By 70 CE the Chinese were using locks for river navigation. The locks were simply a dam with a removable section. Once the section was removed, the rush of the water would carry the boat over the shallows below.
- Lock (water navigation) – Wikipedia
A lock is a device used for raising and lowering boats, ships and other watercraft between stretches of water of different levels on river and canal waterways.
- History and development: Dams and weirs – Wikipedia
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.