Until recent years, the São Francisco was regularly navigated by a type of passenger boat called "gaiola" (Portuguese for "cage"). These were paddle-wheel steamboats, some of them having been Mississippi riverboats and dating from the time of the American Civil War. After the Sobradinho dam was built in Bahia, the conditions of navigability were altered considerably, since the reservoir's large size allowed for the formation of short waves of considerable height. Although the dam has a navigation lock, the waves and currents made traversing the lake difficult for the "gaiolas". At the same time, deforestation and excessive agricultural use of the upper-course waters of the São Francisco and its tributaries greatly reduced the water flow in the middle course, creating sand banks and islands that hindered navigation.