Six thousand years ago the sea reached as far inland as Rosario; the land that has filled that gulf since is the Paraná Delta, the freshwater delta of the Paraná River in north-eastern Argentina, a maze of islands, river arms and marshes that covers about 14,000 km² between the provinces of Entre Ríos, Santa Fe and Buenos Aires before the river empties into the Río de la Plata estuary. It begins near Diamante and Puerto Gaboto, runs for about 320 km with a width of 18 to 60 km, and is still growing: the river delivers about 160 million tonnes of sediment a year, and the delta front advances 50 to 90 m a year into the estuary, depending on the source. Its upper end is protected by the Predelta National Park, created in 1992, and part of its lower islands near San Fernando, just north of Buenos Aires, became a UNESCO biosphere reserve in 2000. Geologically it is very young, and nearly all of it below Rosario was laid down after that sea retreated.
The Mar Entrerriense: a Miocene sea over Entre Ríos
The story starts long before the river reached its present course. During the Miocene, sea levels rose at the same time as phases of Andean mountain building, and the south-western Atlantic advanced north and west from the area of today's Río de la Plata across the plains of the interior. Argentine geologists call this body of water the Mar Entrerriense, the Entre Ríos sea, also known as the Paranaense sea. It was shallow and subtropical, with tidal and deltaic margins, and at its widest it covered much of the present Chaco–Pampean plain, central, northern and eastern Argentina, southern Uruguay, parts of southern Brazil, western Paraguay and eastern Bolivia. To the south it reached north-eastern Patagonia, where its deposits form the Puerto Madryn Formation.
The sea advanced and retreated four times, each advance smaller than the last. Its typically marine fossils, such as sea shells, sharks and whales, do not appear north of the Guayquiraró River, today the boundary between Entre Ríos and Corrientes, which suggests the water became less salty toward the Chaco. In Entre Ríos the marine phase is preserved as the Paraná Formation, exposed in the high river bluffs, or barrancas, between the mouth of the Arroyo Feliciano and the Arroyo Nogoyá, and covered by the mostly continental Ituzaingó Formation laid down after the sea left. River bluffs of this kind still mark the edge of the Paraná's floodplain and form the natural boundary of the delta today.
From the Paranaíba to the Bermejo: where the mud comes from
A delta needs a river with a heavy load, and the Paraná is one of the largest on Earth. It forms in southern Brazil where the Paranaíba and the Rio Grande meet, then flows about 4,880 km through Brazil, Paraguay and Argentina, second in South America only to the Amazon. Its upper course is interrupted by large dams, including Itaipu, which began operating in 1984, and Yacyretá, but much of the load that builds the delta comes from further west.
The key supplier is the Bermejo, the "red river", which flows from Bolivia across the Gran Chaco to the Paraguay River. It carries so much red-coloured sediment that it is not navigable, and its deposits can alter its own course, leaving old channels as wet depressions. The effect on the Paraguay is measurable: its suspended load is about 100 milligrams per litre above the Bermejo and about 600 below it. About half of the 160 million tonnes of suspended sediment the Paraná carries to its delta each year comes from the Bermejo by way of the Paraguay.
The ice-age lowstand: a river that met the sea near Punta del Este
At the end of the last ice age there was no estuary for a delta to grow into. With so much water locked in ice sheets, sea level was far below today's, and the Paraná crossed a dry plain to reach the Atlantic directly, near what is now Punta del Este in south-eastern Uruguay. The land now occupied by the delta and the Río de la Plata was a grassy steppe cut by the river, with a wedge of woodland reaching down from the north along the watercourses. Rainfall at the headwaters of the great Mesopotamian rivers was 25 to 35% lower than today, and with the sea so far away, the first human inhabitants of the region could cross between the river banks with little difficulty.
The rise that changed all this began between 18,000 and 20,000 years ago, in step with the great climatic swings of the Quaternary.
The Querandí transgression, 8,000 to 6,000 years ago
As the ice melted, the sea climbed onto the plain. By about 8,000 years before present it stood around 10 m below its current level. In the following thousand years it rose another 10 m, reaching today's level around 7,000 years ago, and it kept going. Between about 6,500 and 6,000 years ago it peaked at roughly 6.5 m above the present level. Geologists call this the Querandí transgression, and the gulf it created the Mar Querandinense, or Platense sea.
At its maximum the gulf reached as far inland as the line now crossed by the Rosario–Victoria bridge, between Rosario in Santa Fe and Victoria in Entre Ríos. From there a sandy, dune-backed coast ran east across southern Entre Ríos, past the Gualeguay River, Médanos and Ceibas, to the Gualeguaychú–Fray Bentos area, where the Uruguay River then met the sea. The whole of the present lower Paraná and its delta was under salt or brackish water. Only one patch of land stayed dry: the sand dunes of the Alto del Ibicuy, which formed an island of about 11,000 hectares some 20 km from the nearest mainland shore. Behind the coastal sand barrier, drainage was blocked and lagoons such as the Laguna del Pescado and large wetlands formed. Along the north-east coast of Buenos Aires Province, waves piled up ridges of shells, clay and fine sand, now the Atalaya Formation, whose fossils point to a warm estuary.
Falling seas and a delta that fills the gulf
After about 6,000 years ago the sea began to fall. Within a thousand years it dropped 1.5 m, to about 5 m above today's level by 5,000 years ago, and held there for about 1,500 years. Around 3,500 years ago it fell fairly suddenly by another 2.5 m and then sank gradually to its present height, as the climate turned drier and generally cooler. The Ibicuy island was left surrounded by tidal flats, and the gulf began to fill.
The Paraná did the filling. Its sediment settles where the current slows on entering the estuary, building banks that are quickly colonised by rushes, ceibo trees and tall grasses whose roots bind them into islands. Each island has a characteristic shape: its edges, built from natural levees called albardones, stand higher than its centre, which usually holds marsh or small lagoons. As old islands rise and new banks form at the front, the delta advances into the Río de la Plata by infilling, the process Argentine geographers call colmatación. It is therefore not only a delta but the replacement of a former sea gulf by river land.
Upper, Middle and Lower Delta: what the sediment built
The result is conventionally split into three parts that also follow its age. The Upper Delta runs from the Diamante–Puerto Gaboto line to Villa Constitución; the Middle Delta from Villa Constitución to the Ibicuy Islands; and the Lower Delta, the youngest and most active, from the Ibicuy Islands to the mouth. The river divides into several great arms. The Paraná Pavón branches off first, opposite Villa Constitución, enclosing the Lechiguanas Islands, and continues as the Paraná Ibicuy. Further down, the Paraná de las Palmas swings into Buenos Aires Province, while the main course continues as the Paraná Guazú, which sheds the Paraná Bravo and Paraná Miní to the east. The main navigable channel lies on the west side of the delta and takes Panamax-size ships downstream from Puerto General San Martín.
The Predelta National Park protects the point where this begins. Created on 13 January 1992 by Law 24.063, it covers 24.58 km² of low islands, lagoons and swamps 6 km south of Diamante, exactly where the river's sediment starts forming islands and the channel splits into arms. The Isla Botija Nature Reserve, created in 1958, was one of the first protected areas in the lower delta. The delta's wildlife includes marsh deer, capybara, the neotropical river otter, the Pampas cat, the coypu and the red-faced guan. The low elevation and abundant water give the islands a humid microclimate in which some warmer-weather species thrive further south than usual.
Chaná canoes, Tigre and the willow islands
People moved onto the new land as it formed. When Europeans arrived in the 16th century, the delta was home to the canoe-using Chaná, a people under strong Guaraní cultural influence; in Guaraní the delta was called Karapachay, a name that survives in the Carapachay river arm. Tigre, founded in 1820 after floods destroyed earlier settlements, takes its name from the jaguars once hunted there; it became the port for fruit and timber from the islands and is still a timber-processing centre, 28 km north of Buenos Aires. Commercial tree plantations have covered large areas of the delta since the 1960s, and willow forestry is the main activity in the biosphere reserve, where only about 3,600 people lived in 2001.
The Zárate–Brazo Largo bridges and a delta under pressure
Crossing a landscape made of water needed big engineering. Between 1971 and 1977 Argentina built the Zárate–Brazo Largo bridges, two identical cable-stayed road and rail spans 550 m long and 30 km apart, over the Paraná de las Palmas and the Paraná Guazú, raised 50 m above the water so that ocean ships can pass beneath. The delta's youth also makes it fragile. Fires blamed on illegal burning to clear islands for cattle or soy burned at least 65,000 hectares in the first half of 2008 and sent smoke over Buenos Aires; a drought in 2021 dropped the Paraná to a 77-year low; and a rise of only 10 cm in the level of the Río de la Plata could slow or even reverse the delta's advance. To follow the whole river from its Brazilian source, see our complete guide to the Paraná River; for the general processes behind deltas like this one, read how deltas are formed; and the Paraguay River, which delivers the Bermejo's mud, begins in Brazil and first flows through the Pantanal wetlands.