Oh, I didn't mention that I have a very tall column of up/down staircases above it.
So in theory, the area is accessible. That's how my miners dug it out (from above, on the up/down staircase the next z-level up). It's just that the masons for some reason aren't able to build there...
I'm not entirely sure how your setup works, but are you trying to construct the stairs, or dig them out? I think your problem may be that the "hanging" (mid-air) stairs you have so far are constructed by masons, but once you get into the sold rock (the river bed), you need to switch to designating stairs to be dug out with miners. (You could I suppose channel out, remove the ramp, and then construct stairs in the hole instead, but far more work for both you and the dwarves, with more chances for things to go wrong...)
Right, so the way it works is, the Z-level above what the picture shows is another set of 1x3 up/down stairs which were built from above, since they're in the air. What happens next is I told my miners to build up/down stairs in the ice, but I guess you can't do that, so it just mined it out. Now, since there's nothing there but "water" (as in, not water of any depth, but the kind of water you have in a bucket. It has a weight and a value. Anyways, in theory, it's on the ground. I can't dig down anymore because it's at "ground level" (when I hit [k], the list shows 'River' and 'Water,' the latter is mentioned above, though there's nothing actually blocking the way, as shown by the picture).
So the only thing to do is build my stairs from above. Yet, for some reason, they can't path to the construction zone even though the game recognises that it's possible to build there and I was able to designate it. (That is, I didn't get a redded-out "No access to materials" warning when trying to place it.) Does that illuminate the problem any more?
Also, I was digging out water-ice, not normal ice (in mid-Spring, the river will thaw and become 7/7 flowing water again), so any constructions I make there don't last. Thus, I HAVE to construct stairs.