western portion of the bay were impounding sand until 1981, but are now being emptied since the sand source provided by erosion of Punta Salinas's east beach has been eliminated. This loss of sediment supply may result in renewed erosion in the Levittown area.
The west shore of Punta Salinas tombolo was eroding at about two meters per year. The construction of a jetty at the north end of the beach was begun in June 1981. Fill excavated from within the tombolo was added to the berm and beach to extend the shoreline seaward almost 50 meters. This work was completed in 1982. The jetty blocks and alters the wave and current regime which should change the pattern of sediment transport. The 1987 photograph shows the shore position at the south end of the beach at the 1977 shore position and at the north end it is beyond the 1971 position. The beach south of the park was not eroding prior to the jetty construction but between 1987 and 1999 the southern beach has begun eroding.
An increase in the magnetite content of the west shore may indicate changing land use and deforestation in the upper reaches of the Río El Cocal and Río La Plata (Barreto, pers. obs.). The geology of the lower reaches of the river is limestone, biogenic sandstone and alluvial deposits. Volcanic rocks with abundant magnetite are only found in the mountains of the Cordillera Central.
Erosion of the east side of Isla Cabras began between 1971 and 1977 with the loss of more than three meters of shoreline per year at the south end of the east shore. From 1977 to 1981, there was little additional erosion, but between 1981 and 1985 another episode of erosion occurred north of the original area of erosion. Almost 20 meters of shoreline were removed in a four year period (Figure 9). This continued until the entire area of erosion was enclosed with a breakwater to create a public swimming area (Figure 10).
Figure 9. Shoreline changes on the east side of Isla Cabras. Almost half of the width was lost.