Warnings Issued After Break in Folsom Dam : Evacuation: Boaters and hikers are cautioned along a stretch of the American River near Sacramento. About 40,000 cubic feet of water per second is gushing through broken gate.

A huge gate at Folsom Dam broke open Monday, spilling enough water each second to supply a family of five for a year. The break forced evacuation of boaters, hikers and anglers along the American River, but posed no immediate danger to communities downstream.

Dam operators said they may not be able to stop the water roaring out of the broken gate for up to a week, until the water level drops 40 feet to the top of the spillway.

The buckled gate will drain nearly half the water from the reservoir, which holds about 1 million acre-feet that is used for drinking water, agriculture and maintaining wildlife.

The water pouring into the American River at 40,000 cubic feet per second made the river and paved trails along its banks hazardous. The National Weather Service issued a flash-flood warning to clear the downstream area. Law officers ordered recreational users and homeless people to leave the river’s edge.

Folsom Dam is about 20 miles upstream from Sacramento.

There is little chance of flooding because levees should be able to contain the increased flow, said Jeffrey McCracken, a U.S. Bureau of Reclamation spokesman.

And the 40-year-old concrete dam was not in danger of crumbling, said Tom Aiken, dam manager for the Reclamation Bureau.

But the National Weather Service maintained its flash-flood warning throughout much of the day.

“Because of the high degree of recreational activity that occurs on the American River, we wanted to give as much warning as possible,” said Jack Messick, spokesman for the Weather Service. “It is a life-threatening situation for anyone who is rafting or fishing along the American River.”

A greater concern may be the stored water being wasted. Ironically, the reservoir was full for the first time in six years because of this winter’s heavy rain and snowfall. A big water supply in other Central Valley Project reservoirs will allow the federal government to meet its projected deliveries to much of California this year, officials said.

The thundering white-water torrent punching through the 90-ton gate near the top of the dam vibrated the entire structure, including metal walkways and closed spillway gates. Aiken said he did not expect other gate failures.

Even the flow from a second gate opening would not cause the river to top its levees, he said.

The problem started around 8 a.m., as operators were opening the metal gate to continue spilling water into the river after they temporarily shut down the flow to a hydroelectric plant.

“We were opening the gate, and it started to vibrate. We don’t know why,” Aiken said.

Aiken said it could take a year to repair or replace the broken gate. In the meantime, officials said they will search for a temporary plug for the hole.

The damage will cause the loss of about 410,000 acre-feet of water, Aiken said. An acre-foot is the water needed to cover an acre of land to a depth of one foot.

Officers on Monday cleared joggers, homeless people, bikers and others off the popular American River Parkway, which runs along the river from Folsom Lake to downtown Sacramento.

Residents of a Sacramento trailer park near the river were notified to be ready to evacuate, said Michael Heenan, a Sacramento Police Department spokesman.

Folsom Dam, which has eight spillway gates, was built between 1948 and 1956. It is operated by the Bureau of Reclamation as part of the Central Valley Project, which provides water for much of California.

The dam has a concrete gravity section across the American River channel, flanked by long earthen wing dams. The main dam is 280 feet tall.