How does the USGS collect streamflow?

1. A Gaging Site is Established

The USGS selects a suitable site along a river or stream and constructs a station to hold equipment that measures and records the height of the water surface (gage height or stage). A gaging station, such as the Contoocook River streamflow gage (fig. 2), can also include equipment that measures water-quality properties such as temperature, pH, dissolved oxygen, and dissolved chemicals. In addition, some gaging stations contain instruments for measuring weather conditions such as air temperature, precipitation, and wind speed.

Figure 2. Contoocook River streamflow-gaging station in Hopkinton, New Hampshire.

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.

The volume of water passing a specific point in a given interval of time is called streamflow discharge and is generally measured in cubic feet per second. The station does not directly measure discharge, so it must be determined by making measurements of the cross section area of the river and how fast water is flowing (velocity) past that section. Discharge is calculated by multiplying the width, depth, and average velocity of the section of the river.

Velocity is measured by using a current meter (fig. 5). The meter consists of cups that are rotated by the action of flowing water. The speed of the rotation depends on the velocity of the water passing by the cups. A technician records the number of revolutions in a given time interval and determines the velocity at the location of the meter. The stream is divided into segments and average velocity and depth of the stream are measured at each segment. The discharge in each of the segments are then summed to obtain the total stream discharge. Technicians obtain measurements several ways depending on the size and depth of the river. They will wade across small streams, use suspended cableway cars, or stand on bridges to measure streamflow in large rivers.

Figure 5. Technicians are measuring discharge of the river by making observations of width, depth, and velocity using a current meter. Insert shows a current meter attached to a weight.

Figure 6. An example of a rating curve.

4. Relation Between Water Level and Discharge is Made

A stage-discharge relation or rating curve (fig. 6) is used to relate river level to associated streamflow discharge. The rating curve for a specific stream location is developed by making successive discharge measurements in order to define and maintain a stage-discharge relation. These discharge measurements and their corresponding stages (or gage heights) are then plotted on a graph for each station. Continuous discharge throughout the year can be determined from the rating curve. Factors such as ice cover, debris, and vegetation growth can affect the stage-discharge relation and the data must be checked periodically to ensure accuracy.