If you go down deep enough the earth temperature is stable at 12 C. But I’m pretty sure that’s like thousands of feet down. Geothermal rigs drill 500+ meters (1600 feet) down. Having a hard time finding a source on the stable earth temp though.

I mentioned 6 feet because it’s well below the frost line where I am, but I guess in that part of Iran they probably don’t have to worry about that.

Mostly I was just wondering if you had found something on how deep those waterways were actually built.