All three regiments of the NK 2nd Division @-@ the 4th , 17th , and 6th , in line from north to south @-@ crossed during the night to the east side of the Naktong River into the 23rd Regiment sector . The NK 2nd Division , concentrated in the Sinban @-@ ni area west of the river , had , in effect , attacked straight east across the river and was trying to seize the two avenues of advance into Changnyong above and below Lake U @-@ p 'o . On August 31 , 1950 , Lake U @-@ p 'o was a large body of water although in most places very shallow .