I found one of the easiest ways to simplify the game without hacking or removing many sources of Fun is to lower the population cap. I found that once my fortress grew to 100 dwarves or beyond, I couldn't figure out how to manage all of them and keep them working productive jobs. So instead they all just sat around throwing parties and pestering my productive dwarves. Until you get enough experience to handle a fortress that size, try lowering the cap to 40 or 50 dwarves.
To do this in v0.31.08, edit data/init/d_init.txt and change the value of this line:
[POPULATION_CAP:200]
Note: your actual population may still go above the cap you set, you just stop getting new immigrant waves after you reach or surpass that cap. And apparently births don't respect caps. Also I think (but I haven't tested) that you can change this mid-game if you save and quit first.
Well you can't tunnel off the side of the map because the edge tile is never diggable, so that's out anyway. So it turns out you can turn an edge tile into fortifications which will drain water. First smooth it, then carve fortifications. (Thanks Nakedible!)
However, for those who consider this cheating or who are using a non-infinite water source, the only way is, indeed, a pump stack. This, while time consuming, isn't as terrible as you might think. (You'll probably have to do this later to bring magma up from the deeps, anyway.) The wiki has a description of optimal pump stack construction.
This will allow you to build a stack of mechanically operated pumps without putting extra machinery on each level. You only have to power one of the pumps in the stack. (Assuming that power source provides enough power for the whole stack.) Combine this with the (somewhat cheating) dwarven water-wheel reactor and you have a relatively painless pumping solution.
I like to put two gears between the power source and the pump stack and attach a lever to the gear nearest the pump stack. This allows me to turn the whole thing off if chaos ensues for any reason or I need to do maintenance to the system.
The alternative is of course a genuine water wheel setup or windmill setups, but these options will force you to be more choosy about where you locate the construction. Any axles or gears that lead from your power source to the pump stack consume power!
You might want to try building a smaller pump stack before going for one that spans 50 levels or more. Keep in mind it's very easy to flood your fortress with Fun!
Best Answer
The game tries to prevent you from building all the way to the edge of any map you embark on, probably to prevent your blocking of caravans, new immigrants, and goblin invasions.
The restrictions it puts on building close to the edge have limitations that other DF players have discovered:
1) You can build walls 5 tiles from the edge.
2) you can build fortifications 4 tiles from the edge.
3) you can channel 2 tiles from the edge.
4) you can build floors all the way to the edge.
5) You can build a drawbridge and then lower/extend it all the way to the edge.
If you want to create a drain for your river, you have two choices; find a way to empty the river over the edge of your map or channel it to some aquifer tiles, which can absorb any amount of water.
One way to empty water over the edge of the map is to dig down at the map edge and attempt to find one of the cavern layers under that point. If the cavern connects to the map edge, any water dumped down that hole will flow out of the map through the cavern.