The San Juan delta, Colombia: tides, circulations, and salt dispersion

摘要

The San Juan River delta (Colombia) with ail area of 800 km 2 is the largest delta environment oil the Pacific coast of South America. It consists of active distribtaries maintained by an average discharge of 2500 m(3) s(-1), is tide dominated, and has relatively narrow estuarine mixing zones <17 km wide and typically similar to7 kill wide. Water level and current time series in two distributary months indicate that the tide is semidiurnal with a form number 0.1-0.2 and a mean range of 3 m. Processes at tidal frequencies explain 75-95% of the water level variability with the remaining low-frequency variability attributed to meteorological forcing and river processes. The tidal phase for the main diurnal and semidiurnal Constituents progress from north to south along the coast. Only the Southernmost distributary experiences significant tidal asymmetry as a result of strong river discharge and shallow depths. In the northernmost distributary, shallow water constituents are insignificant. Tidal currents were more semidiurnal than file water level. with form number 0.09-0.13. Tidal ellipses indicated that currents were aligned with the channels and mean amplitudes < 1 m s(-1) the delta distributaries, circulation modes varied from seaward flow at all depths during intermediate runoff conditions to gravitational circulation during rising and high discharge periods. In San Juan and Chavica distributaries, the Currents were ebb-directed, while in Charambira they were flood-directed. The circulation appears to be controlled by the morphology of the distributaries, which were weakly stratified and only sometimes moderately stratified. The net salt transport was directed seaward in San Juan and Charambira, and landward at Chavica, indicating an imbalance ill the salt budget, and signifying non-steady state behavior. The net longitudinal salt flux in the San Juan delta is largely a balance between ebb-directed advective flux, and flood-directed tidal sloshing. Along the distributary channels, fringing vegetation is controlled by freshwater discharge, longitudinal distribution of salinity, and morphology. In the most active distributaries, Chavica and San Juan, the vegetation setting is strongly shaped by the short estuarine zone, and mangroves only occur 5km upstream of any distributary mouth, whereas in the tide-dominated distributaries, Charambira and Cacahual, dense mangroves intrude 14 and 17 km upstream. respectively. Also, salt dispersion, tidal intrusion, salinity distribution, and mangrove extent in the San Juan delta agree qualitatively With the productive coastal fishery at the tide-dominated distributaries. (C) 2002 Elsevier Science Ltd. All rights reserved. [References: 62]