I'm using ArcMap 10.3.1
I have a polygon shapefile for areas upstream of 5 monitoring sites. I used
merge to merge all these polygons together. You can download the resulting shapefile from here.
I used this shapefile in
R v. 3.2.5 for further analysis. In the result of this analysis, below, there was a line cutting through the polygons
I checked the shapefile again in ArcMap 10.3.1 to find that after merging, there were traces outside the polygons.
I tried to remove them but I couldn't.