Spatiotemporal Response of Currents and Mixing to the Interaction of Tides and River Runoff in a Mesotidal Estuary

Tidal river management requires consideration of the influence of both tides and freshwater on hydrodynamic patterns. Most previous studies have focused on large and high discharge estuaries, whereas tide and river runoff interactions in small, shallow, and low discharge estuaries have received limited attention. Understanding these interactions remains a challenge due to the impacts of disturbances and changes in processes, which may differ from those of large estuaries. In this study, we investigated the influences of tides and river runoff on circulation and mixing processes in a small estuary with an area of less than 50 km 2 and a discharge range of 20–130 m 3 . In this study, we applied mixing evaluation and wavelet analysis to the salinity, temperature, density, and three records of acoustic Doppler current profilers that were placed along a tidal floodway (the Ota River diversion channel, Japan). Our findings revealed that mixing was controlled by neap–spring tidal oscillation, with high discharges significantly influencing mixing during the neap tide. Moreover, the results of the wavelet transform indicated that during the spring, tidal straining increased and induced intense mixing, while during the neap, river discharge had a greater influence on circulation. The interactions between tides and river runoff influence the appearance of a convergence zone and estuarine turbidity maxima that are located approximately 4.8 km from the river mouth.