Extra:
It's not in a particularly wet place aside from the river. The common rains happen due to the plants sweating what they absorb very quickly due to heat.
It's also worth noting that even if floods and general problems due to too much water don't happen, problems with too
little water WILL happen. Locking the depository due to the lack of deep roots as well as tossing the water into the atmosphere is going to lower the water levels in a good chunk of the country, causing - at the very least - a mass scale energy crisis, since we essentially run on hydroelectric powerplants:
http://www.eia.gov/todayinenergy/detail.php?id=16731
This has happened in the recent past, mostly in the southern regions (graph's in portuguese, but blue means major energy crisis while greenish gray means mild energy crisis):

The cause, as mentioned? Low water levels on reservoirs.

You probably don't care about this last bit (at least I personally don't, so I wouldn't expect you to either), but in the case you do, this also means a large scale genocide of river-based indigenous tribes, which are visibly focused on the amazon:
Edited 1/10/2017 06:03:07