The Danube Delta is the second largest river delta in Europe, after the Volga Delta, and is the best preserved on the continent. The greater part of the Danube Delta lies in Romania (Tulcea County). Its approximate surface area is 4,152 km2 (1,603 sq mi), of which 3,446 km2 (1,331 sq mi) is in Romania. With the lagoons of Razim–Sinoe (1,015 km2 (392 sq mi) with 865 km2 (334 sq mi) water surface), located south of the main delta, the total area of the Danube Delta is 5,165 km2 (1,994 sq mi). The Razim–Sinoe lagoon complex is geologically and ecologically related to the delta proper and the combined territory is listed as a World Heritage Site.
The modern Danube Delta began to form after 4000 BCE in a bay of the Black Sea, when the sea rose to its present level. A sandy barrier blocked the Danube bay where the river initially built its delta. Upon filling the bay with sediment, the delta advanced outside this barrier-blocked estuary after 3500 BCE, building several successive lobes: the St. George I (3500–1600 BCE), the Sulina (1600–0 BCE), the St. George II (0 BC–present) and the Chilia or Kilia (1600 CE–present). Several other internal lobes were constructed in the lakes and lagoons bordering the Danube Delta to the north (Chilia I and II) and toward the south (Dunavatz). Much of the alluvium in the delta and major expansion of its surface area in the form of lobes resulted from soil erosion associated with the clearing of forests in the Danube basin during the 1st and 2nd millennium. Geologist Liviu Giosan told The New York Times:
Probably 40 percent of the Delta was built in the last 1000 years. Finding that was like a eureka moment.
At present the delta suffers from a large sediment deficit, after the construction of dams on the Danube and its tributaries in the later half of the 20th century. However, construction of a dense network of shallow channels in the delta over the same period attenuated the deficit on the delta plain but increased erosion along the coast. The Danube Delta is a low alluvial plain, mostly covered by wetlands and water. It consists of an intricate pattern of marshes, channels, streamlets and lakes. The average altitude is 0.52 m, with 20% of the territory below sea level, and more than half not exceeding one meter in altitude. Dunes on the most extensive strand plains of the delta (Letea and Caraorman strand plains) stand higher (12.4 m and 7 m respectively). The largest lakes are lakes Dranov (21.7 km2), Roșu (14.5 km2) and Gorgova (13.8 km2).