A silver eel slides into a spinning metal runner about the size of a trash can lid.
It does not get chopped.
It comes out the other side, tail flicking, and keeps swimming.
This happened inside a test facility in Alameda, California, not in a river.
A high speed camera caught most of these passes.
What the footage speaks to is a question hydropower has carried for decades, whether the blades that make the electricity can stop being the thing that kills the fish.
A blunter blade changes the impact on contact
Conventional propeller blades are thin and knife shaped because that geometry pulls the most electricity out of moving water. The tradeoff has always been fish. A thin leading edge strikes hard on contact, and the narrow gaps between a spinning blade and its housing can pinch anything that drifts too close.
The redesign, called the Restoration Hydro Turbine, swaps that profile for something thicker and rounder, with a forward sweep from hub to tip. Instead of a cutting edge, a fish meets something closer to the blunt side of an oar.
The runner was built around that single idea, a glancing blow rather than a slicing one. The design also minimizes the gaps between the blades and the turbine walls, reducing the chance that a fish is trapped between moving and stationary parts. High speed video of passage events revealed an absence of pinching or laceration injury risk inside the turbine.
A test tank in Alameda stood in for a river
None of this happened inside a working dam. It took place in a closed loop test facility built for the purpose, a recirculating hydraulic test facility in Alameda, California, where a runner can spin at realistic speeds while biologists watch live animals cross through it.
So the lab stood in for the river. The speeds were set to match operating plants rather than a gentler laboratory approximation, which matters because passage claims are easy to overstate at low velocities.
The fish work was also not self reported. Fish biologists from Pacific Northwest National Laboratory tagged and measured the eels before passage and inspected each animal in a water filled tube afterward, so the outcome was independently measured.
All 131 test eels survived the 667 RPM runner
In the peer reviewed trial, a 21.6 inch runner was operated under about 33 feet of hydraulic head at 667 revolutions per minute. A total of 131 eels were passed through it and 43 were held as controls, with eel lengths ranging from about 13 to 26 inches.
A subset of 37 eels was X rayed for internal injuries. The 48 hour survival rate for both treatment and control groups was 100 percent, with no major internal or external injuries after the hold period. The work was published in the Transactions of a fisheries science journal. A separate test reported by the Department of Energy reached the same result on a smaller scale. Forty seven American eels of 13 to 19 inches passed through a 21.6 inch unit under about 33 feet of head at 670 revolutions per minute, again with 100 percent survival, and the same agency reports that laboratory biologists confirmed 100 percent survival of all 60 rainbow trout passed through the turbine.
Some of the eels tested were longer than the runner was wide. One engineer involved in the design work put it this way: the turbine was meant to treat a fish “like a boat hull meeting a wave, not a blade meeting a target.” For comparison, the eel paper notes that survival through conventional Kaplan and Francis turbines may range from 40 percent to 95 percent.
Sediment tests pointed to a 78.5 percent wear drop
The forward swept shape appears to help the hardware too, not only the animals swimming through it. In tests following IEC Standard 62364, the company reports that its thicker blade profile showed up to 78.5 percent less wear in high sediment conditions than conventional blades, which it translates into roughly five times the blade life. That figure comes from a company white paper, not a peer reviewed journal, and should be read as such.
It matters because silty rivers grind a standard blade down fast, shortening its working life and raising repair costs. A runner that both protects fish and resists grit is a rarer combination than either benefit alone, part of why the design has drawn interest beyond its installed projects in Maine, Oregon and Austria.
None of that erases the scale of the real problem. Much of the aging hydropower fleet in the United States still turns older blade designs, and retrofitting even a willing plant takes time and money.
Eels still face years of travel past one wheel
A single turbine passage is not the whole migration. The same eels that cross this runner safely may still meet dams with no downstream passage design at all, warming water, or habitat lost long before they reach an intake.
Even so, the result is a measured thing rather than a rendering or a promise. A fish facing certain conventional runners has had, at the low end, something close to even odds. In this tank it had a documented path through, counted one eel at a time.
For a river that has to keep both its current and its creatures, that is a small, verifiable start worth watching as more plants decide whether to swap the wheel.
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