The USGS Current Water Data site is one I visit often for updates on water conditions around Pennsylvania and other states. There are many locations that I fish with gauges on the water, and if not, there tends to be at least one in the same watershed. This information is critical for making a decision on where and when to fish.
Aside from the information that can be learned from a quick glance (such as the color-coded meanings), once you click on a gauge, more historical data is available to you. Of particular value, I tend to use the water flow and temperature information the most. With the dog days of summer upon us, a gauge with recent temperatures can help you decide on fishing for trout or smallmouth bass!
For a quick tip, I keep an updated record on Excel of all of the fishing trips I make, and one of the first pieces of data entered is the USGS stream flow (in cfs), then the report continues. This gives me one additional piece of information to examine before making my next trip to that waterway.
I am sure the site will be as beneficial to you as it has been to me: