Selecting stream/flowline length.

This is what I'm trying to do:

I have a single point along a river. I want to select upstream of that river a certain length (20 miles) and buffer. I know I can select by location, but that gives me a straight line distance. I want to select 20 miles along the polyline length. It seems like it should be simple, but I can't find the solution.

thanks in advance.