You may or may not have them in your part of the world, but lock and dam systems are important in ours. The Mississippi River is the nation's central freight highway and of untold economic importance. You just don't see them much from the road, with a few exceptions.
When our tour boat slid into L&D 27, the last one on the the river's southbound course, big steel gates closed behind us and the water level of the lower river. This is the lock for small boats - we went through the big one on our way back. In front, a horizontal gate slowly lowered, letting in water from the north and raising the boat some 15 feet / 4.5 meters. I've seen these things from the side before (the next one up has a visitor's center) but never been through one on a vessel.
If you are terribly curious, there is a long discussion about how these work around the world at https://en.wikipedia.org/wiki/Lock_(water_navigation) .