Montevideo is situated on the north shore of the Río de la Plata, the arm of the Atlantic Ocean that separates the south coast of Uruguay from the north coast of Argentina; Buenos Aires lies 230 kilometres (140 mi) west on the Argentine side. The Santa Lucía River forms a natural border between Montevideo and San José Department to its west. To the city's north and east is Canelones Department, with the stream of Carrasco forming the eastern natural border. The coastline forming the city's southern border is interspersed with rocky protrusions and sandy beaches. The Bay of Montevideo forms a natural harbour, the nation's largest and one of the largest in the Southern Cone, and the finest natural port in the region, functioning as a crucial component of the Uruguayan economy and foreign trade. Various streams criss-cross the town and empty into the Bay of Montevideo. The coastline and rivers are heavily polluted and of high salinity.