And if they dam your river or worse, poison it without you noticing?
If they can dam the river in a way that's effective, I will be very impressed with the AI 
But that's only 1 out of 5 possible renewable sources of water that I can think of (river/brook/stream, cave river, aquifer, muddy pool, ocean/lake). I suppose that's not 5 depending on how you split them. Underground pools are there, but not renewable. Ice is technically renewable due to buggy behavior (1/7 water->ice->7/7 water) or such. If they can poison it and you know they can do that, then you would have to seal off your intake and run off of reserves. Which could be very effective in a siege, but requires changes be made so that water is actually required for something, as it is currently, it's not needed for much. Assuming of course that they have unlimited poison, and can poison it as long as they're there. And that they can get to the water to poison it, which isn't very easy for all the other sources of water (poison an ocean? an aquifer? They might not be able to reach the other ones unless they can break through walls.
I think this is the direction we should be heading. Make it so that farming requires water even when farming on soil.
Goblins should poisin aquafiers and try to dam above ground rivers. This way the fortress actualy has to worry about supplies getting low. (something that would cause bad thoughts. Perhaps the game , as it already reconizes when the fortress is under siege, could change the regular "Mc Ulrist is worried about the low supplies" to the more siege related "Mc Ulrist is worried about the terrible effect the siege has on our supplies" kind of thoughts.
Ofcourse, with the new underground improvements the player had acces to masive underground lakes, but those lakes are protected by the never ending attacks of underground critters. Killing / wounding your dwarves, causing bad thoughts all over the place, and at that moment the dwarfs realize that there isnt enough water to treat the wounded / irrigate the farms.
This would make sieges more difficult and cause siege related bad thougts without resorting to the, in my opinion, 'gamey' sollutions like arbitrary mood penalty's
Also: Iam sorry for raping the english language.
