My goal is to determine the possible area covered by interpreted Tertiary drainages. To keep my example simple, I have a DEM and a River polyline. I would like to see the area a polygon (or raster) would cover if it was 100 feet above my River polyline. Being that the river decreases in elevation as it drains, the polygon would cover most of the valley but some might spill over the top and could possibly show where old drainages might have occurred. I have attempted to google this but have not found anything. Seems simple enough to do.
Can someone lead me in the right direction?
I am using ArcMap 10.5