It would be constructive, since it creates more than it destroys.
See this article for more information on the Amazon:
http://www.freshwateraquariumplants.com/amazonbiotope/amazonriverfacts.html
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they are made by both
Volcanos I guess
hurricanes can be both constructive or destructive.
An earthquake in some sense is both constructive and destructive.
Plains are neither Constructive or destructive. they do not harm or majorly help. if there had to be one it would be constructive, it would help wildlife create stuff.