Question: When did the refugee crisis start in the continent of Volga River?

full_context: Document 1: Ottoman Empire: Economic and political migrations made an impact across the empire. For example, the Russian and Austria-Habsburg annexation of the Crimean and Balkan regions respectively saw large influxes of Muslim refugees – 200,000 Crimean Tartars fleeing to Dobruja. Between 1783 and 1913, approximately 5–7 million refugees flooded into the Ottoman Empire, at least 3.8 million of whom were from Russia. Some migrations left indelible marks such as political tension between parts of the empire (e.g. Turkey and Bulgaria) whereas centrifugal effects were noticed in other territories, simpler demographics emerging from diverse populations. Economies were also impacted with the loss of artisans, merchants, manufacturers and agriculturists. Since the 19th century, a large proportion of Muslim peoples from the Balkans emigrated to present-day Turkey. These people are called Muhacir. By the time the Ottoman Empire came to an end in 1922, half of the urban population of Turkey was descended from Muslim refugees from Russia.
Document 2: Italy: In 2016, Italy had about 5.05 million foreign residents, making up 8.3% of the total population. The figures include more than half a million children born in Italy to foreign nationals – second generation immigrants, but exclude foreign nationals who have subsequently acquired Italian citizenship; In 2016, about 201,000 people acquired Italian citizenship (130,000 in 2014). The official figures also exclude illegal immigrants, that were estimated in 2008 to number at least 670,000.Starting from the early 1980s, until then a linguistically and culturally homogeneous society, Italy begun to attract substantial flows of foreign immigrants. After the fall of the Berlin Wall and, more recently, the 2004 and 2007 enlargements of the European Union, large waves of migration originated from the former socialist countries of Eastern Europe (especially Romania, Albania, Ukraine and Poland). An equally important source of immigration is neighbouring North Africa (in particular, Morocco, Egypt and Tunisia), with soaring arrivals as a consequence of the Arab Spring. Furthermore, in recent years, growing migration fluxes from Asia-Pacific (notably China and the Philippines) and Latin America have been recorded.
Document 3: Thuringia: Environmental damage in Thuringia has been reduced to a large extent after 1990. The condition of forests, rivers and air was improved by modernizing factories, houses (decline of coal heating) and cars, and contaminated areas such as the former Uranium surface mines around Ronneburg have been remediated. Today's environmental problems are the salination of the Werra river, caused by discharges of K+S salt mines around Unterbreizbach and overfertilisation in agriculture, damaging the soil and small rivers.
Document 4: Tuvalu: The rising population has resulted in an increased demand on fish stocks, which are under stress; although the creation of the Funafuti Conservation Area has provided a fishing exclusion area to help sustain the fish population across the Funafuti lagoon. Population pressure on the resources of Funafuti and inadequate sanitation systems have resulted in pollution. The Waste Operations and Services Act of 2009 provides the legal framework for waste management and pollution control projects funded by the European Union directed at organic waste composting in eco-sanitation systems. The Environment Protection (Litter and Waste Control) Regulation 2013 is intended to improve the management of the importation of non-biodegradable materials. In Tuvalu plastic waste is a problem as much imported food and other commodities are supplied in plastic containers or packaging.
Document 5: Nigeria: Waste management including sewage treatment, the linked processes of deforestation and soil degradation, and climate change or global warming are the major environmental problems in Nigeria. Waste management presents problems in a mega city like Lagos and other major Nigerian cities which are linked with economic development, population growth and the inability of municipal councils to manage the resulting rise in industrial and domestic waste. This huge waste management problem is also attributable to unsustainable environmental management lifestyles of Kubwa Community in the Federal Capital Territory, where there are habits of indiscriminate disposal of waste, dumping of waste along or into the canals, sewerage systems that are channels for water flows, etc.
Document 6: History of immigration to the United States: The history of immigration to the United States details the movement of people to the United States starting with the first European settlements from around 1600. Beginning around this time, British and other Europeans settled primarily on the east coast. Later Africans were imported as slaves. The United States experienced successive waves of immigration, particularly from Europe. Immigrants sometimes paid the cost of transoceanic transportation by becoming indentured servants after their arrival in the New World. Later, immigration rules became more restrictive; the ending of numerical restrictions occurred in 1965. Recently, cheap air travel has increased immigration from Asia and Latin America.
Document 7: Caspian Sea: The Volga River, the largest in Europe, drains 20% of the European land area and is the source of 80% of the Caspian's inflow. Its lower reaches are heavily developed with numerous unregulated releases of chemical and biological pollutants. The UN Environment Programme warns that the Caspian "suffers from an enormous burden of pollution from oil extraction and refining, offshore oil fields, radioactive wastes from nuclear power plants and huge volumes of untreated sewage and industrial waste introduced mainly by the Volga River".The magnitude of fossil fuel extraction and transport activity in the Caspian also poses a risk to the environment. The island of Vulf off Baku, for example, has suffered ecological damage as a result of the petrochemical industry; this has significantly decreased the number of species of marine birds in the area. Existing and planned oil and gas pipelines under the sea further increase the potential threat to the environment.
Document 8: Volga Delta: The Volga Delta is the largest river delta in Europe, and occurs where Europe's largest river system, the Volga River, drains into the Caspian Sea in Russia's Astrakhan Oblast, north-east of the republic of Kalmykia. The delta is located in the Caspian Depression—the far eastern part of the delta lies in Kazakhstan. The delta drains into the Caspian approximately 60 km downstream from the city of Astrakhan.
Document 9: Greece: A study from the Mediterranean Migration Observatory maintains that the 2001 census recorded 762,191 persons residing in Greece without Greek citizenship, constituting around 7% of total population. Of the non-citizen residents, 48,560 were EU or European Free Trade Association nationals and 17,426 were Cypriots with privileged status. The majority come from Eastern European countries: Albania (56%), Bulgaria (5%) and Romania (3%), while migrants from the former Soviet Union (Georgia, Russia, Ukraine, Moldova, etc.) comprise 10% of the total. Some of the immigrants from Albania are from the Greek minority in Albania centred on the region of Northern Epirus. In addition the total Albanian national population which includes temporary migrants and undocumented persons is around 600,000.
Document 10: New York City: Newtown Creek, a 3.5-mile (6-kilometer) a long estuary that forms part of the border between the boroughs of Brooklyn and Queens, has been designated a Superfund site for environmental clean-up and remediation of the waterway's recreational and economic resources for many communities. One of the most heavily used bodies of water in the Port of New York and New Jersey, it had been one of the most contaminated industrial sites in the country, containing years of discarded toxins, an estimated 30 million US gallons (110,000 m3) of spilled oil, including the Greenpoint oil spill, raw sewage from New York City's sewer system, and other accumulation.
Document 11: Russia in the European energy sector: The Russian Federation supplies a significant volume of fossil fuels and is the largest exporter of oil and natural gas to the European Union. In 2007, the European Union imported from Russia 185 million tonnes of crude oil, which accounted for 32.6% of total oil import, and 100.7 million tonnes of oil equivalent of natural gas, which accounted 38.7% of total gas import.
Document 12: Greece: Greece, together with Italy and Spain, is a major entry point for illegal immigrants trying to enter the EU. Illegal immigrants entering Greece mostly do so from the border with Turkey at the Evros River and the islands of the eastern Aegean across from Turkey (mainly Lesbos, Chios, Kos, and Samos). In 2012, the majority of illegal immigrants entering Greece came from Afghanistan, followed by Pakistanis and Bangladeshis. In 2015, arrivals of refugees by sea have increased dramatically mainly due to the ongoing Syrian civil war. There were 856,723 arrivals by sea in Greece, an almost fivefold increase to the same period of 2014, of which the Syrians represent almost 45%. An estimated 8% of the arrivals applied for asylum in Greece.
Document 13: Rhine: Until the early 1980s, industry was a major source of water pollution. Although many plants and factories can be found along the Rhine up into Switzerland, it is along the Lower Rhine that the bulk of them are concentrated, as the river passes the major cities of Cologne, Düsseldorf and Duisburg. Duisburg is the home of Europe's largest inland port and functions as a hub to the sea ports of Rotterdam, Antwerp and Amsterdam. The Ruhr, which joins the Rhine in Duisburg, is nowadays a clean river, thanks to a combination of stricter environmental controls, a transition from heavy industry to light industry and cleanup measures, such as the reforestation of Slag and brownfields. The Ruhr currently provides the region with drinking water. It contributes 70 m3/s (2,500 cu ft/s) to the Rhine. Other rivers in the Ruhr Area, above all, the Emscher, still carry a considerable degree of pollution.
Document 14: Southern Europe: The Late Middle Ages represented a period of upheaval in Europe. The epidemic known as the Black Death and an associated famine caused demographic catastrophe in Europe as the population plummeted. Dynastic struggles and wars of conquest kept many of the states of Europe at war for much of the period. In the Balkans, the Ottoman Empire, a Turkish state originating in Anatolia, encroached steadily on former Byzantine lands, culminating in the Fall of Constantinople in 1453.
Document 15: European migrant crisis: The European migrant crisis, or the European refugee crisis, is a term given to a period beginning in 2015 when rising numbers of people arrived in the European Union (EU), travelling across the Mediterranean Sea or overland through Southeast Europe. It is part of a pattern of increased immigration to Europe from other continents which began in the late 20th century and which has encountered resistance in many European countries.
ground_truth: 2015
sub_queries: In what continent is the Volga River located? Answer: Europe
when did the refugee crisis in Europe start Answer: 2015