Analysis of information sources in references of the Wikipedia article "Saint David's Buried Gorge" in English language version.
The tunnels ran only as far as St. David's Buried Gorge, an ancient river valley that crosses the modern Niagara River a few kilometres below the Falls. Since the loose silt filling the gorge couldn't be safely tunnelled through, the tunnels have to veer back up to the surface and the water completes its journey in a canal. The Americans, meanwhile, were also building tunnels to feed a new power plant across the river from the Beck stations, using the even more disruptive cut-and-cover technique, essentially digging a giant trench then covering it.
