[fragmede]

The UK locks themselves are water movement though, so if you wanted to power something for "free" on those, you'd attach it to there. Oh man did I love watching those as a kid. You wouldn't do a fan though, you'd have a line and a buoy and use the up and down motion to drive a linear electromagnetic generator to harvest energy.

[fennecbutt]

Yeah but it's not really a lateral flow of water when the locks fill up and empty.

[lexicality]

you do get lateral flow up/downstream during filling and emptying, but it's generally considered rude to do it fast enough to cause turbulence