A Decanter of Endless Water puts out a little under 2 billion gallons of water in a year, and it can be salt water at the user's option. It's been said that the classic adventure setting, Blackmoor, came to be because someone dropped such a decanter into a hole, and the water eventually created a huge swamp.
All you've done is the reverse.
So what would I do, as the DM?
Congratulations, you've just created a navigational hazard, a new Maelstrom if you will, and laid the foundation for some future adventure. probably by another group. For your group? Life goes on.
As a player I never, *ever* presume that my character is the first person in the history of the world to come up with some neat and nifty way to destroy a monster, and economy, or a world.
Do any of you ever read the (extremely intermittent) webcomic
Dungeons and Denizens? The main character is "Tech Support" in a dungeon, resetting traps, making sure all the wandering monsters know their schedules, feeding the pit monsters and cleaning up after the slime molds.
Consider having a secret society in your world, the Caretakers Guild, who basically does the same thing on a global basis. They close those nasty ol' dimensional rifts that PCs sometimes leave open when an inconvenient TPK comes by, they spread the rumors of monsters and treasures, write up appropriately obscure treasure maps, and make sure that there are some heroes scheduled to avert the monthly apocalypse. They're the ones who make sure the lightning crashes at dramatically significant moments, they build appropriately rickety rope bridges over bottomless chasms, and make sure that there's a Dragon or a great mystery around when you really need one.