If you're already a river angler, you're probably already in the know. If you're new to the scene (such as I am) consider this link pure gold.
The site is maintained by the US Geological Survey, and holds real time information about water levels on most major rivers and streams. If you're new at this or targeting a small body of water, you'll still have to see it in person. After you've fished a listed river, go check the site. You'll quickly correlate "the water was too high" to "the water was x feet deep." I've also checked the depth on days fellow bloggers remark about the water being "in perfect condition."
Once you know, in feet, how deep good water is, make an informed decision before you hit the road. If it's good, rock on! If it's bad, go to a lake instead.