| The San Juan River has the most extensive and best developed delta on the Pacific coast of South America, measuring 800 km2. The river drainage basin measures 16,465 km2 and consists of mountainous terrain west of the western Cordilleras, and is located in one of the areas with the highest precipitation in the western hemisphere, 7 to 12 m yr −1. As a result, the San Juan River has the highest water discharge (2,550 m3 s−1), sediment load (16 × 106 t yr−1), and basin-wide sediment yield (1,150 t km−2 yr−1) on the west coast of South America.; The San Juan delta growth began approximately 5,000 years B.P. The morphodynamics is shaped approximately equally by contributions from fluvial, tidal, and wave forces and tectonic events. The semidiurnal tide (form number 0.1–0.2) has a mean range of 3 m and experiences significant tidal flow asymmetry in the San Juan distributary. The delta is a moderate marine energy environment with predominant swells 63% of the time, mostly from the southwest, with a significant height of 1.7 m, and a mean period of 6.0 ± 2.6 s. Analysis of delta progradation indicates that during the 1848–1992 period, the morphology of the delta was characterized by a general advance of the delta shoreline. During the past decade, processes such as rapid erosion along the delta shore, narrowing of barrier islands, breaching of a new inlet, and the increased occurrence of non-storm washover events are indicators that relative sea level is rising, currently at a rate of 2.6 mm/yr, due to earthquake-induced subsidence and eustatic sea level rise. The morphology and recent evolution of the San Juan have been controlled by (1) high precipitation rates and high fluvial discharges; (2) tectonic activity in the drainage and receiving basins; (3) strong climatic and oceanographic manifestations related to ENSO; (4) significant wave energy from the southwest; (5) an increase in relative sea level and eustatic sea level; and (6) the absence of human-induced manipulations in the drainage basin. For these reasons, San Juan is unique when compared to other deltas in South America. |
