The river at Hanover Pond Dam does not sound like a power plant.
It sounds like a drainpipe the size of a school bus.
Water spirals down a steel trough and turns a giant corkscrew instead of a spinning blade.
Somewhere inside that slow turning metal, a migrating eel is riding the current downstream.
It comes out the other end unmarked.
That is not what usually happens inside a hydropower turbine, and the reason comes down to the shape of a single moving part.
Why a slow screw barely touches a fish at all
A conventional turbine runner works by spinning fast, with thin bladed edges cutting through the water column many times a second. A fish caught in that path can be struck, pinned against a housing, or carried through a zone of sharp pressure change. Lacerations and sectioning are the injuries most often linked to direct contact with a runner.
An Archimedes screw turbine flips that logic. Instead of slicing, it lifts and carries.
The corkscrew shaped runner turns slowly, and the water travels down the trough in blocks with enough space inside them to hold a fish. So a fish riding downstream tends to glide through one of those open pockets the way it would ride a slow eddy.
Researchers attribute the low injury rates at screws to that slow rotation and to the absence of extreme pressures and shear forces, rather than to anything exotic in the materials. The mechanism is mostly about dwell time and gap size.
A small Connecticut mill dam becomes a test case
Hanover Pond Dam sits on the Quinnipiac River in Meriden, Connecticut, a former mill town on a river that once carried more than a hundred Industrial Revolution era dams. The screw installed there is about 46.5 feet long and 11.65 feet in diameter, set in a concrete trough sloping down the face of the dam.
It was the first hydroelectric facility in the United States to use the design for power generation. Migratory species in the project area include American eel, sea lamprey, alewife, blueback herring and American shad, and they push upstream to spawn, then drift back down later as juveniles or spent adults.
A conventional small turbine at a site like this would usually be screened to keep fish out of the machine. Here the intake trashrack was built with nine inch clear bar spacing so that fish can pass through the turbine instead, and federal licensing required the operator to evaluate how they fared on the way down. The screw became the migration route itself.
The numbers that came back from the river
The institute that certifies the site reports a 100 percent survival rate for migrating fish at Hanover Pond, including alewife, American shad and American eel. “Mortality or injury of fish passing downstream through the turbine was not observed,” the certification record notes, after researchers used a balloon tag recapture technique in November 2018 that allowed fish to be collected immediately below the machine.
All but one released eel were recaptured within six minutes, giving recapture rates of 100 percent for treatment and control groups. You can read the certification record at the Low Impact Hydropower Institute.
A second fish safe machine in Maine backs this up. At the Freedom Falls project, juvenile alewives passing through a compact turbine showed 98.2 percent immediate and 100 percent 48 hour survival once corrected for control mortality, in work published in the North American Journal of Fisheries Management.
Those numbers sit at the good end of a wide range. A review of turbine passage across many sites put mean fish mortality at 22.3 percent, and for a fish about 10 inches long estimated mean mortality of 11 to 22 percent at Kaplan turbines against 2 to 8 percent at Archimedes screws.
Where the slow screw still struggles
Even so, the screw is not a universal fix, and the species matters. At a large screw on the Albert Canal in Belgium, forced passage produced average mortality of 37 percent for bream and 19 percent for roach, against 3 percent for eel. Elongated, flexible bodies fare better in the pockets than deep bodied fish do.
Behavior matters too. At a screw on a lowland river in England, seaward migrating silver eels descended without immediate mortality, but some milled and turned back at the entrance.
Under high flows, most took an alternative route over a weir instead. There is also a scale limit worth noting here.
Archimedes screws suit low head, low flow sites like small mill dams far better than tall dams moving heavy volumes of water, the kind this desk has covered at Blenheim-Gilboa in New York. Scaling the slow corkscrew up to those volumes remains an open engineering question.
What a small mill town suggests about the rivers ahead
None of that erases what happened at a modest dam in a town most people drive past without a second look. Meriden’s stretch of river now carries both electricity and fish through the same piece of turning metal, a small reversal of the usual tradeoff between power and passage.
Plenty of other low head mill dams sit in the same awkward middle. They are too small to justify the cost of full removal, yet large enough to interrupt a run of migrating fish.
The same interest in gentler hardware shows up elsewhere, in how old turbine blades get shredded into material for new projects. Most of these results come from a handful of small sites and forced passage trials, not a nationwide survey, and bigger rivers with deeper bodied fish will test the idea harder.
But for one modest dam in Connecticut, the fish moving through its slow spinning screw suggest a gentler machine can still keep the lights on.
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