The problem with moving water to a higher location is that it wants to move back down...sometimes catastrophically so. If Melkor has created a reservoir higher up Taniquetal than Valmar, there is the potential for a dangerous flood if the dam breaks.
An example would be the Johnstown Flood (1889):
https://www.nps.gov/jofl/learn/historyculture/south-fork-dam.htm
For a more sinister example of a unique type of danger, there is Lake Nyos (in Cameroon). It also is in danger of causing a massive catastrophic flood if seismic activity destroys its natural dam, but....it has the fun addition of accumulating CO2 from volcanic activity in the area. I don't think that 'chained' Melkor has access to his volcano-making abilities, though, and don't know what Tulkas would destroy.
https://en.wikipedia.org/wiki/Lake_Nyos
An example would be the Johnstown Flood (1889):
https://www.nps.gov/jofl/learn/historyculture/south-fork-dam.htm
For a more sinister example of a unique type of danger, there is Lake Nyos (in Cameroon). It also is in danger of causing a massive catastrophic flood if seismic activity destroys its natural dam, but....it has the fun addition of accumulating CO2 from volcanic activity in the area. I don't think that 'chained' Melkor has access to his volcano-making abilities, though, and don't know what Tulkas would destroy.
https://en.wikipedia.org/wiki/Lake_Nyos