The Rio de la Plata is an estuary 220 kilometers long and 35 to 230 kilometers wide. The upper river has a fresh-water environment where the Uruguay and Parana River systems converge. The Parana River is building a delta in the upper Rio de la Plata. The hydraulic system has an average yearly discharge of 25 × 103 cubic meters per second, which is controlled in part by wind and tidal action.
The mouth of this estuary is open to the sea. Because the tide, tidal currents, wind, and waves affect the fluvial discharge, the river flow is not continuous.
More than 2,000 measurements of suspended load show that solid sedimentation is jointly affected by tide and currents, and apparently the salt water influence on sedimentation is less than that of dynamic action. The collection of more than 1,000 bottom samples shows that over the estuary bottom lie silty sand and silty clay. The river sediment supply consists mainly of silt and clay. In the upper estuary the sand is scarce and localized to banks, bars, and northern coast beaches. In the outer estuary a sand carpet extends from the inner continental shelf into the estuary. These are relict sands of the last Holocene transgression which invaded the estuary between 7,000 and 3,000 years B.P. After Holocene time fluvial mud facies covered, in part, the transgressive sands. The south coastal environment is estuarine, with its maximum development in Samborombon Bay. The north coast is rocky and sandy with pocket beaches. The regional estuarine environment is mostly fluvial, but the mixing of ocean waters creates a gradual change from fluvial in the upper river to fluvio-marine and marine in the outer river.