Why has no one mentioned the fact that diagonals kill water pressure, saving your fort from flooding while managing to leave the spice water flowing?
I thought I had? And, if I hadn't, it's been mentioned in the threads I first referenced.
Which is not to say you can't end up flooding your fort with depressurised water. And when tapping from the side of a river you're taking water of the lowest (full-tile) pressure possible and, once you let it through the diagonals for long enough to trickle up to 7/7ths it's back to the 'original pressure', and you can still quite easily floor your fort with that water-source if you're careless enough.
"Draining an aquifer for a well", from OP? Well, I'd personally just place a well or three over a dug-into aquifer slot. Doing the same "for general purposes"... well, if it's for busting through to levels below there are a number of techniques already out there (check wiki on auifers, if you can't find on the forum?), and for getting a flow of water to go elsewhere I'd use the same technique as the aquifer-well but with then use a pump (or pumps) to lift it out of the hole you dig and send it where it needs to go[1] (also an unmentioned option for tapping, or with enough effort
draining, your aboveground river).
The difference between an aquifer and a river is that the aquifer is "omnipotent", whereas it's possible to disrupt the flow upstream (e.g. mass draining through a floodgate array out to a sink) to get access to a pretty dry riverbed for either full blocking purposes or installing more sophisticated tapping techniques (once you let the flow continue again), the aquifer 'happens' from
any exposed 'aquifering' block of rock that you haven't dug away (and, if you have, possibly its undug neighbours), regardless of works done elsewhere. This can be a problem
or irrelevant
or handy even, depending on what you want (or
don't want) from the area.
There's quite a lot of subtleties to each and every situation and need. To repeat my last contribution: "Shucks, it all depends on what you want to do..."
[1] Yes, you can dig to its edge, on that level, and then 'tap it' like you do with a river (from side or channel from above), but the precise edge of the aquifer isn't always so easy to identify with the "damp rock" buffer", so you may need to be more careful.