I have the following question:
In the picture below you see 2 lakes. Lake 1 is the one with the city and lake 2 is the lake in the upper left.
Now I want to drain water from lake 2 and let it flow into lake 1. But lake 2 is on the edge of the known world and if I let it flow into lake 1, will it work? Or do I get tons of errors.
Should work but your city will get flooded.
you could lower the water level in the lower lake with some mining in the lake.
Can you share the seed to this map? 
I wanted to try something like that. :>
That’s an amazing world spawn
It looks as though the water levels between the two lakes are equal; so that should work fine – you’ll lose a fraction of a block worth of water height as some water moves to fill the dug-out gap, but given how much you have in the big lake I doubt it will even be enough to notice.
I would suggest you dig the connecting “river” in small sections to prevent large water flows; otherwise the game gets very laggy trying to merge lots of different sections together. It’s also a good idea to dig almost up to the other lake, and then let them merge all along that face all at once; rather than digging the whole river and allowing the water to “meet in the middle”, since that can again cause a fair bit of lag.
You may get a single error if the water fails to equalise properly; although that may be enough to make the Lua throw a tantrum… worth taking a backup save just in case.
Good luck, and let us know how it turns out 
As you can see I cut the upper level of the island, otherwise the difference between water and land would be to much. I will try it tomorrow and I’m defenitly saving it. 
World seed: 6969
I did testing on my map with a lake higher than my town. Water drained for as deep as my drainage was. The pool it drained into was 45 x 18. I drained a whole level of my large lake into that area and it did not increase further.
Made a large empty whole/pool next to the lake at the same level it was and connected them. The water filled up to the level of the lake and nothing more, also the large lake did not drain.
Loaded your map (6969) and had a look. The lake you want to drain into the one around your town is of the same level so,… it will not drain nor floor your town. Don’t know of errors though, sorry.
When I was playing dwarf fortress I used to start on maps with a volcano\high magma chamber and I would dig conduit tunnels down from it down to a soft earth area so I could fill in building molds I dug in the ground, I waited a bit for it to cool down and so i can clear the earth from around it and what would be left there would be the casted buildings made out of pure Basalt.
And so - with the opening of a valve I had a casting operation for a full blown Castle which I called cast’le because I dig puns, you dig it?
I wish we had valves, floodgate and retractable bridges… I’d even stop with the puns, I swear!
Sounds like a w’hole lot of work. I dig it…
