The greater part of the Danube Delta lies in Romania (Tulcea county), while its northern part, on the left bank of the Chilia arm, is situated in Ukraine (Odessa Oblast).
The approximate surface area is 4,152 km2 (1,603 sq mi), and of that, 3,446 km2 (1,331 sq mi) are 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 reaches 5,165 km2 (1,994 sq mi).
The Razelm - Sinoe lagoon complex is geologically and ecologically related to the delta proper and their combined territory is part of the World Heritage Sites.
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 sediments, 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 to present).
Several other internal lobes were constructed in the lakes or 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 a major expansion of its surface area in the form of lobes resulted from soil erosion associated with clearing of forests in the Danube basin during the 1st and 2nd millennium.Geologist Liviu Giosan told The New York Times that:
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 dams on 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 at 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.
The Danube branches into three main distributaries into the delta, Chilia, Sulina, and Sfântul Gheorghe (Saint George). The last two branches form the Tulcea channel, which continues as a single body for several kilometers after the separation from the Chilia. At the mouths of each channel gradual formation of new land takes place, as the delta continues to expand.
Sulina, the central and thus the shortest arm, which consequently led to its extensive use for traffic and severe transformation. At its mouth is located the main port and the single settlement with urban characteristics of the Romanian part of the delta. Because of the alluvium deposited at its mouth, a channel gradually advancing into the sea (presently it has 10 km), was built in order to protect the navigation.
Sfântul Gheorghe (Saint George in English), in the south, is the oldest and more sparsely populated. Its alluvium has led to the creation, beginning with 1897, of the Sacalin islands, which today measure 19 km in length.
The climate of the Danube Delta is continental with strong influences from the vicinity of the Black Sea and its prevalent amphibian environment. It is the driest and sunniest region of Romania. The mean annual temperature is 11 °C (-1 °C in January and 22 °C in July), with mean precipitation between 400 mm/year and 300 mm/year, decreasing from west to east.
The evaporation is around 1000 mm/year, amplified by the strong and frequent winds, resulting in long periods of drought in the summer. The northwest winds cause frequent storms in spring and autumn. In the interior of the delta the continental character of the climate is very pronounced.
The Danube Delta falls within Pannonian steppe ecosystem of eastern Europe, with Mediterranean influences. As a young region in full process of consolidation, the Danube Delta represents a very favourable place for the development of highly diverse flora and fauna, unique in Europe, with numerous rare species.
. It hosts 23 natural ecosystems, but due to the extent of wetlands the aquatic environment is prevalent; the terrestrial environment is also present on the higher grounds of the continental levees, where xerophile ecosystems have developed.
Musura Gulf, north of Sulina, and Saint George Gulf are considered the most representative for this type of ecosystem.
Situated on major migratory routes, and providing adequate conditions for nesting and hatching, the Danube Delta is a magnet for birds from six major eco-regions of the world, including the Mongolian, Arctic and Siberian.
There are over 320 species of birds found in the delta during summer, of which 166 are hatching species and 159 are migratory. Over one million individuals (swans, wild ducks, bald coots, etc.) winter here.
It comprises the Danube arms, as well as a series of more important streamlets and channels. It is an environment rich in plankton, worms, mollusca, grubs, spongiae, with numerous species of fish, such as the carp, pike perch, sheat fish, and freshwater sturgeons (sterlet, Vyza and Danube mackerel).