Throughout history it has often been desirable to allow boats to travel up streams farther than shallow water would normally allow them to go. For centuries this problem has been solved by using dams, gates, and locks.
Since 1941, dams built in San Antonio's downtown river channel have maintained the consistent water levels needed for boat travel. Completion in 2009 of the dam and locks at Brooklyn Avenue extended travel farther to the north. The locks are basically enclosures with gates on either end. The gates are opened or closed to raise or lower the water level inside the locks using gravity flow. A boat moving upstream enters the lock at the lower end through open gates. The gates are then closed and valves opened, allowing water to flow into the lock from the upper pool, floating the boat up to the higher level. The upstream gates are then opened, and the vessel moves into the upper pool. On the return trip, the process is reversed.
The Brooklyn Avenue dam and lock system allows passenger barges to move from downtown San Antonio all the way north to the Pearl Brewery and back. Since there are two locks, boats can move upstream and downstream at the same time.