Note: If you're a player in my Rhine game, please stay out of this thread 
What happens if two spells directly oppose each other?
For example, Maga A casts a sun-duation Rego Aquam spell to make a small pond calm and quiet on a windy day. Magus B comes along and casts a different Rego Aquam spell to make the pond water form small waves. Does Magus B's spell work? Would it be a different answer if Magus B's spell was cast first, and Maga A tried to calm the pond down afterwards?
The actual scenario for my game involves the entire Rhine river, but is more or less analogous. The PCs have found out about Schwall of Bjornaer's plot to awaken the Rhine spirit. They told Falke of Bjornaer about this plan, and she's good enough at Aquam to invent a suitably powerful ReAq spell in time. But will Schwall/Rhenus just blow through her spell as though it were a normal calm day on the Rhine? This doesn't answer the question of whether Falke's spell is a long-term solution, but I'd like to understand if it can work in the short-term at least.