THE WORLD DELTA DATABASE

THE DANUBE DELTA

Delta ID   # 06.

DISCHARGE WATER 85000M3/S, SEDIMENT 67X106 TONS/YR

DRAINAGE BASIN AREA 8.99X105 KM2

The Danube River was known to the Greeks as the Ister River, and Herodotus called it "the greatest of rivers". Napoleon referred to it as the "king of rivers." The Danube is the second largest river in Europe; it is approximately 2,900 km long and drains an area slightly larger than 779,500 sq km. The river rises in the Black Forest Mountains of Germany and empties into the Black Sea. At the Romanian border, the river once cut a channel through the mountain ridge that joins the Carpathian arc with the Balkan Mountains, and a large interior sea was formed. The geology of the drainage basin is complex; the western part of the basin is dominated by Precambrian and Paleozoic sediments, the southern basin is Mesozoic in age, and the central and eastern part of the basin is dominated by Neogene sediments. The western part of the drainage basin lies within the West Molasse and Southwest German basin, while the central basin lies within the Central Pannonian and Caspian - Balkanean basins. The northern basin is bordered by a major zone of faulting, while the southern border of the basin displays a large number of earthquake epicenters. Not until Recent geologic times has the Danube lowered its channel through the gap to drain this interior sea. The drainage basin has an extremely dense tributary pattern (Figure 37). The density of the tributary pattern is 0.22 km stream length per 500 sq km and the average rainfall is 808 mm, with a maximum of 1,678 mm (July) and a minimum of 457 mm (January). The river, from its headwaters to the mouths of the river in the Black Sea is 2,536 km in length. The basin geology is complex. Relief in the basin is generally low, averaging only 292 m. The average elevation of the drainage basin is 462 m, with a maximum of 2,600 m and a minimum of 60 m. The average annual rainfall is 808 mm, with a maximum of 1,678 mm and a minimum of 457 mm. The wet months are March through August, while the dry months are December through February. Most of the western basin lies within temperate broadleaf and coniferous forests and the central basin consists primarily of temperate grasslands and savannas.

The alluvial valley of the river system is well-defined [06-i05] and meandering of the channel is quite common. Numerous channels exist and from the satellite images, it is apparent that changes in the river course is quite a common occurrence. The average annual discharge is 6,499 m³/sec, with a maximum of 8,938 m³/sec and a minimum of 4,447 m³/sec (Vorosmarty, et al, 1998). Floods generally begin in late March and continue into the latter part of July. Lowest discharges occur in September and October. Settlements and population density is quite high within the alluvial valley and much of the area is under cultivation .

The delta area (4,345 km²) of the Danube was created in Recent times. Sediment discharge averages 122 million tons/year, of which 54 million tons consist of bed load (Samajlov, 1956). To the north and west of the river delta, primarily Pliocene and Miocene sedimentary rocks form north-south-trending low hills capped by Pleistocene loess up to 200 m high. East of the river and south of the delta, Paleozoic and Mesozoic sediments form high rolling hills that attain elevations of 450 m.

The main channel of the Danube is highly migratory within the lower part of the alluvial valley, so that numerous meander scars are present within the valley. Small stretches of river braiding are found along the valley course. As the Danube turns abruptly east to form its delta plain [06-i07], sedimentation has blocked numerous valleys of the north- south-trending topography, forming elongated freshwater lakes. The major distributaries of the Danube consist of three major channels, the St. George to the south, the Sulina in the middle, and the Kilia to the north (Almazov et al., 1963) [06-i01]. The St. George arm is 120 km long and has widths ranging from 200 to 500 m, while the Sulina arm, prior to 1860, had a length of 100 km and a width of 250 m. The Sulina was artificially diked in the period 1860-1895 for navigation purposes. Kilia, the youngest of the distributaries, having formed within the past 600 years, now receives the major part of the flow. It is slightly longer than 100 km and ranges in width from 300 to 700 m. The distributary channels are bordered by well-developed natural levees that are quite narrow, generally less than 250 m wide. Downstream, the natural levees decrease in width and height. Sinuous distributary channels result from migration as the channel cuts through former beach-ridge trends. Like distributaries in many deltas, migration of the channel is controlled by the presence of coarser material. The Kilia distributary to the north is the youngest part of the delta. Numerous bifurcated channels and overbank splays are present, most of which are too small to show up in the satellite image. Offshore of these young prograding distributaries, slope is extremely low (0.152 degrees), the coastline is relatively muddy, but sandy beaches are located along the entire delta coast. A large sandy barrier spit is present south of the St. George distributary.

Several abandoned distributary channels are present within the delta plain [06-g01]. They indicate that the channels shift with time in response to subtle changes in slope, supplying sediment to all parts of the delta. In general, the delta consists of an older upstream part and a younger downstream part. Separating the two units is a large beach-ridge complex seen in the satellite image. These beach ridges display both progradational and transgressive characteristics. They are obviously associated with the prograding river mouths of the St. George, Sulina, and older distributaries. Once the Kilia distributary became the dominant channel, wave reworking of the older distributaries resulted in the formation of transgressive beaches at these river mouths. The Danube beach ridges have heights of 5 to 10 m, and many contain coastal eolian dunes.

Predominant in the delta plain are roseau cane (marsh cane, Phragmites) marshes and freshwater lakes. The marshes are an important resource of the delta, providing a major nesting place for waterfowl. The marshy areas of the delta plain are the largest continuous marshland in Europe which includes the greatest stretch of reedbeds probably in the world. The roseau cane forms thick root mats and results in organic content being exceptionally high in the delta deposits. Much of the delta area is occupied by freshwater lakes up to 3 to 4 m deep. These lakes are initially filled by overbank flow of organic-rich clays. As filling with organic-rich clays proceeds, the lakes become isolated from the overbank splays, and thick floating organic mats form the final fill.

Alongshore drift north and south of the delta has built linear barrier islands and spits, which enclose broad brackish and marine estuaries and lagoons. The barrier islands have relatively steep shore faces, and many are characterized by coastal eolian dunes. The estuaries are important biological environments in that they form the spawning ground for many economically valuable marine species.

Tides in the Black Sea are virtually absent and the only water variations result from wind driven surges. Wave power is moderate, with average wave power of 0.033 x 10⁷ ergs/sec/m coast. 06-g02 shows the distribution of wave power along the delta front for each month of the year. Wave energy is highest in the months of March – May and September through November. The subaerial delta is much larger than the subaqueous delta (ratio of subaerial/subaqueous delta is 8.6). The abandoned delta is approximately three times the size of the active delta.

In order to detect changes in open water and agricultural and industrial use within the Danube River delta, comparisons were made between a satellite obtained in 1987 and one in 2001 [06-i06, 06-i04]. Changes in these parameters were completed for the 14 year period. The delta area covered by the geo-registered images was 3,066 sq km (757,625 acres) and included a significant portion of the entire delta plain. Thus before man influenced the delta plain, there was approximately 757,000 acres of wetlands within this portion of the delta plain. By 1987, there was a total of 402,083 acres of open water within the delta plain, or a reduction of some 53% of the delta plain wetlands by formation of open water due to subsidence, changes in channel geometry and obviously some influence by man. Some 14 years after the 1987 satellite image was obtained, the open water was reduced to 389,105 acres, mostly because of filling of open water areas by man. Agricultural and industrial use of the delta plain comprised some 121,925 acres on the 1987 image. By 2001, the total wetlands loss due to agricultural and industrial expansion was 155,379 acres or a loss of 33,454 acres in the 14 year period. Assuming that most of the delta plain was intact before major modifications by man, a total of 524,008 acres of wetlands were loss because of increasing open water and modification by man until 1987. From 1987 to 2001, an additional 20,476 acres of pristine wetlands were lost primarily the result of modification by man. Thus, since the delta was first occupied by man, a total of 72 percent of the wetlands in the delta plain has been destroyed and the average rate of wetland loss is 1,462 acres/year.