reverse-direction flow

例句：

1. 1. The waterflows back into the river from thelake.
   水从湖里流回河里。
2. 2. The machine reverses direction when itreaches its limit.
   机器到达极限时会反转方向。
3. 3. The flood caused a backflowof sewage into the city'smain drains.
   洪水导致污水回流到城市的主排水道。