CHICAGOCHICAGO (Reuters) - The flow of grains and other commodities through the U.S. Pacific Northwest has stalled because of a broken river lock at the Bonneville Dam on the Columbia River, along the Oregon-Washington border.

The stoppage adds another worry for U.S. farmers at a time when wheat prices have tumbled to multi-year lows, reflecting fierce competition for export business amid burdensome U.S. and world grain supplies.

Typically, grain export terminals along the Columbia River load and ship about half of all U.S. wheat exports, mostly bound for Asian markets such as Japan, South Korea, Taiwan and the Philippines, according to U.S. Department of Agriculture export inspections data.

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The Bonneville navigation lock was closed and emptied over the weekend after crews with the U.S. Army Corps of Engineers, which operates the facility, noted the lock appeared to be leaking water below its gates. Inspectors found a crack in a concrete sill at the base of the gates that prevented the lock from closing properly, a Corps spokesman said.

The breakdown halted Columbia River traffic at Bonneville, including barges bearing grain from eastern Oregon, Washington and Idaho to Pacific Northwest export terminals.

More than 100,000 tons of product and at least one cruise ship have been stranded above the Bonneville lock, Kristin Meira, executive director of the Pacific Northwest Waterways Association said in an email. Other commodities shipped on the river include minerals and wood products.

The Army Corps said it was not yet clear how long it would take to fix the lock.

"We are pretty optimistic that sometime next week we will be up and operating, so I think it will hold collateral damage to a minimum," said Damon Filan, manager of Tri-Cities Grain, a grain elevator on the Snake River, which flows into the Columbia.

"If it turns out to be something bigger and longer term, then we will all try to figure out a way to start trying to move grain by rail," Filan said.

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Demolition of the cracked sill was expected to be completed late on Tuesday, said Chris Gaylord, a spokesman for the Corps' Portland District. The Corps expected to establish a rough timeline for repairs once the demolition site is cleaned up, possibly on Wednesday, he said.

The U.S. trade war with China has slowed export demand for U.S. soybeans and corn, leaving more rail capacity available for wheat, Filan said.

"There are lots of rail cars sitting ... due to all the trade wars and the lack of demand," Filan said.