The Ornico River is a major river that flows in an arch formation for approximately 1700 miles. The River starts in the Guiana Highlands and flows to its mouth in the Atlantic Ocean. It mostly flows through Venezuela, except for a frontier between Venezuela and Colombia. The Ornico and its 436 tributaries make up the northernmost of South America’s four major river systems. The Andes Mountains border the River to the west and the north, the Guiana Highlands to the east, and the Amazon watershed to the south. The river basin has an area of about 366,000 square miles and makes up ⅘ of Venezuela and ¼ of Colombia. The river mostly flows through dense rainforest or vast grassland areas of the Llanos. The Ornico River is the world's eighth-largest river and South America's second-largest