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[Melasoul]

No, no goblins on him, just a lone croc. Also, it couldn't have come from below. I hadn't hit the caverns yet and had dug this whole thing myself.

So fully submerged grates don't block creatures? That seems kind of wrong. And yeah, I know there is no drain but that was kind of the point, as it was just supposed to be a standing, refillable pool. I thought about putting a one level drain at the bottom of the pit just to keep it from spilling over into the hallway but thought it to be more effort than it was worth. And without that drain even if I had the grate diagonally it would still fill up and be rendered useless right (since I have the grates before the floodgates)?

So yeah, seems like I'll be abandoning this for a well, or doing it deeper for my well and walling it off. Too bad, I kind of liked the idea of a spring fed swimming pool and watering hole.

So does this mean that there is no good way to filter a flow of water?

What about dropping it a few z-levels through a horizontal grate? Would it count as 'water filled' while the water ran through it, assuming that it never becomes a standing pool? (this is just my curiosity now as a system like that seems possibly very... fun inducing)

[Sphalerite]

Floor grates permit water to pass without letting creatures through.  They can be smashed by building-destroyers on the same level as the grate, but not form the level below it.  What I do when I want to make a completely secure water supply is pull the water up from the water source through a grate by using a screw pump.  The pump goes one level above the water source.  Channel the floor at the pump entrance side and build a floor grate over it.  Connect the pump outlet side to your cistern.  When you need water, have a dwarf run the pump manually, or if you feel really ambitious rig up a water wheel or windmill or dwarven water reactor to power it.  This prevents anything from getting in from outside, and having the water be manually pumped means you're less likely to accidentally flood your fortress.

[Teneb]

So fully submerged grates don't block creatures? That seems kind of wrong.

It's not that they don't block creatures, they do. But due to a bug, they can get pushed through the grate/bars/fortification by pressurized water.

[Sphalerite]

It doesn't take pressurized or moving water for a creature to pass through a fortification.  Swimming creatures will go through completely submerged fortifications as if they weren't there at all, even in still, unpressurized water.

I think the problem with grates and fortifications is that they don't actually act as physical barriers.  As far as the game is concerned, creatures, items, and fluids can still pass through them.  I've seen thrown debris from traps get lodged in fortifications many times.   What I think is happening is that the pathfinding code is marking that space as unusable for pathing, so a creature trying to get somewhere will never voluntarily path through a fortification or wall grate.  However, water deeper than 4/7 is also marked as unusable by the pathfinding code, which is why your dwarves won't path through water even if you mod them to not need to breathe.  But animals which can swim, like cave crocodiles, have some alternate pathfinding logic which lets them move around randomly through water tiles.  I suspect that same system also unintentionally lets them move through fortifications and wall grates, but only when they're underwater.

[daggaz]

What ive been doing is just F...XF....R, F=fortification, X=floodgate, R=river, . = space.   This usually ensures that the water is low enough to keep building destroyers off your floodgate, but has an extra defense just in case.  The spacing between the floodgate and the second fortification is to ensure that if something does make it thru, it doesnt get trapped ON your floodgate, rendering it inoperable.   The set up requires a permanent drain, or the whole channel floods up to 7/7. 

[Sphalerite]

Instead of a floodgate, consider using a single-tile drawbridge.  It will block liquids just like a floodgate.  It's immune to building-destroyers.  It will also atomsmash anything but a very large creature when you close it, so you don't need to worry about objects or cave crocodiles jamming it open.

[Teneb]

You may want to make it a 2x1 bridge instead, since a 1x1 bridge uses the same tile for open and closed states. Makes you wonder if it's up or down if you leave it alone for over an in-game year.