Recently we sent a FISHBIO hydrologist out on the water with cbec inc., eco engineering to take measurements of water velocity in a Central Valley river. Fastened to the transom of the boat is an Acoustic Doppler Current Profiler (ADCP). The ADCP (Teledyne RD Instruments) has been used for decades in oceans, estuaries, and rivers to gather detailed information on water flow. It is a type of sonar that uses pings of high-pitched sound to calculate the velocity structure of a river channel or an ocean current using the Doppler effect. In this case the instrument measured minute variations in water velocity throughout the entire water column, which were then used to calculate channel discharge (i.e. flow) for that reach of the river.
Photo source: FISHBIO