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  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 harbor, 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 crisscross the town and empty into the Bay of Montevideo. Its coastline near the emptying rivers are heavily polluted.