This Spanish-language infographic by the Fundacion Solon lays out 14 reasons to say no to mega hydroelectric plants:
deforestation
flooding
relocation of communities
lack of consultation
obstruct free movement of boats and fish
contaminate water (e.g., mercury)
potential for dam collapse
not clean energy
higher costs and lower return than promised
will increase national debt
corruption
will not be profitable
there are other alternatives
is an obsolete model of development