino National Forest.
Meanwhile, the existing Prado Dam and system of levees along the river's course provide less than 70-year flood protection to the more than 1 million people living and working below it, according to a 1976 report by the corps' Los Angeles district office.
"Hundreds of thousands of homes, businesses and factories, and hundreds of schools would be inundated by the standard project flood under (existing) conditions," the report said.
"With only eight hours warning time, complete evacuation before the peak flow would be impossible. Untold numbers of lives could be lost in the floodwaters. . . .
"In addition . . . there is the possibility that during the maximum probable flood, the dam itself might be overtopped and perhaps even fail. The loss of life and property damage could be several times as severe as the standard project flood."
The proposed All-River plan would increase the storage capacity of Prado Dam by raising its height 30 feet, building a smaller upstream dam, and improving the lower Santa Ana River channel and its tributary, Santiago Creek. Together, these would give urban Orange County and parts of Riverside and San Bernardino protection from a 170-year flood at a cost of $1.1 billion in 1985 dollars.
Many of the local objections that previously have stymied the project's approval appear to have been resolved with a new proposal to build a $304-million dam in a steep, narrow canyon in the San Bernardino Mountains about two miles north of Redlands.
The Upper Santa Ana River Dam would replace the proposed Mentone Dam in the East Highlands area of San Bernardino. Residents and local officials had rejected the Mentone Dam as unsightly and too close to major earthquake faults to be safe.
If Congress were to authorize the plan as part of a massive water projects bill and if President Reagan withheld his threatened veto and signed it by 1986, construction could begin in 1989, Majors said. It would take from seven to 20 years or more to complete.