When I first read the headline, I thought: what a splendid way to combine two green issues, but how would it work? Then it dawned on me: "canal" in America implies irrigation rather than navigation. So turning the Neath and Tennant canals into a cheap energy source was clearly not on. However, the idea is clearly a goer in the appropriate areas, as this article explains.
It has the beneficial side-effect of reducing water loss due to evaporation from direct sunlight. This was carried to the extreme by the ancient Persian qanat (a useful word for Scrabble-players because it does not need a U). Several of these underground aqueducts are still in use today.
Less ecological is the Libyan Great Man-made River, a project begun by Gaddafi and completed by 2007. The infrastructure was attacked by NATO in 2011 and has since been prey to campaigns by warlords and neglect, but is still delivering some water. The source of the water, piped to reservoirs in the north of the country is a fossil aquifer which is not being replenished and may be exhausted by the end of the century, though other estimates give lifespans of up to one thousand years.
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