4 ▁following ▁a ▁d yn astic ▁dispute . ▁ ▁In ▁ 4 2 6 , ▁the ▁Emperor ▁Tai w u ▁of ▁Northern ▁Wei ▁made ▁a ▁surprise ▁attack ▁on ▁T ong wan ch eng . ▁Although ▁a ▁brief ▁inc urs ion ▁into ▁the ▁city ▁succeeded ▁only ▁in ▁burning ▁the ▁main ▁temple , ▁the ▁surrounding ▁hinter land ▁was ▁dev ast ated . ▁ ▁The ▁city ' s ▁site ▁was ▁on ▁the ▁fert ile ▁upper ▁reaches ▁of ▁the ▁W ud ing ▁River , ▁but ▁the ▁river ▁and ▁lake ▁died ▁up , ▁possibly ▁due ▁to ▁def or est ation ▁that ▁might ▁be ▁trac ed ▁back ▁to ▁Tai w u ' s ▁dev ast ation . ▁ ▁The ▁city ▁was ▁then ▁gradually ▁buried ▁by ▁the ▁s ands ▁of ▁the ▁desert . ▁ ▁This ▁' wand ering ' ▁( W ud ing ) ▁gave ▁the ▁river ▁its ▁current ▁name . ▁ ▁The ▁X ion gnu ▁continued ▁to ▁live ▁in ▁the ▁region ▁until ▁the ▁ 7 th ▁or ▁ 8 th ▁century . ▁ ▁In ▁ 7 8 6 ▁the ▁city ▁was ▁bes ie ged ▁by ▁Tib et an ▁forces , ▁and ▁it ▁was ▁inv aded ▁by ▁J urch en ▁soldiers ▁in ▁ 1 2 0 6 . ▁ ▁There ▁is ▁no ▁record ▁of ▁the ▁site ▁in ▁Chinese ▁records ▁after ▁the ▁early ▁ 1 5 th ▁century . ▁ ▁The ▁city ▁was ▁only ▁properly ▁survey ed ▁by ▁the ▁Chinese ▁in ▁the ▁ 2 0 0 0 s . ▁ ▁The ▁city ' s ▁Y ong ' an ▁Platform , ▁a