The U.S Army Corps of Engineers built a scale model of the San Francisco Bay and the Sacramento-San Joaquin River Delta system in 1957. The three-dimensional model is more than 1.5 acres in size and is capable of simulating the tides and currents found in the bay. It is emptied every night and gradually refills with fresh salt water. The salinity exactly matches the areas of the bay it depicts. It is updated to accurately reflect current conditions as dredging is done. There are ranger-guided tours of the model and anyone can wander around and see the exhibits. Rather than a physical model of the bay, it is a simulator of bay and river conditions, and is used to reproduce (to the proper scale) the rise and fall of tide, flow, and currents of water, mixing of salt and fresh water, and indicates trends in sediment movement.