Effects of tides, wind and river discharge on the salt intrusion in a microtidal tropical estuary

Data from three 25 h hydrographical surveys conducted in a microtidal, tropical estuary were used to study seasonal changes of salt intrusion and its response to runoff and wind stress pulses. The tropical estuary was the Jamapa River estuary, located in the southwestern Gulf of Mexico. This tropical region features wet and dry seasons, and is sporadically affected by >10 ms−1 northerly wind events (3 to 5 days long) during the dry autumn and winter. During both, wet and dry seasons, the daily average river discharge commonly exhibits pulses of few days long, associated either with the passage of cold fronts or of tropical storms and hurricanes. Data collection campaigns took place on April 14 of 2010 (dry season), October 15 of 2010 (wet season) and January 11 of 2011 (dry and northerly wind season). In each campaign, an axial transect covered the entire estuary with 13 hydrographic stations, periodically sampled during a diurnal tidal cycle. The main findings can be summarized as: (1) The salt-wedge position was modulated primarily by river discharge. A maximum intrusion length of 5 km along Jamapa River estuary was observed under minimum river discharge. A critical value of river run off (Qc) for the presence or absence of a salt-wedge was found to be 70 m3s−1, which was close to the mean annual river discharge. The position of the salt wedge responded to runoff pulses over time scales of the order of hours. (2) Both, down-estuary winds (local action) and NW-winds (along-shore, remote action) pulses enhanced salt wedge intrusion and overwhelmed tidal action on the salt wedge position. (3) Tidal influence was evident on the position of the salt wedge and on the salinity of the bottom layer (only at the estuary entrance), under wind calm conditions.