As we were walking along we came across a river we couldn't pass, so we turned back.

turn 的意思是"转向、改变方向"，这里指改变行进方向。 back 的意思是"向后、往回"，表示返回原路的方向。 turned back 合起来表示"折返、返回"，指因为遇到障碍而原路返回的动作。