A hydroacoustic instrument that measures water velocity throughout the water column by emitting acoustic pulses and analyzing the Doppler shift of signals reflected from suspended particles. ADCPs have revolutionized river discharge measurement.
An Acoustic Doppler Current Profiler (ADCP) is a sophisticated hydroacoustic instrument that measures water velocity at multiple depths simultaneously by transmitting acoustic pulses (typically 300 kHz to 2 MHz) and analyzing the Doppler frequency shift of signals backscattered from suspended particles and bubbles moving with the water. ADCPs use three or four acoustic transducers oriented at different angles (typically 20-30 degrees from vertical) to resolve three-dimensional velocity components. When mounted on a boat or tethered platform that traverses a river cross-section, an ADCP measures velocity throughout the depth profile while simultaneously tracking bottom depth and boat position (via GPS or bottom tracking), enabling calculation of total river discharge in a single transect. The USGS adopted ADCP technology in the 1990s and it has largely replaced mechanical current meters for discharge measurements in medium to large rivers. Moving-bed conditions in sand-bed rivers require GPS referencing to correct bottom-track errors. ADCPs are also deployed as stationary instruments for continuous velocity monitoring at fixed stations, and horizontal ADCPs (H-ADCPs) measure velocity in a horizontal plane for index-velocity discharge computation. Typical measurement accuracy is 0.25-1% of the measured velocity. Side-looking and upward-looking ADCP configurations expand the range of deployment options for lakes, estuaries, and ocean environments.