An acoustic velocity meter (AVM) has been developed to measure river discharge continuously. The AVM has breakthroughs as follows: 1) accurate time with GPS clock signals, 2) high signalto-noise ratio with tenth order M-sequence, 3) thrifty power consumption, reduction in size and weight. The AVM installed at the Ohtagawa diversion channel, where the water level and salinity change significantly, successfully measured the cross-sectional average velocity and discharge for a spring tide. In

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... ion, the intrusion of saltwater was detected from the sound speed data of the AVM. The cross-sectional average velocity and discharge give close agreement with those deduced from an acoustic Doppler current profiler.