Wouldn't it be easier to simply dig an access tunnel to your moat, remove the ramps and leave, walling off the tunnel as you go?
With the ramps being talked about being flooded by the watersource, I don't think that's an option.
If the water doesn't freeze, and you're not in the mood to actually dam the river, I would probably choose (say) the inside part of the moat (i.e next to wall) as the breaching point, do that by normal mining by a dwarf. You still have two other 'ground level' tiles with mass below, and one (over where you've just dug) that's a floor only. Go in and channel the floor-only, then the 'middle' block away, then the outer block. You're left with a single ramp.
As in the following side-view:
O I
u n
t n
e e
r r
____#
##### The bank, still complete, wall at 'ground level' shown to right.
____#
### # The bank, breached, but still bridged (at least up until the wall)
____#
###~# Water immediately starts flowing through it
___ #
###~# Channel the floor tile
__ #
##\~# Channel the next on (get a ramp in the middle of the eventual moat)
_ #
#\~~# Channel the outer tile (lose the central ramp, gain one at the outside)
But there are probably better ways of doing it.
(Hmm, what happens if you channel down onto a smoothed then forifification-carved wall? Wouldn't work on a soil layer, but as fortifications aren't Z+1 walkable, I bet that a channel on the level above doesn't leave anything to walk on, and it's effectively solid. ICBW. I don't have DF at hand, right now , to test. And you'd still have problems w.r.t. getting all your fortifications carved simultaneously, so that the water leaking through the first doesn't adversely affect the willingness of the dwarf(s) tasked to carve the second and third.)