How to classify a DEM base on the lowest elevation of a stream (stream generated by flow accumulation of same DEM)?

[JalilGhaznawi]

How to classify a DEM base on the lowest elevation of a stream (stream generated by flow accumulation of same DEM)? Or should I say, I want to find those elevation of DEM that are close to the floor of the stream. It's not a simple classification because the stream floor does not have same elevations in its whole lenght, meaning that there is a big elevation difference between start and end point of stream (Stream has alot of slope!) Any solution please... Thanks And sorry for my poor English❤

[DeanDjokic]

I think you might be referring to the HAND (height above nearest drainage) concept (if you search this topic, you will find a lot of good references).

The tool that allows you to derive HAND surface based on preprocesed DEM (you will need: DEM, flow direction, and stream rasters) is the "Flow Distance" Spatial Analyst tool.  Use "Vertical" as the "Distance Type" in the user interface.

This function generates for cell in the raster a difference between the value at that cell and the value of the cell in the river "nearest" to the cell - thus "height above nearest drainage".  Review the function help - there are several options and the choice might depend on the type of analysis you are going to do with that surface.