| RulzLawyer |
"can tread on any liquid as if it were firm ground."
This sentence I think covers it. You can walk on firm ground that is horizontal, or in the case of the "running water" being falling just a matter as to what degree, it would be what the character could normally traverse, up to possibly a 60* slant. If the person can't walk up a wall or cliff, they can't walk up the waterfall.
"The subjects can walk, run, charge, or otherwise move across the surface as if it were normal ground."
"Whatever, I'll just walk under the waterfall itself and become partially or wholly submerged to be launched toward the surface at 60 feet per round. How high is the waterfall?"
Unless the character drops the spell, he couldn't walk under the waterfall, because you can't move into normal ground barring digging.
Now, having said that, I also think that this is a fantasy world, and GMs should reward players who think outside the box. If he was under the waterfall, and THEN cast the spell, I would give the character a chance to "ride" the waterfall up to the top. Assign a DC and have them roll to be able to float like an air bubble up to the top of the waterfall.