[1] The continuous measurement of river discharge for long periods of time is crucial in water resource studies. However, the accurate estimation of river discharge is a difficult and labor-intensive procedure ; thus, a robust and efficient method of measurement is required. Continuous measurements of flowrate have been carried out in a wide, shallow gravel bed river (water depth % 0.6 m under low-flow conditions, width % 115 m) using Fluvial Acoustic Tomography System (FATS) that has 25 kHz broadband transducers with horizontally omnidirectional and vertically hemispherical beam patterns. Reciprocal sound transmissions were performed between the two acoustic stations located diagonally on both sides of the river. The horizontal distance between the transducers was 301.96 m. FATS enabled the measurement of the depth-and range-averaged sound speed and flow velocity along the ray path. In contrast to traditional point/transect measurements of discharge, in a fraction of a second, FATS covers the entire cross section of river in a single measurement. The flow rates measured by FATS were compared to those estimated by moving boat Acoustic Doppler Current Profiler (ADCP) and rating curve (RC) methods. FATS estimates were in good agreement with ADCP estimates over a range of 20 to 65 m 3 s À1 . The RMS of residual between the two measurements was 2.41 m 3 s À1 . On the other hand the flowrate by RC method fairly agreed with FATS estimates for greater discharges than around 40 m 3 s À1 . This inconsistency arises from biased RC estimates in low flows. Thus, the flow rates derived from FATS could be considered reliable.Citation: Kawanisi, K., M. Razaz, K. Ishikawa, J. Yano, and M. Soltaniasl (2012), Continuous measurements of flow rate in a shallow gravel-bed river by a new acoustic system, Water Resour. Res., 48, W05547,

In the present paper, the cross-correlation, multiple regression, salinity and velocity decomposition techniques were employed to analyze the short and long-term variations of salinity and flow rate in the Ota estuary. We considered the effect of the freshwater discharge, the tidal range, the activities of wind and the mean sea level on the salinity variations. Although the freshwater discharge plays the most important role in the variations of salinity, the results show high correlation between the along-channel component of the wind and the estuarine salinity changes. The direction of the net salt transport was landward during spring tides and low flow conditions. Though, the direction of net salt flux was seaward during most part of the observation period.

In the present study, the variations of the salinity and water discharge in the main branches of the Ota Estuary were investigated using the Fluvial Acoustic Tomography System (FATS). The Ota Estuary has a branched section of the Gion and Oshiba. The response of the salinity variations at the branches can be described as a power-law of river discharge. The exponents -0.53 and -3.86 at the Gion and Oshiba implied significant differences in the salt intrusion variations in these branches. The mechanisms of the salt intrusion at the site study were analyzed using the salt decomposition method based on the FATS data. The results indicated that the advection is the dominated seaward mechanisms at the both branches. However, the variations in the landward directed mechanisms were influenced by the triple correlation between the salinity, water velocity and water level fluctuations.