tokens: Once a long-standing frontier of the Roman Empire , the river passes through or touches the borders of 10 countries : Romania ( 29.0 % of basin area ) , Hungary ( 11.6 % ) , Serbia ( 10.2 % ) , Austria ( 10.0 % ) , Germany ( 7.0 % ) , Bulgaria ( 5.9 % ) , Slovakia ( 5.9 % ) , Croatia ( 4.4 % ) , Ukraine ( 3.8 % ) , and Moldova ( 1.6 % ) .