KAHLOTUS, Wash. - The giant navigation lock at Lower Monumental Dam is nearly empty of water, its concrete walls dry.

The lock will stay that way for the next 14 weeks, as long-needed repairs will bring cargo shipping to a standstill.

It is the longest shutdown in the history of the Columbia and Snake river system, as lock gates on three different dams are replaced between Dec. 10 and March 13.

"The gates have developed cracks and corrosion," said Steve Hartman of the U.S. Army Corps of Engineers, which is managing the work.

The lock at Lower Monumental Dam is 700 feet long, 84 feet wide and 100 feet deep. It takes 43 million gallons of water to lift or drop barges to the other side of the massive hydroelectric dam. The lock was built in 1969, and this is the first time a gate is being replaced, the Corps said.

The downstream gate, which is eight stories tall, is made of steel and weighs 1.2 million pounds. Moved up and down for 41 years, it has been weakened by age and has become brittle.

"The gate is cracking itself apart," Hartman said.

Similar gates are also being replaced at The Dalles Dam and John Day Dam, both downstream on the Columbia River. The budget for all the gates is about $50 million, and was funded by the federal government's stimulus package, the Corps said.

Hartman said the closure had been in the planning stages for years, and was scheduled for the slowest time of the year in shipping.

The Columbia-Snake system is typically shut down for two to three weeks each March for routine maintenance, but an outage of this length is unprecedented.

Forty percent of the nation's exported wheat, from 13 states, is barged down the system each year to West Coast ports, where it is loaded on ships for Asia. Petroleum products, fertilizers and pesticides move upriver to farm communities.

"It's the third-largest grain export gateway in the world," said Kristin Meira, spokeswoman for the Pacific Northwest Waterways Association, which represents businesses that use the river for transport.

The association supports the extended closure, Meira said. That's because the unplanned failure of a lock gate could close the river for a year as a new gate is designed, manufactured and installed, she said.

"We are pleased that there will be three new gates on the system in one closure," she said.

The wheat industry worked with buyers in Asia to make sure they had enough grain to weather the shutdown, Meira said.

"The big buyers of Pacific Northwest wheat purchased early and planned to store the wheat on their end," she said.

The full impact of the 14-week closure on the economy is not known, but Washington State University has said it will study the issue.

More than 8 million tons of cargo, valued at up to $2 billion, moves by barge each year on the system, said Glenn Vanselow, executive director of the waterways association.

The new gates will extend the life of the navigation locks by 50 years, Vanselow said.

The gate for Lower Monumental Dam was manufactured in Vancouver, Wash., and barged up the river. One of the nation's biggest cranes was erected next to the dam to lift the new 1.5 million pound gate into place.

In addition to the lock gate, engineers are also replacing a big chunk of the lock wall known as Monolith 15, which has cracked and deteriorated after years of being bumped by barges, engineer Steve Thompson said.

Football-sized pieces of concrete have fallen onto barge decks, creating a hazard for workers, he said.

"Big barges are a tight fit," Thompson said.