If it were possible to develop a turbine that did not need such a water-head to operate, and that could sit in the riverbed, then a dam would be unnecessary.
如果有可能制造出一种涡轮无需水的落差来运转，并且可以放置在河床上，那么就不再需要大坝了。
2
The water closed over his head.
水没过他的头顶。
《牛津词典》
3
I bashed him on the head and dumped him in the water.