Water Channels, what good are they?

I’ve noticed I have an option to dig water channels, do they do anything? I assume I’d have to fill them myself or wait for rain? Do zombies have trouble crossing? Can I cover them with a plank? Do they irrigate farms in some fashion?

the only positive answer is that zombies have trouble crossing them (since it cost more movement points than regular floor) you need dig that kind of channel from a river, if you try it normal ground it shows a message of why it doesnt let you do it,

You can use them for the same things as any infinite water tile: Collect water, wash clothing and also to place a water wheel to generate power.

Basically the answer to all these questions is no. As @Ragno said, you have to find a river and dig from there (or from an adjacent water channel); you can’t fill them in any other manner.
It slows down monster, NPCs and you(r character) in the same way as any water tile would, unless they can fly. But the slowdown is not that impressive.

Interesting, what good is a water wheel if we don’t have real building power type stuff?

There’s a water mill for grinding grain, but there’s also a stationary water wheel for use with stationary power “vehicles”