The USGS runs a network of stream gauges across the country which throw data onto the Internet in near "real time" (updated once an hour).  This info has been available for years over the web, which is essential info if you're about to drive to a stream and want to make sure it's not blown out.

The USGS recently made available a KML which will display this same information within Google Earth's map.  It's a little unreliable, but it's nice when it works.  The KML you download here is configured to show streamflow across the east coast from Maine to Georgia.  If you need additional states, or would like them state by state (presumably it would respond a little quicker, and be less funky), you can configure and download your own KML(s) from their website:

http://waterwatch.usgs.gov/new//index.php?m=real&w=kml

When you're mucking around their site, be sure you're getting what they call "real time" streamflow.  They also have KMLs that display historic streamflows, which probably aren't what you want.