None of the trophies in Horizon Zero Dawn: The Frozen Wilds are hard, but Drained the Flood can be confusing. While it might seem like you get this trophy after finishing the quest in the dam and draining the water, it actually happens after that.
When the water is drained, head downstairs and a new path will be open. Go down the new path and there will be a drain puzzle. Get the water to the exit and the trophy will pop.