A critical navigation lock at the Bonneville Dam on the Columbia River has shut down because of cracked concrete, and that has put huge barges that transport millions of tons of wheat, wood and other goods from the inland Pacific Northwest to the Pacific Ocean for export on hold.

The closure comes at the peak of wheat harvest and could be devastating for farmers who ship to Asia via barges that fill up at more than two dozen grain elevators along the river network.

Many of those barges are now stranded above the dam, unable to reach deep water export terminals on the Pacific Ocean.

The crack in the concrete sill was discovered late last week and the lock was drained of all water over the weekend. On Monday, crews were working to demolish the faulty concrete section so repairs could begin, said Chris Gaylord, a spokesman for the U.S. Army Corps of Engineers in Portland, Oregon.