How the U.S. Geological Survey Monitors Water

Science Center Objects

There are many pieces of equipment, both mechanical and electronic, that are installed at stream-monitoring sites all around the world to measure, record, and transmit both water-quantity and water-quality information. The U.S. Geological Survey (USGS) monitors "real-time" streamflow and water-quality conditions for thousands of streams nationwide.

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Corrugated-galvanized-steel stilling well and shelter.

Corrugated-galvanized-steel stilling well and shelter.

USGS hydrologists monitor water resources

Long-term monitoring of water resources requires knowledge, skills, and commitment developed over decades by the men and women of USGS. USGS streamgagers and hydrologists must develop and follow precise scientific procedures to assure the quality of water measurements and to properly interpret the meaning of water data. High-quality data is essential for the wise management and use of water resources. Technological advances have improved the amount and timing of data, and made stream monitoring more complex, but these have not reduced the need to physically measure the stream over the range of possible flows, particularly during floods, whenever they occur.


Float-tape gage example

Float-tape gage with analog or SDI–12 shaft encoder.

Stream-stage monitoring instrumentation

This picture shows one type of USGS gage structure which is attached to a stilling well (the vertical pipe), and the shelter contains measurement and data-transmission instruments. Equipment in the gage house continuously measure stream height (stage) every 15 minutes. Stilling wells are used at locations where they can built next to the stream, but other systems monitoring water pressure can be used in different situations.

The box on top of the pipe contains a measuring instrument (data logger) that has a pulley with a metal tape holding a float at one end. As the water in the stream rises and falls, the water in the vertical metal pipe also moves and the float on the wire goes up and down with the water level. As the wire moves, the pulley turns, which changes the stream-stage (gage height) reading. The stream-stage readings are recorded by the data logger. Many gages have a solar-powered GOES satellite system that transmits and uploads stream-stage and water-quality measurements directly to USGS databases to give virtually "real-time" readings of stream stage, streamflow (computed using the stream stage/streamflow relation), and water-quality measurements.


Water-quality monitoring instrumentation

Streams in urban settings can be affected by the pressures of urban development, and thus, the potential for water-quality problems are high. Possible sources of problems are:

  • Sediment runoff from construction sites
  • Potential pollution from runoff from roads and parking lots
  • Inflow of warmer water from impervious surfaces (roads, parking lots)
  • Fertilizer (nitrogen and phosphorus) runoff from yards and gardens
  • Bacteria and pathogens from animal and human wastes
  • Runoff containing pesticides and pharmaceutical residue
  • Industrial wastes
  • Trash

It is important to monitor water quality not only to establish baseline water-quality information, but also to allow for timely notification when water quality changes. USGS installs automated water-quality monitoring sites which might consist of a stilling well and instrument shelter. The stilling well contains probes and instruments submerged in the stream and the instrument shelter houses the data-transmission and automated stream-sampling and storage equipment.

Photo of continuous water-quality sensor at USGS gage site

Continuous water-quality monitoring equipment at a USGS gage site.

Continuous water-quality monitoring equipment at USGS gage sites contains probes that continually measure temperature, specific conductance, dissolved oxygen, and pH. They sometimes also contain tubes to collect water samples during storm events and store the samples in the refrigerated housing structure. It is important to sample the water during storm events, because it is during high water that some water-quality problems, such as high levels of bacteria, sediment, and nutrients (nitrogen and phosphorus) occur.


Sources and more information

 • Discharge measurements at gaging stations, USGS Techniques manual
 • A Day in the Life of a USGS Water Scientist
 • Effects of Urban Development on Floods
 • USGS Real-Time Streamflow Data FAQs