OK hmm. First time I did it, I had something like
F : T : T: D : R where F is Fortification block, T is open tunnel, D is a dirt filled block, and R is a river block. I then channeled above the D and the adjacent T block. My dwarves were idiots tho, and ended up jumping in and dying (tho I know how to fix that now). The channels ran fine tho, and I got plenty of flow further down all the way past the floodgates i installed and to the tile that leads down into the drop chutes for the waterfalls. Thing is, I didnt like how the open channels looked at the edge, and I would really like to avoid building floor plates.
Second time I did it, I jammed the fortification block all the way up against the river embankment:
F : D : R , and just channeled the D block from above. This gave me a ramp directly inbetween my fortification block and the river current. Unfortunately, the water did not flood the tunnels completely, and got shallower and shallower. Some of them were close enough, and I got good waterfalls, others were only half way decent, and some falls were completely dry. I have a feeling that it might be because I was too close to a natural waterfall in the river (were the depths began to lessen the closer you got), but just want to make sure before I spend that much time again.
Question: If I do end up tunneling in from underneath the river, will I risk getting more pressure than normal at the bottom of my drainage tunnel to the end of the map? Also, will I risk draining the entire river? I am making nine channels, the first time i did this, there was no appreciable drop in the main river. Thats important because it is a major defensive feature for my fort. All I really want is to make sure my waterfalls are running nicely.