If you click on a state (e.g., Pennsylvania) you can zoom in and then click on a particular gage to see the current flow state and its change with time (hydrograph) and build your own graphs, etc.
Here is the page for the Conestoga River at Lancaster, PA which at least right now (Sunday afternoon August 28, 2011) is on the steep rising limb of the hydrograph:
Location is here at this link
This is a critical USGS function that is under threat of severe reduction due to budget cuts.
I noticed that sometimes you have to reload a few times or just wait for the main map to appear. I don't know if they are getting heavy loads due to public interest or what is going on.
Hat tip to Michelle Cooke for this idea.