Extract River network with Qgis GRASS

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I´m working with Qgis2.6.1 and GRASS. I have used the r.watershed module to create a flow accumulation map. With r.mapcalc I have extracted the river network, then i´m using r.thin to thin out to a single pixel width the lines on the binary raster map. Then i´m using r.to.vect.line to convert raster to vector lines. Everything looks good however the network doesn’t have smooth corners...please look at the picture, my rivers are blue.Can I change something in this procedure to diminish this effect?

Thank you

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