A rebuilt fish ladder at Bonneville Dam gives Pacific lamprey smooth curved walls and small resting holes along a stretch where only about half of the ancient fish were getting past each dam

On a gray February morning at Bonneville Dam, the upper fish ladder looked more like a demolition site than a wildlife corridor.

The old walls were gone.

Workers in hard hats were building frames for the new ones.

And somewhere below in the Columbia River, Pacific lamprey were waiting out the winter before their long push inland from the Pacific Ocean.

The ladder had to be ready.

The question was whether a new shape of concrete could do what decades of the old shape had not.

Why half of every run was stopping cold

Fish ladders on the Columbia were built around salmon, and regional fisheries managers have long reported that many of those salmonid ladders pass only about half of adult Pacific lamprey that reach them. Multiply that across a full migration and the math turns brutal fast. A fish that slips through at fifty fifty odds at the first dam faces something close to the same coin flip at the second, and the third, and every dam after that.

Pacific lamprey must pass eight dams to reach hundreds of miles of historic spawning grounds along the Snake River. On the mainstem Columbia they must traverse nine before arriving at Chief Joseph Dam, which cannot be passed at all.

The old ladder design sat at the center of the problem. A lamprey does not power through fast water the way a salmon does. It attaches with a sucking disk mouth and works its way up a surface, so square corners, open channels and high velocities leave it with few places to grip between rests.

What the rebuilt ladder looks like

The renovation aimed to reduce sharp corners, lower water speed, cut overall travel distance, and give lamprey places to rest along the way. That last detail matters because lamprey do not swim through a fish ladder so much as climb it.

The old walls were torn down and replaced by new concrete walls extending across the channel except for a small angled break in the middle. The new walls also have small holes at the bottom sides, letting a lamprey press its disk against the surface and move through at water level, hugging the wall rather than fighting through open flow. Because the new concrete is smooth and free of 90 degree angles, engineers expect the animals to rely more on that suction.

The new design echoes what fish would have encountered before dams, when millions of lamprey climbed the slick, water worn rocks of waterfalls including Celilo Falls, now submerged beneath the pool behind The Dalles Dam. The smooth curved concrete is an attempt to bring something of that surface back.

What a decades long collapse looks like in numbers

Returns of this roughly 450 million year old native fish have shrunk to about 10 percent of historic numbers following dam construction, warming water and other pressures on the river. Columbia River treaty tribes have documented the decline across generations, watching counts that once pointed to runs above a million fall to tens of thousands in a season.

Tribal programs have translocated more than 70,000 adult lamprey over the past 25 years. In a recent season crews set a record by collecting about 17,000 at Bonneville, The Dalles and John Day dams and hauling them to water above each one. Translocation is essential but laborious, compensating for what the ladders were failing to do on their own.

Anyone curious about what river recovery looks like after barriers come down can follow the Chinook salmon story on the Klamath, where sonar counted more than 21,000 fish across two migration seasons.

The two year study now running

The renovation was expected to cost about $8 million. A similar renovation at John Day Dam benefited both salmon and lamprey, and the Corps said it would conduct a study comparing passage before and after the Bonneville rebuild.

“We had years of research showing that lamprey were struggling to pass this section. There was a real bottleneck,” said Erin Kovalchuk, the Corps project manager, while watching workers build frames for the new walls.

That bottleneck is physical. A fish that uses its mouth as a grappling hook needs a surface the hook can hold, and for decades the ladder offered neither the texture nor the geometry to allow it. The study running now is the real test: ladder shape can be changed, but the proof is in the count of animals that make it through under their own power.

What comes next for a fish older than trees

Lamprey lineages reach back hundreds of millions of years, predating dinosaurs and even trees, and have outlasted multiple mass extinctions. That deep resilience is part of what makes their decline inside a single human lifetime so striking to the biologists and tribal members who track them.

The Bonneville rebuild is one piece of a longer effort. Work on the Bradford Island side of the dam includes modifying the ladder’s exit section, rounding corners, cutting new lamprey orifices and adding more refuge boxes. Each modification chips away at the same basic problem: concrete infrastructure designed around salmon that left little room for a disk mouthed fish with different needs.

Results will not arrive until lamprey have run the new ladder for two full seasons. What biologists already know is that roughly one fish in two turning back at each dam leaves a species little room to recover. If the new geometry pushes even a fraction more through, the math that has been working against Pacific lamprey for generations starts, at last, to work the other way. For anyone wondering how industrial materials shape wild waterways, the rebuilt ladder at Bonneville is an unusually legible answer.