Photo by Joel SaxTwo weeks ago, after five days of pounding rain and two days of grace, a damkeeper on the Santa Ana River looked over the Prado Dam. He noticed a wet spot, some puddles. Seepage. The water wasn't coming out in a jet or even a weak spurt. “Dribble, dribble” is how one observer later described it. The man on watch had to get that pressure off the dam before the dribble grew into a spurt and opened a crack. Before the crack turned into a chasm, the chasm into a canyon.
And then he noted an abrupt shift in water moving through the dam's floodgates. His entries in the Army Corps of Engineers logbook tell the story: At 8:21 p.m. on Thursday, Jan. 13, he clocked the water at less than 5,000 cubic feet per second (cfps). One minute later, the rate doubled to more than 10,000 cfps, the largest discharge in the dam's history. Full capacity.
Though the previous week's rainfall set records, it wasn't enough to account for the sudden pressure on the dam. Something was wrong.
White water gushed from the square mouths of the dam. Just downstream, an outlying neighborhood of Corona, which developers had built on the debris from a Pleistocene landslide, was evacuated by order of the city's fire chief.
The media blamed it on the rain. They should have blamed developers.
The early-January storm wasn't unprecedented for residents of the Santa Ana River watershed. Storms in 1862, 1938 and 1969 were worse. Prado was nowhere near full capacity.
True, the January storm filled local creeks and generated landslides. But the 1938 flood—whose ravages occasioned the building of the dam in 1941—broke records. For 15 days, it rained. On just one of those days—March 2, 1938—10 inches of rain fell. Two of those inches came in one hour. The next day, an eight-foot-high wall of water surging out of the lower Santa Ana River Canyon tore houses from their foundations and slammed them against bridges. The debris formed a levee. The river jumped its banks and erased the town of Atwood—which used to stand downstream from Yorba Linda. Greg Knott, a Cal State Fullerton geomorphologist, recalls his grandfather telling him about “houses floating down the Los Angeles arroyo with the candles still burning.”
None of the rainy days two weeks ago equaled that, and yet Prado's floodgates were releasing more water than ever in the dam's history.
That's because of what's happening upstream. In 1941, most of the 2,450-square-mile Santa Ana River watershed was covered by small towns, citrus orchards, vineyards, dairies, livestock pasturage and gravel pits. Aquifers slurped up the rainfall and kept surface water levels to a minimum. Urban development in the intervening decades changed the face of the land. The cities of San Bernardino, Riverside and Ontario expanded. New towns such as Rancho Cucamonga and East Highlands popped up. Asphalt and concrete replaced open tracts of stream-rounded rock and dirt.
Development waterproofs the land. Irvine-based hydrogeologist Essi Esmaili says the problem of urban expansion is well-known to his colleagues. “You get more water re-routed into the storm channels than would otherwise have happened,” he explained. Neither Esmaili nor Knott felt undue concern about Prado. Knott, however, stressed it was very difficult to manage a flood plain. “You can't really win,” he said.
Californians, he explained, don't get that they live among the most-dangerous watersheds in the United States. Developers regularly lay out new subdivisions on available flat plots with little consideration for the flood dangers. “We build in flood plains here because we look out and see it dry,” Knott went on. “People should be aware of this. It's going to happen.”
And thanks to development, more runoff from Los Angeles, San Bernardino and Riverside counties backs up behind Prado Dam. When developers pave a wetland, they must mitigate the loss by creating another such environment nearby. Erasure of aquifers, on the other hand, requires only the approval of local politicians. When the mobile-home parks and new housing went up below Prado in Corona, it took only a decision of the city council. In Orange County, thousands of acres of aquifer were potentially lost last November when the Board of Supervisors approved the Rancho Mission Viejo Project.
Meanwhile, Prado is already pounded by more runoff than it's designed to take. The Army Corps of Engineers is retrofitting the dam to handle the increased flows. The extreme makeover—planned and approved under President Clinton—will raise the height of the dam 30 feet, add an intake tower, construct dikes around the catch basin to protect private property and raise the spillway by 20 feet. Maximum discharge from Prado's floodgates will triple to 30,000 cfps.
Prado's makeover won't be finished until 2008.
Before then, of course, it's possible we'll see a replay of early January's storm—or worse. Earlier floods wiped away Orange County's orchards and fields. Future floods would inundate a very different, very urban region.
