|Check out the carry-on sized panel on top
Two days before Christmas, I corresponded via email with a USGS rep about the solar panel on top. He works with a team that runs more than 200 USGS stream units in MD, DE and DC. He told me that the vast majority are powered by 20-40 watt solar panels. The panels power water level sensing, information gathering and satellite radio signal transmitters. The data are transmitted hourly and are uploaded automatically to web pages for EACH station.
If you walk up to the station, you will see a small green and white USGS sign about the unit. It reads, "The recorded water level and corresponding rate of flow are used for flood forecasting, reservoir operations, design of bridges and culverts, interstate and intrastate water-rights claims, and many other projects". 01648000 has been recording since 1929.
For those interested in learning more about USGS stuff in their area, here's the station map. In DC alone, there are 11 stations like old reliable (pictured). Heck, there's even one a mile south of the one I saw. I need to start keeping my eyes peeled.