Surface flow modulation by river discharges in a tropical micro-tidal estuary

Abstract

The typical bidirectional pattern of subtidal flow observed at the mouths of estuaries is produced by inflows during high-tide (flood) and outflows during low-tide (ebb) periods. This flow pattern can be modulated in the presence of a strong river discharge, as observed for the Alvarado Estuary (western Gulf of Mexico) during the rainy season. Herein, six transect repetitions were obtained using a towed Acoustic Doppler Current Profiler (ADCP) at an average speed of approximately 2 m/s during a diurnal tidal cycle from September 20 to 21, 2009. The water column during this period was strongly stratified at the mouth of the estuary due to the amount of discharge from the Papaloapan River. Furthermore, the temperatures within the estuary were directly related to the distance from the Papaloapan River. Based on salinity data obtained at four locations along the estuary, the Alvarado Estuary could be considered for the first time as a transitional system from oligohaline to mesohaline (salinities between 5 and 18). The currents and backscatter intensities of the ADCP revealed concentrated areas of suspended particles in the deepest area of the estuary during flood tide periods. During ebb tide periods, estuarine water primarily exited through its mouth (∼90%), followed by the artificial channel (∼10%) located in the northernmost area of the estuary. The residence time during this season was 3.99 days. Finally, the typical estuarine flow pattern observed during flood and ebb tide periods was only depicted near the bottom of the mouth of the estuary. However, the near-surface flow was always toward the ocean (outflow), regardless of the tidal stage.