Now, the USGS river discharge maps are available state-by-state via a Google Maps interface. For those of you who don't know what the graphs mean, investigate what "discharge" and "gage height" are. If you don't want to do the investigation, then - in brief - "discharge" is how much water is flowing past a certain point measured as volume/time (e.g., cubic feet per second), and "gage height" is how high the water is in the river measured as length (e.g., feet). Due to hydraulic geometries of rivers, the shape of these two graphs is usually similar.
The cool thing about this post is not that there is a lot of river data out there (of course, that is cool), but that the data are now connected with a Google Maps interface, making it much easier to figure out where the sites are on the ground. (Which - if you are working with these sorts of things - is a really nice thing, let me tell you.)
There is also the ability to get a GIS shapefile of all the sampling locations, as well as looking at the data on Google Earth KML.