The USGS Water Science School
The U.S. Geological Survey (USGS) has thousands of these stream-gaging systems installed nationwide. It is still the most common way to measure the heights (and, indirectly, the amount of water flowing at any given time) of streams. The technology is changing. The more modern systems detect the heights of streams and transmit that information via satellites to a USGS office.
In the past, the most common type of stream gage worked by recording stream height on a moving paper roll. Nowadays gage heights are stored electronically in an instrument in the box on top of the big pipe (stilling well).
Here's how some of these gages generally work: The big metal pipe going down into the water is a stilling well. The box on top (made of metal to resist bullets!) contains a measuring instrument (data logger) that has a pulley with a metal wire holding a float at one end. As the water in the stream moves up and down, the water in the vertical metal pipe moves also. The float on the wire goes up and down with the water. As the wire moves, the pulley turns, which changes the gage-height reading. Then the gage-height readings are recorded automatically in the data logger. When a technician visits the site they can either take data-storage card back to the office, or download the data on site.
The most modern gage houses have a satellite-transmission system that can upload data right to the USGS office -- essentially a real-time system.
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