Rivers.run provides water levels, water temperature, and other information, using data collected from over 10,000 gauges. (You can see a full list at https://rivers.run)
Rivers.run also knows plenty about whitewater rivers, and can tell you if a river is at a good level for paddling - just ask rivers.run if a river is running or paddleable, etc, and it will tell you how current levels compare to optimal ones.
Currently, rivers.run only works with gauges from USGS (United States Geological Survey). Additional sources of water levels may be supported in the future.
Information on rivers.run is crowdsourced - so if your favorite river is not available, is missing information on runnable levels, or has other issues, you can add it. Just go to https://rivers.run/FAQ or send an email to firstname.lastname@example.org to learn how to get started.