After crossing a river, you should get far away from it.
“In order to tempt the enemy to cross after you,” according to
Ts‘ao Kung, and also, says Chang Yü, “in order not to be impeded
in your evolutions.” The _T‘ung Tien_ reads 敵若絶水 “If _the
enemy_ crosses a river,” etc. But in view of the next sentence,
this is almost certainly an interpolation.