Until the 1990s, the U.S. Geological Survey (USGS) had been making streamflow measurements using the same type of equipment for more than 100 years. The Price AA current meter was developed by USGS engineers in 1896. Until recently, the majority of all streamflow measurements made by the USGS were made using this instrument. In the mid-1990s, a new technology emerged in the field of inland streamflow monitoring. The acoustic Doppler current profiler (ADCP), originally developed for oceanographic work, was adapted for inland streamflow measurements. This instrument is transforming the USGS streamgaging program.
The ADCP transmits an acoustic pulse through the water column. A 'Doppler shift' is measured as the signal is reflected off of particles in the water, such as sediment and microorganisms. Based on the assumption that the particles in the water are traveling at the same velocity as the water itself, a water velocity is computed.