Corps installing fish passage smolt slides at Chittenden Locks in Ballard

Seattle District
Published April 4, 2019

U.S. Army Corps of Engineers officials are installing smolt slides in two spillway gates to help juvenile salmon traverse the Hiram M. Chittenden Locks in Ballard.  

As part of an overall project to improve fish passage, the Corps began installing smolt flumes in 2000 to help provide juvenile salmon – smolt – safe passage through the Locks and into Puget Sound. In 2017, the Corps tested a prototype smolt slide to replace the aging flumes. The prototype was successful and the Corps built two more slides. This is the first year all three slides will replace the four flumes.

The slides look just like the name implies, with the small 4- to 6-inch fish gliding down from Lake Washington to Puget Sound. The slides, and previous flumes, main purpose is water conservation, but also provide a surface route for smolt.

“Although salmon have been navigating the 102-year-old Locks for as long as it’s been operating, studies in the 1990s indicated they were having a difficult time,” said locks fisheries biologist Kaitlin Whitlock. “The locks’ primary passage routes during the migration period were deep in the water column and not easily found by juvenile salmon which stay closer to the surface.”

Before the flumes, the spillway and fish ladder were the only surface outlets available. The spillway could only operate a short time before it had to close to conserve water and was not open for most of the smolt migration period. The fish ladder didn’t provide enough flow to attract the smolt.

The slides provide the perfect combination: just the right amount of flow to attract them but not too much, which allows operation for the entire migration period.

“We’ll monitor the migration and use the slides until they are no longer migrating,” said Whitlock. “Typically sometime between mid-July and September.”

Even with the slides, some smolt still end up going through the large lock filling tunnels. The large lock filling culverts contain bends, fast moving water and sharp barnacles. Together, these three hurdles likely injure salmon. To decrease possible injury, each year during the annual maintenance pump out in November, Corps employees scrape the barnacles growing on the filling tunnel walls.

During the 2019 pump out Corps officials will begin replacing the original valves, called Stoney gate valves, which control water through the culverts. The new design will allow for much slower water velocities.

“The new valves and smolt slides, should allow much safer passage to the vast majority of juvenile salmon transiting the locks from the Lake Washington basin to Puget Sound,” said Whitlock.

The valve replacement project will require four 45-day outages. “The first of the outages will begin this year,” said Operations Project Manager Jon Hofstra. “We know this will affect our commercial users quite a bit, but in the long run it means a more reliable lock system and less chance of an extended outage.”

The current extended outage schedule is:

2019 – October 12 through November 30
2020 – February 12 through April 2
 October 12 through November 30
2021 – February 12 through April 2

For current information about activities at the Locks, visit the Locks’ Web site at or follow the Locks on Facebook and Twitter: and

Bill Dowell

Release no. 19-011