The Danube, an Empire Boundary River: Settlements, Invasions, Navigation, and Trade Pathway

The Danube River basin was home to some of the world’s earliest human cultures such as the Danubian Neolithic cultures including the Linear Pottery culture. The Vucedol culture was famous for ceramics during the third millennium BC. Early settlement required fortresses and castles to be built on the Danube River to defend the settlements from invading forces. These included the Persia, Roman and Ottoman empires. Cities included Vienna, Budapest, Belgrade and Ruse. The risks include stream capture, settlement, invasions, navigation, trade, pollution, industrial and urban wastewater, over fishing, food insecurity, locks and dams, shoreline erosion and flooding. The primary objective was to document the settlement history on the Danube River which became Eastern Europe’s pathway for settlement, invasions, navigation and trade in the Danube basin has put the river at risk for more than 2500 years. River capturing of the Danube by the Rhine River has reduced the length and flow of the Danube. The Danube River has had a huge economic, social and environmental impact on 10 European countries. However, with 10 countries sharing the river it has been difficult to manage and mitigate the risks and threats to the Danube River and its water quality.


Introduction
The Danube River played an important role in the political evolution and set-Journal of Water Resource and Protection tlement patterns of central and southeastern Europe [1] with riverbanks lined with fortresses and castles. The river formed the boundary between great empires, and the river served as a vital commercial highway between nations. During the 7 th Century, the Danube became the pathway for the transport of invading forces. A traditional trade route in Europe since ancient times, the Danube continues to be an important trade artery.
Today, approximately 2415 km of the Danube River are navigable. The river is an important source of drinking water and hydropower. It has been harnessed for hydroelectric power particularly along the headwaters. The cities along its banks include the national capitals of Budapest (Hungary), Vienna (Austria), and Belgrade (Serbia) which have depended upon the Danube for their economic development [2]. The River has long been celebrated in music.
After the Volga, the Danube is Europe's second longest river. It is located in Eastern and Central Europe and was once a long-standing frontier of the Roman Empire. Originating near the Black Forest in Germany, the Danube presently flows southeast for 2850 km and passes through 10 countries which is more than any other river in the world: Slovakia, Austria, Hungary, Serbia, Germany, Croatia, Romania, Bulgaria, Moldova and Ukraine, draining into the Black Sea. Its watershed extends into 9 additional countries ( Figure 1).
The Danube has a drainage basin of some 817,000 sq km with the river basin expanding unevenly along its length. The 300 branching tributaries form a dense, deep water river network with more than 30 navigable tributaries. The river width varies unevenly along its length. It covers about 47,000 square km at the Inn River confluence, 210,000 sq km after joining with the Drava River. More than half of the entire Danube basin tributaries collect two-thirds of the total runoff volume from the Alps and other mountain areas.
The primary objective is to document the natural and anthropic risks to the Danube River including stream capture, settlement, invasions, navigation and trade. Environmental risks include pollution, industrial and urban wastewater, over fishing, a threat to food supply, urban development, locks and dams, shoreline erosion, flooding.

Geology
The Rhine and Danube rivers, compete for water in southern Germany. The Rhine is the only river headwater in the Alps Mountains and flows north towards the North Sea. The Rhine River is at a much lower elevation than the Danube River. The headwater percolates into underground channels at the Danube Sink in the Swabian Alb. The Swabian Alb is largely shaped by porous limestone with many sinkholes. The catchment subsurface river transports much of the upper Danube River water to the Rhine River. During the summer, the Danube carries little river water.
Most of this water resurfaces only 12 km south at the Aachtopf, Germany. The average flow feeding the Rhine is at a rate of 8500 liters per second. The European K. R. Olson, E. Krug Water Divide applies only for those waters that pass beyond this point, and only during the days of the year when the Danube carries enough water volume to flow east over the limestone sinkholes in the Danube Sink. Since such large volumes of underground water erode much of the surrounding limestone, it is estimated that the Danube upper course will one day be lost to the Rhine as a result of stream capture [1]. The Rhine started at the southwestern tip of the Black Forest before the last ice age in the Pleistocene. The waters from the Alps were carried east by the so-called Urdonau or ancient Danube. Parts of this ancient "RIVERBED" can still be seen in the Swabian Alb landscape. Much of the upper Rhine valley had been eroded and the headwaters from the Alps changed direction and began flowing to the Rhine [3]. The upper Danube River's headwater river is much smaller than the ancient river.

Hydrology
The constant shift of deposits in different parts of the riverbed forms shoals. The The climate of each basin segment determines the temperature of the river waters. In the upper segment, the summer water temperature is low since the water is derived from the Alpine snow and glaciers. Summer temperatures vary between 22˚C and 24˚C in the middle and lower reaches while winter temperatures drop below freezing. The middle and lower reaches become icebound during severe winters. In the winter ice drift combines with the spring thaw resulting in floating ice blocks which accumulate on the river islands and restrict the river flow often creating major floods.
As a result of the introduction of stream-training equipment, including dams and dikes, the natural regime of river runoff frequently changes. The sediment load of the river is greater in the winter months than the summer. The content of organic matter in the river water and sediment is relatively low. As the river waters flow past industrial and urban areas the pollution level increases. The river chemistry changes with the addition of each city sewage discharge and as a result of agricultural runoff water is high in sediment and nutrients.

The Route of Emperors and Kings
The Route of Emperors and Kings got its name from the Holy Roman Emperor

Current Danube Watershed, Navigation, Danube
Commission and Impact on Environment and Economy of 10 European Countries

Navigation and Canals
The Austrians were granted the right to navigate the middle and lower Danube

Danube Delta
The Danube Delta is the largest river delta in the European Union (EU

The Economy
The Danube is of great economic importance to the 10 countries ( Figure 1) that it borders or flows through: Ukraine, Moldova, Romania, Serbia, Croatia, Hungary, Bulgaria, Slovakia, Austria, and Germany [5]. These countries use the river for freight transport, the generation of hydroelectricity, industrial and residential water supplies, hydroelectricity, fishing, and irrigation. The most important economic use (highest and best use) of the Danube is for the movement of freight.

Ecology and Environment
The International Commission for Protection of the Danube River [6] consists of 14 member states (Germany, Austria, the Czech Republic, Slovakia, Slovenia, Hungary, Croatia, Bosnia and Herzegovina, Serbia, Bulgaria, Moldova, Romania, Montenegro, and Ukraine). The ICPDR established programs for the whole Danube river basin including tributaries and groundwater resources [6]. The Commission's goal is to promote and coordinate sustainable and equitable water management including conservation, improvement and rational use of waters and the implementation of the EU Water Framework Directive [7].

Transport
The Danube was an important 19 th century waterway but it was annually swept by ice that could lift a large ship out of the water or break ships in two. The Danube "Corridor VII" is still an important EU transport route [7].

Fishing
Fishing on the Danube, once important in the Middle Ages, is now dramatically reduced although some fishermen are still active on the river tributaries and fishing is still an important industry in the Danube Delta. The Upper Danube ecoregion has about 60 fish species and the Lower Danube-Dniester ecoregion has more than 120 species. There are six sturgeon species including beluga (Del-phinapterus leucas), Russian sturgeon (Acipenser gueldenstaedtu), sterlet (Acipenser rutenus), bastard sturgeon (Acipenser nudiventis), starry sturgeon (Acipenser stellatus) and European sea sturgeon (Acipenser sturio). The sturgeon have nearly disappeared from the river. The huchen (Hucho hucho) is endemic to the Danube basin [8].

Sultans Trail
The hiking Sultans Trail runs along the river between Vienna and Smederevo in Journal of Water Resource and Protection

Drinking Water
The Danube is a source of drinking water for about 20 million people. Drinking water comes from purified Danube water. Cities such as Ulm and Passau, also use some water from the Danube [1]. In Austria and Hungary, most water is drawn from ground and spring sources. Most countries find it too difficult to clean the river water because of extensive urban and agricultural pollution. In the Danube Delta of Romania the water is cleaner and cities, like Sulina, a free port at the mouth of the Danube, still obtain drinking water on a regular basis from the Danube. Sulina has been occupied for 7 centuries, although it has been losing population during the last decade perhaps as a result of periodic flooding and the river drinking water becoming more polluted.

Conclusions
The historic Danube was used by the Ottoman and Roman Empires as a boundary river. Its riverbanks are lined with castles and fortresses that had been used to protect the empires from invading forces. The Danube became the southwestern and central European trade route, most of which is navigable. The Danube headwaters are being captured by the Rhine River via limestone sinkholes and underground river flows. This is a classic example of stream or river capture.