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In ancient times river transport was common, but rivers were often too shallow to carry anything but the smallest boats. Ancient people discovered that rivers could be made to carry larger boats by making dams to raise the water level. The water behind the dam deepened until it spilled over the top creating a weir. The water was then deep enough to carry larger boats. This dam building was repeated along the river, until there were "steps" of deep water.
This however created the problem of how to get the boats between these "steps" of water. An early and crude way of doing this was by means of a Flash lock. A flash lock consisted essentially of a small door in the dam, which could be quickly opened and closed.
When the gap was opened, a torrent of water would spill out, and the boat would be hauled through the opening against the water current with ropes, and when the boat was through, the opening would be quickly closed again.
This system was used extensively in Ancient China and in many other parts of the world. But this method was however highly dangerous, and risky, and many boats were deluged by the torrent of water.
The type of lock seen today is known as a Pound lock as described earlier in this article, which work by raising or lowering the water level within a double gated pound. It is believed that this type of lock originated in Medieval Europe.