Figure 3. Diagram of a streamflow gage with a bubble system used for measuring river stage.
Figure 4. Stream-gaging equipment inside the gage house that is used to measure, record, and transmit river levels.
2. Water Level is Measured and Recorded
The gage height (or stage) of a river is most commonly measured through the use of a stilling well or a bubble system.
Stilling well gages are used when a gage can be built immediately adjacent to a river. The well is connected to the stream with pipes such that when the water level changes in the stream, the level simultaneously changes in the well. A float in the well is then connected to a recorder or data collection platform.
A bubble system can be used when construction of a well immediately next to a river is not feasible (fig. 3). The bubble system requires a long open-ended pipe extending from the river to the gaging station. One end of the pipe is attached securely to a pier below the water surface. Pressurized gas (usually nitrogen or air) is forced through the pipe from inside the gage and out a submerged opening called an orifice. Because the pressure in the pipe is determined by how deep the water is over the orifice, a change in the height of the river produces a corresponding change in pressure in the pipe. This change is then converted to an electronic signal by a transducer inside the gage house. Data from the transducer is fed to a recorder, or data collection platform, which records the corresponding river height (fig. 4).
An outside reference gage, typically a horizontal graduated ruler called a staff gage or inclined staff gage (fig. 3), is read periodically to verify that the recorded gage heights from the stilling well or bubbler systems are the same as the water level in the stream.