Making Streamflow Measurements
The flow, or discharge, in major streams is measured at gaging stations by hydrographers (hydrologic technicians) who visit each site about every 6 weeks. Discharge is the amount if water that moves past a specific point for a period of time and is measured in cubic feet per second (cfs). For example, if a stream has a discharge measurement of 15 cfs, this means that 15 cubic feet (about 112 gallons) of water is flowing past that section of the stream channel each second. Discharge is determined by measuring a cross-section of the stream channel (area) and multiplying it by the speed (velocity) of the water. A cross section of a stream is determined by multiplying the width of the stream by the depth of the stream. Because stream bottoms are not smooth, USGS hydrologic technicians make measurements at several spots across a stream. Velocity is also measured at several depths, because the velocity of the water differs from the top of the stream to the stream bottom.
Current meters are one way of measuring water depth and velocity along a cross section. A current meter is a rod with a set of cups on one end that spin in the water as the water flows by. The cups click after each rotation. Hydrologic technicians count the number of clicks over a specified period of time to determine the velocity of the water. Many USGS offices now use an Acoustic Doppler Velocimeter (ADV) called FlowTracker which attaches to the top of a wading rod. FlowTracker measures water velocity using a 2D or 3D probe that sends a burst of data through the water. This data is reflected back to the ADV from sediment particles in the water. The FlowTracker processor averages the data together and returns a velocity measurement.
Acoustic Doppler Current profilers (ADCPs) are another way to measure water velocity. ADCPs use the Doppler effect to measure velocity. The ADCP sends a sound pulse into the water then measures the change in frequency when the pulse is reflected back by sediment or other particles in the water. The ADCP can also measure water depth by measuring the time it takes for a pulse to be reflected back from the river bottom. ADCPs are mounted on a small boat. The boat is then hooked to a cable or rope that spans the stream.
Sometimes, the only way to measure discharge is from a bridge. A crane is used to drop a current meter and weight down to the stream below. In the photo, the current meter is right above the yellow weight. The crane is weighted on one side to keep the crane from tipping over the bridge when the weight and current meter are lowered. A hydrologic technician then counts the clicks from the current meter to determine water velocity. The crane is then rolled along the bridge to take the next measurement.
Cableways are used to make discharge measurements on streams and rivers that are too wide, too swift, or too dangerous to wade. Cableways are anchored on both banks of a stream. Scientists use a cable car to reach the middle of the stream then lower a current meter attached to the car to the stream. The video below shows a USGS scientist crossing the Carson River in a cable car.
For many streams, USGS uses a streamgage to estimate streamflow. A streamgage uses floats inside a stilling well or a pressure transducer to measure river stage. A data logger, inside the gagehouse, stores the river stage data until the data is transmitted back to the USGS office. Data is transmitted back to the USGS office at a predetermined interval (usually every 1-4 hours). At the USGS office, a rating curve relating stage to streamflow is applied to determine discharge. The stage and discharge measurements are both then sent to NWISweb.
A staff gage is used in streams and lakes to allow hydrologic technicians a way to quickly measure the stage, or height, of the water.
Additional detailed information about streamflow measurements is available in USGS Factsheet 2005-3131 and USGS Factsheet 2011-3001. A detailed description of equipment and methodology used for stage measurements is available in USGS Techniques and Methods 3-A7. Techniques and standards for making discharge measurements are available in USGS Techniques and Methods 3-A8. Techniques for determining levels at gaging stations are available in USGS Techniques and Methods 3-19.