romania is the biggest country in southeast europe by population. it has an area of . it is the twelfth-largest country in europe. most of romania's border with serbia and bulgaria is made by the danube. the danube joins the prut river. the prut river makes the moldovan–romanian border. the danube then flows into the area of the black sea inside romania. this makes the danube delta. the delta is a biosphere reserve and a biodiversity world heritage site. some of the other major rivers in romania are the siret, the olt, and the mureş. the siret river runs from the north to the south of moldavia. the olt river runs from the carpathian mountains to oltenia. the mureş runs through transylvania from the east to the west.