The Plate River, or Rio de la Plata, is not a river but a large, funnel-shaped estuary formed by the confluence of the rivers Uruguay and ParanÃ¡. These rivers and their tributaries drain about one-fifth of the land area of South America. At 180 miles (290 km) long and 136 miles (220 km) wide at its mouth, the Plate discharges about 6.5 million gallons (25 million liters) of water per second into the Atlantic Ocean. As well as transporting this vast amount of water, the estuary receives about 2 billion cubic feet (57 million cubic meters) of silt each year from its input rivers. This mud accumulates in great shoals, so that the water depth in most of the estuary is less than 10 ft (3 m). Constant dredging is therefore needed to maintain deep-water channels to the ports of Buenos Aires, which lies near the head of the estuary, and Montevideo, which is close to its mouth. Surface salinity varies uniformly through the estuary, from close to zero in its upper parts to a value just below average ocean salinity near its mouth. Deep down, a wedge of salt water penetrates deep into the estuary. Biologically, the Plate is highly productive, yielding large annual masses of plankton, which support large numbers of fish and dense beds of clams. It is also a habitat for the La Plata dolphin, a rare, long-beaked species of river dolphin.