These gates are the doors that allow boats and barges to enter and leave the lock. Gates are located on each end of the lock. Once the doors are closed on both ends, the water level can be raised or lowered solely by gravity.
The locks were also symbolic doors when the original canal was built in 1821. It created a doorway to the Great Lakes, and by extension, opened the continent allowing relatively fast, inexpensive and convenient travel. It is the canal that made Rochester a city, along with many other towns in its path.