Even though I set a high traffic area in a safe part to cross,
Knowing waterfalls as I do, were you trying to get them to cross at a safer 7/7ths deep area, at all? They'll only ever attempt to path through... 4/7ths, I think... maximum depth (at the point at which they calculate their path), and from your more recent posts that
now you have a bridge, it doesn't sound like there was anywhere to cross
except the dangerous lip of the waterfall[1].
...and restricted a huge chunk of area on and around the head of the waterfall,
Restricted as in "Restricted priority"? Much apart from burrows (as already explaining) not affecting the intermediate pathing of jobs (although by restricting the woodcutters to burrows
without the unnoticed accidentally set-for-woodcutting tree, and the hunters
probably to burrows that are only on the near of the river), Restricted Priority just makes it more costly to path through, not "never go here". If it's the only route (and I still think it was) to try to path through the fluctuating low-water-level edging at the top of the waterfall, then the pathing solution going through that Restricted Priority area will still be as dangerous as without.
The quickest way I've heard of to prevent inadvertent nature walks across similarly dangerous rivers is to designate a floor or three to be constructed on the near-side bank, directly above the ramps that might at some point have formed part of the path down onto the river-bed. These days, with ramps alongside all channelled-down surface water areas, the aforementioned floors (which will safely be built only from the top of the bank, not the bottom), the ramps underneath will be rendered unpathable (directly, in either direction) with the bank-side you have just extended, and thus no valid path at all will be experienced.
If you
do have access to the other side, because of seasonal water fluctuations (e.g. freezing upstream at the source, zero/delayed freezing over the waterfall so that it drains away) then as soon as you have access to the other bank (and it is safe to do so) set another lot of bank-extending walls from the other side. Or just make your bridge (whether the literal building, or composed of successive floor-constructions) all the way across the river at this waterfall/cliff-edge location from the start, thus covering up both sides' ramps. (Modify according to your local circumstances, of course, because there might actually need to be a much wider length of bank protecting if the low-water values extend well up the river basin, and a bridge
only at the end might end up attracting builders across the shallows that you had hoped to avoid.)
[1] Which also, as it goes above and below the 4..5/7ths dividing line, or whatever the point of decision might be, would have had even chances of causing travel-cancellation upon about to attempt to move into a now-too-deep bit.