Creating a polygon outside another but using existing as boundary

Question

I need two polygons. I already have one (the river edge boundary as seen below)

What i want to do is create another polygon which covers the part of the raster which does not fall inside the river polygon. It will therefore need to snap to the outside of the river polygon which is a complex shape. I have tried the auto-complete polygon tool but not having much luck. Furthermore, i have part of the river polygon which is separate from the rest. This would need to be classed as the river polygon and not outwith.

Answer 1

I would recommend using Raster to Polygon to create a polygon from the raster.

Then you can use Erase to remove the area covered by the river polygon(s). The output will be your desired nonriver-raster polygon. Note: this tool require an advanced license