I've river polygon data representing the water quality. This polygon data were derived from polyline dataset according to a buffer distance in the attribute table. Unfortunately I don't have this line dataset. So my "possibly naive" question is - is it possible to "re-buffer" the polygon data set to have polyline data again? Maybe there is a way to draw the centerline of each polygon?

I prefer to work with qgis 2.6 but also have ArcGIS 2.2 available.