6.5 Billion Gallons at Prado Dam Dumped : Drought: O.C. officials see waste, but the water is sent to the ocean to protect a rare bird and for flood control.

Amid the worst drought in California history, 6.5 billion gallons of water are being dumped from Prado Dam into the Santa Ana River and then the ocean off Orange County, both to guard against floods and to protect the habitat of an endangered species of bird, officials said Friday.

Obliged under federal law to protect the nesting area of the least Bell’s vireo, of which just 400 breeding pairs remain, dam officials Thursday began releasing the storm water down the Santa Ana River.

Without the discharge, the birds, which nest low to the ground in thickets of brush, would be flooded by the rising water, officials said.

The least Bell’s vireo is a tiny gray-and-yellow songbird not much larger than the hummingbird. Native to the California and Mexico coastlines, the birds migrate to Southern California from Baja in late March. The Prado Basin is one of their last remaining breeding grounds.

“Which is more important--the bird or the water?” asked bird-watcher Dale Dillon, who served on a panel aimed at preserving the bird’s habitat. “That’s a tough question that I wouldn’t want to have to answer. Let’s hope there’s some way to find enough water to drink and to save the bird at the same time.”

While water officials bemoaned the loss of 6.5 billion gallons to be dumped by Monday, they said the water could neither be stored nor diverted to help alleviate the five-year drought. The water is not potable because it is contaminated by runoff and pollutants. There is no pipeline system to divert it for agricultural use, and storing it could pose a flood threat.

“We just don’t have any way to hold onto the water, since the law does not allow us to collect it in the reservoir,” said Grigor Grigorian, chief of the Reservoir Regulation Section of the Army Corps of Engineers in Los Angeles. “But unfortunately, in these days of a drought, that water is awfully valuable.”

The water would be valuable for replenishing the ground-water basin, which provides about three-quarters of the water consumed in northern Orange County, said Lawrence Kraemer, a board member of the Orange County Water District.

“I’d love to see them hold a lot of that water,” Kraemer said. “Close the gates and let us hold it. But you have to always keep in mind that it is a flood-control dam, so they can’t hold that much back, or you’ll flood people downstream with the next storm that comes.”

Prado Dam--in Riverside County near the junction of the Corona and Riverside freeways--was built in 1941 to control the Santa Ana River, which is considered the worst flood threat west of the Mississippi, according to Corps officials.

If storm water surged over the top of the 540-foot dam, it could drench large areas of Newport Beach, Tustin, Santa Ana, Orange, Anaheim and other communities under several feet of water.

The Orange County Water District does not have a reservoir. It has considered building one but rejected the idea because of its $60-million price tag.

Orange County has four holding tanks along the Santa Ana River, which were being filled with storm water Friday, said James Van Haun, spokesman for the Orange County Water District. The captured rainwater will then be allowed to sink into the ground to replenish the ground-water table.

“We’re taking advantage of the free water,” Van Haun said.

The storm brought an average of 4.3 inches of rainfall in 48 hours, the county Emergency Management Agency reported Friday, with more than 12 inches falling on Santiago Peak, the highest point of Saddleback.

“That’s significant,” said Van Haun. “It’s definitely positive.”

But, he warned, “It would take 10 to 12 of these types of storms” to undo the effects of the long-lasting drought.

“One major storm is not going to do it, though it’s a welcome relief,” he said.

To improve the county’s long-term water supply, water district officials want to expand Prado Dam’s capacity. It can now hold 195,000 acre-feet. One acre-foot is about 326,000 gallons, enough for a large family for a year.

Since 6 p.m. Thursday, Grigorian said, the dam has been releasing 2,500 cubic feet of water per second. At that rate, it will have discharged 20,000 acre-feet, or 6.5 billion gallons, by Monday, he said.

The water district has been urging the corps to hold more water back, Kraemer said.

“Our contention is after the flood danger season is over, they can hold more water back there,” he said. “At this time, because of the flood danger and because of the bird, they refuse to hold it. If we were in April, they might shut the gates and hold more for us, because the flood danger would be over.”

The least Bell’s vireo, once common in California, has fallen victim to predators and development and is protected by both state and federal law. It is an especially vulnerable species because it is unusually selective about where it breeds.

“It will only build nests in the willow trees found in the bottom of riverbeds,” said Dillon, a bird-watcher and maintenance supervisor with the county Environmental Management Agency.

Experts said there are just 300 to 400 breeding pairs left, because most of the riverside thickets where they once bred have been dammed, drained or paved. Without intervention, Dillon said, the species would perish.

“It’s like the push to save the California condor,” he said.

Times staff writers Marla Cone and Janice Jones contributed to this report.