Real-Time Data for the Nation
The link above links to the national info center for streamflow in the USA. You can either click on the map to zoom in on a particular area or use the pull down menu on the far right top to select a geographic area...such as your state. You can usually then select a river gauge in your area for live data from it, plus the recent history.
I use this all of the time in planning river trips. It is a lifesaver when you know that certain rivers can only be paddled at certain river levels. I try to keep a log of the river levels on my trips for future reference...especially if the river was borderline too low or too high.