A technique for accurate measurement of the average water velocity in estuarine environments has been developed. The technique is based on measuring the travel time of 200-kHz ultrasonic signals. A high degree of precision has been obtained using a cross-correlation technique, and long, detailed time series have been recorded. However, in some geometries the precision is degraded by multipath interference. Measurements of current speed in California's Sacramento-San Joaquin River Delta system will be discussed, with particular reference to the effects of multipath interference. Data were collected throughout a large flood in Dec. 1996-Jan. 1997. An analysis of the variation of the strength of the ultrasonic signal has been carried out to determine the attenuation of the signal due to the load of suspended sediment, and the results will be presented. The current-velocity time series recorded in the San Francisco Bay also show wave trains from boat wakes. These wakes will be discussed.