The closure, which began this week, is part of routine upkeep for one of the busiest boat locks in the world. The small lock remains open for boat traffic, and the large lock is scheduled to reopen on Nov. 20.

During the drawdown, heavy machinery is lowered into the empty chamber. Crews are repairing metal rods to combat corrosion, replacing and repairing the large lock culvert valve that controls water flow and helps manage water levels, and conducting inspections, according to the Army Corps of Engineers.

A key task is removing barnacles from the walls of the large tunnels that move water in and out of the locks. The sharp shells can cut salmon as the fish swim against the walls on their migration between Puget Sound and Lake Union or Lake Washington.

The Ballard Locks draw about 2 million visitors each year, and last year about 27,000 vessels passed through, according to the Army Corps. The locks function like a boat elevator, using gates to raise or lower vessels between the lower water level of Puget Sound and the fresh water of Salmon Bay.

For residents and visitors, the drained chamber offers a rare view.

“It’s so deep. It’s deeper than you realize ’cause you don’t usually see it completely empty,” said Carlyn Orians, who lives in West Seattle.

Phil Hammond, a Ballard resident, said the locks are always worth a look. “There’s always something different to see. A big boat coming through, little boats coming through. You get the herons roosting in the spring. I see seals and sea lions. I see beavers out here,” he said.

Alex Dee, visiting from California, called the sight remarkable. “It is amazing. It’s not every day you get to see a lock emptied out. There’s tractors and stuff down there. And wow, the idea of cleaning barnacles,” Dee said.

Orians noted that the maintenance period comes with a noticeable smell, but also a unique spectacle. “You notice a smell, but it’s actually cool to see the equipment down there and the maintenance going on and the birds pecking at the walls to see what’s exposed that they can get at,” she said.

The annual drawdown is a reminder of the locks’ dual role: keeping marine traffic moving while trying to reduce harm to the region’s salmon runs, which face mounting pressure from habitat loss, warming waters and other threats. For now, boaters should plan around the large lock’s closure and use the small lock until the chamber refills later this month.