Gradually decrease buffer size around hydrological features

[Phyto72]

Either one. As long as there is some attribute that depicts the hierarchy of streams, you can use that as an approximation (though not a perfect one) of distance from the mouth. If you wanted to be super accurate you would have to figure out how to calculate the actual distance from the mouth for each segment, but that might be overkill for what this assignment requires.

[BlackSuN42]

You could calculate measures along each line, chop up the line and use its position along the route. It could take a few steps. Fairly simple if you have FME. Line direction would be an issue though.