水坝是防堵水的障碍物。
因为他们不知道水有多深，不知道礁石块儿在哪里，不知道障碍物在哪里。
They didn't know how deep the water was. They didn't know where the coral heads were. They didn't know where the obstacles were.
当水跌过障碍物，它会自己回卷，形成一股强劲的可以困住船或者游泳者的回流。
When water drops over a obstacle, it curls back on itself, forming a strong upstream current which may be capable of holding a boat or swimmer.
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