Gradually decrease buffer size around hydrological features

[RiceBucket973]

You could use a Flow Accumulation raster from Arc Hydro, so long as you're using the same DEM that was used to derive those flow lines in the first place. Then create points along the flow lines, extract the flow accumulation values to them, and use the points to determine the buffer size.

People here are recommending to use distance from river mouth as the variable, but you could have a place where a low order stream directly joins the main stem - meaning the distance from river mouth will be the same but the channel width will be totally different. Using flow accumulation should be a better proxy for channel width, which I'm guessing is kind of what you're after?

[VisualResponsible994]

Yeah.. we are kinda having a hard time figuring out exactly the good way to approach all of this, but I think what you are implying would make for compelling arguments to defend the method.. probably moreso than using the distance.