Time to clean up...
First off, you are dumping water on top of other water. This will only place water on top of the water that's already there, which will cause flooding. It won't magically get absorbed (in this situation).
If you want to clean up the circuit, try pumping the water back into the brook on the brook's level. Set up some pumps on the same level to keep things going in the right direction. Water will stack, but it won't shoot upwards unless it is pressurized by elevation difference (I'll get into this later).
I'm not entirely sure if that will work though, so be careful if you attempt it. Otherwise, I'd suggest just finding a nice, quiet spot to pump it off the map. Invest in a chasm next time you plan on having extensive waterworks.
The "horseshoe" thing that Hoborobo is talking about is false (or incomplete) information based off of a real bug. It refers to an event where a seemingly innocent water pressure system can turn into a death trap that will flood vast portions of the world if left to its own devices.
Here's how it works (or at least how it used to. I don't know if it got fixed recently or not).
###########
########~~~~~--> Source
Exit<--###~#
####~###~#
####~###~#
####~~~~~#
##########
Seems harmless enough, right? A simple means of providing water which has been pressurized by an elevation change.
The problem is that down at the bottom, where it makes that turnaround, something goes horribly wrong. the water will take on far more pressure than expected, and will also create a new source for itself. Anything at or under the exit's level will be drowned by an unending force of highly pressurized water. Shutting off the source will have no effect.
At least, that's what I remember of it. I never experienced it myself, and I haven't heard much about it lately. Just do a few searches for "U-bend bug", and you should find more information on the subject. I might be wrong about a couple aspects, but I think that's the basic gist of it.
However, that's not the problem here. If it were, your dwarves would all be dead by now.
All you need to is redesign your water output thingy so that it punts it off into a more manageable place.