I've been playing, admittedly, for only a few days with a finally moderately successful fortress. However, I found one issue with channels:
You can't fill them in or otherwise get rid of them.
Any reason? (Or maybe, I haven't figured out how yet?)
One problem this presented me was building a Floodgate and the dwarf gets stuck behind it (ok, so the AI needs fixing too....but I knew that watching a dwarf die of starvation after being surrounded on 7 sides by water (the other space also surrounded on 7 sides by water) from a failed feild flooding (I can't nflood it).
Anyway, dwarf gets stuck after putting in a flood gate, so I tell someone to remove it and I get a channel left behind: no big deal, place the floodgate one sqare over (and hope the dwarf doesn't trap himself again) and then wait to connect it, yadda yadda. Next spring comes around/I get it test worthy, and oops, that extra channel space meant I just flooded the farm-room permenently (farm area floods back to river, which then floods back the other way? Not sure what the cause is yet).
So the suggestion is a way to remove channels and maybe a way to "reclaim" perma-flood areas by building flood-berms/banks/some low wall type structure. Dwaves can climb onto it to build another berm on the water side (turning it into a surface to walk on) and continueing to the river (river can't be damned) and then by digging out the walls again (not gaining any material to use later like normally digging a wall) you can reclaim flooded areas and re-work your irrigation mistakes (as new players won't know exactly what caused the problem or why and will have to abandon just to start over--very annoying).
Thoughts?