Handy Compilation Of Water Data
The USGS collects an insanely vast amount of information about, well about a lot of things, but in particular about water. In fact, they monitor water conditions at about 1.5 million sites across the country including all 50 States, the District of Columbia, Puerto Rico, the Virgin Islands, Guam, American Samoa and the Commonwealth of the Northern Mariana Islands.
That's a lot of data. Fortunately, they also compile and present that information in a variety of useful ways. The main water-data web resource is located here. That page provides links for, among other things, real-time or recent surface-water, groundwater, or water-quality data.
To see a handy list of information for California's rivers, click this link.