Earlier this year, the USGS has released a web mapping service called Streamer.
It’s a fairly straightforward tool. A user can either trace a channel upstream or downstream. From there, users can get a summary or detailed report that has some basic information such as how many states the river is in, or how many USGS gages are on the trace.
A bit more background on the tool is available here: http://nhd.usgs.gov/newsletters/News_July_13.pdf