Hard: Station military in drowning chamber to level swimming, then wrestle carps to death.
Yeah, that's the option that isn't working properly. I like my axedwarves and airdrowning carp in a river's gonna be difficult.
It's not that difficult. I've gotten it down to a fine science, and a method which is also extremely useful for damming rivers. It's something similar to the
drain method, but simpler and superior as far as I understand it.
Dig a chamber underneath the river, at least two tiles wide and as long as you like. Channel out one side of this chamber, and put grates over the open space. If you intend to dam this river, build a raising bridge on the other tiles, so that it blocks water flow when activated. One level below that, dig out a chamber reaching to the map edge and carve fortifications in it. If you have a chasm/bottomless pit available, you can use that instead. When all this is prepared, head to the chamber below the river and carve ramps or otherwise breach the river bottom, causing the entire river to drain into your chamber; this will airdrown carp in the river proper, and any carp which fall down the ramps will be caught on the grates, where they will also airdrown. Diagram below.
|x|
|.|____
|.#####| Grates ^
|[====]| Bridge | Water flow.
|vvvvvv| Ramps |
Requires nothing more impressive than a few mechanisms, a bridge, and some grates, nothing that can't be made quickly and easily, and the river drains PDQ.
It requires more materiel than the drain method outlined on the wiki, but it's faster and easier, IMHO, and does not require fiddling around with pumps that I still don't perfectly understand.