As Ulfarr says, the bucket brigade is usually a better (and safer) method to muddy floor than a river redirection.
You'd dig out the floor where you want the farm, as well as the level above it. Then you channel two tile wide slits in the upper floor for as long as you want them, and then designate each "open air" as a pond zone (you need to change the default, so the designation is a 3 key combination, if I remember correctly). Once a floor tile has been muddied, either because water was dumped on it, or because water flowed in from a neighboring tile, you remove the pond zone above that tile.
If you want to redirect a river, I would use a drawbridge rather than a floodgate, because the latter can be destroyed by building destroyers (and I'd use two drawbridges with an airlock in between to ensure nasty critters won't get in). Hook up the drawbridge to a lever (or each drawbridge to separate levers) placed in the tunnel you intend to let the river fill. This tunnel should reach one tile away from the river itself. Once the drawbridges have been set up (test the levers to ensure you've hooked them up correctly and the drawbridges have been designed as raising rather than retracting), channel away the tile separating the river from your tunnel to let the water through (or breach it with a miner into the river if you don't care about your miners drowning).
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Topic: Help Setting up a Farm (Read 750 times)