Solved.
Thanks for all of your help.
If you're interested, here's what I ended up doing:
z-1z-2z-3My solution is probably terrible, but like I said I'm just today starting to understand the way water works in DF. All I did was dig another reservoir under the first and added like 30 floor grates onto the first one, cut off by walls and a floodgate (I probably would have done this earlier, but I didn't know that water evaporated even if it is not outside).
If the first reservoir is too full, all I do is open the floodgate near the floor grates so that the water begins to drain further underground. If I do this is short bursts, the water can evaporate before the water reaches 2/7 or higher.
(By the way, if all you're going to say is that it looks sloppy, don't post anything)
As for trapping fish, this design is useless for vermin fish. Which is what your limited to because of the brook (only true rivers hold large fish)
I ended up trapping like nine salmon, but that's about it. I don't know if salmon are vermin but it seemed to work kinda.
If you really need to get rid of the water, I'd recommend just digging a small reservoir somewhere out of the way that it drains into. If you put a well over this, the dwarves can slowly drain it to the point where you can simply let it evaporate. Alternatively, you could dig out a wide space for the water to pool into and evaporate from.
Haha, I wish I'd known I could do that before I posted this.
...you should dig a separate system that loops back into the river that separate from the main water for the fort.
I wanted to do that from the start, but I thought it could be done just by carving out two tunnels. The one on the top was meant to go back around and into the brook somehow, but in the future I guess I could just pump it back in from the surface.
The water is moved a few tiles to the east and deposited into a channel that leads to a large underground reservoir.
"A" channel. Singular. Unless the water is pumped thru (and nothing says it is), narrowing that will help.
If that's only 1 tile wide already, adding the diagonal will help.
(Maybe you were the one not reading closely.)
I don't think I get what you're saying, but it sounds like I was the one who was unclear
EDIT:Hey even though I figured this thing out, while we're on the subject of irrigation and stuff, I was wondering why if you build a moat that is 3 tiles thick and then put a bridge across, the bridge always breaks? I figured it was just because it collapses after the 2 bridges supporting it are raised, but is there something else going on or some way to get around it?