Posted on November 9, 2026
By Andrew Byrne
At Allegheny River Lock and Dam 4, the U.S. Army Corps of Engineers Pittsburgh District completely drained the lock chamber, deployed dozens of personnel, cranes, boats, barges, and heavy equipment, and halted navigation through the vital inland waterway link for months because of a piece of wood.
Let us explain – it is an important piece of wood.
It is a timber miter sill, a sealing surface at the bottom of the lock chamber that helps the downstream gates form a watertight barrier. American navigation locks have used timber sills since the nineteenth century, when engineers relied on oak’s durability and ability to form a tight seal.
The design worked for nearly a century until a severe winter freeze in 2026. Ice moving down the Allegheny River damaged the sill, creating gaps that allowed water to escape.
The damage left the lock operational, but vulnerable. Aging filling valves had to compensate for water escaping through the damaged seal. With some valves already facing challenges, additional problems could leave the chamber unable to reach the upstream water level, effectively shutting down navigation.
And because the lock has only one chamber, there is no alternate route.
Enter the Medium Capacity Fleet, a Huntington District-operated team of specialized maintenance workers, divers, and equipment operators whose floating cranes, barges, and workboats travel between navigation facilities to tackle major repairs.
Their first task at Allegheny River Lock and Dam 4: get the river out of the river.
Dewatering a lock chamber is not as simple as dropping a huge (some may even say large) shop vac into the chamber and waiting. The fleet had to empty millions of gallons of water from the chamber to complete the work. Crews installed 16 bulkheads, 13 structural struts, and two needle-dam systems to isolate the 360-foot-long chamber, creating a temporary barrier that allowed the fleet to pump the water out of the chamber.
Installing the necessary equipment to begin the dewatering required intensive diving operations, including work near the dam’s crest, where turbulent water presented significant safety challenges.
“The valve bulkheads especially were high risk because the divers had to go in right near the crest of the dam where the water is most turbulent,” said Chris Smidl, a Pittsburgh District project engineer.
The operation brought together experienced divers from the Pittsburgh and Huntington districts. Water management personnel also coordinated releases from eight upstream flood-control reservoirs to temporarily reduce river flows and create safer working conditions.
“There was a lot of communication, group effort, everybody coming together to make it work,” Smidl said.
With the chamber dewatered, crews could finally address the important wood.
Crews are replacing the sill with a modern steel-and-concrete assembly, which uses a bolted rubber seal. The new design provides greater impact resistance while maintaining the watertight barrier that, until recently, depended on a piece of oak from a design chosen when Warren G. Harding was president.
Once a century-old lock chamber is drained, replacing one piece of wood and calling it a day would be a spectacular waste of a perfectly-adequate dewatered lock chamber, so the Medium Capacity Fleet seized the opportunity.
“We’re taking advantage of it and doing additional maintenance work that otherwise would not be possible,” Smidl said.
Before work started, the fleet expanded its task list with several other items, including replacing all eight emptying valves, their shafts and bushings. The work addresses aging equipment that could otherwise impede the lock’s reliability.
According to Smidl, the district moved from concept to construction readiness in approximately three months, which was only possible because the Pittsburgh District maintains an inventory of replacement components – some of which take years to acquire.
Planning and completing the work at a rapid pace goes beyond checking a box on a maintenance list.
The Allegheny River is an important transportation route for industrial equipment, construction materials, and oversized cargo for industry up and downstream of the lock. Smidl said the district adjusted its schedule to accommodate critical equipment shipments before closing the chamber.
However, without the repairs, additional equipment failures could shut down navigation indefinitely. More extensive failures could threaten upstream water levels, potentially affecting municipal water intakes, industrial operations and aquatic habitats.
Once repairs are complete, crews will reinstall the gates, refill the chamber and dismantle the temporary dewatering systems. Smidl hopes the improvements will keep Lock 4 operating for another 20 to 30 years before similarly extensive maintenance becomes necessary.
The fleet is expected to complete the work by December 2026.
