 Two roads ran through the regimental sector from the Naktong River to Changnyong . The main road bent south along the east bank of the river to Pugong @-@ ni and then turned northeast to Changnyong . A northern secondary road curved around marshland and lakes , the largest of which was Lake U @-@ p 'o , to Changnyong . In effect , the 1st Battalion of the 23rd Regiment guarded these two approach routes to Changnyong . 
