I notice your project projection is WGS84 (EPSG:4326). The distance buffer is always in the units of the projection (i.e. not necessarily meters!). In this case it will be decimal degrees. This is why your buffer is so big (note the scale differences in your two screen shots).
Reproject your GT_Rios data to an SRS which has meters as its units and then do your buffer operation (preferably put the whole project into the same projection). I think your screen shots are in Portuguese (if it is Spanish I am so sorry! Please forgive my ignorance) but I don't know whether you are in Portugal or Brazil (or somewhere else) so I can't suggest a projection. You should be able to find something suitable with a bit of research or by posting a new question to this board.
Also, I personally would also increase the "Segmentos a aproximar" value from 5 to say 32 as this will give you a smoother buffer (though you do get a lot more vertices).