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The São Francisco is naturally navigable all through the year between Pirapora (Minas Gerais) and the twin cities of Petrolina (Pernambuco) and Juazeiro (Bahia), a length of 1,371 kilometres (852 mi). However, there are large variations in depth depending on the rainfall. Because of the diversity of physical characteristics over the course of the navigable stretch, it may be divided into three substretches, as follows: From Pirapora to Pilão Arcado (Bahia), a length of 1,015 kilometres (631 mi); differences in height up to 6 metres (20 ft) may occur due to rains and drought. From Pilão Arcado to the Sobradinho Dam; the latter's reservoir is 314 kilometres (195 mi) long, with a surface area of 4,214 square kilometres (1,627 sq mi) and a comfortable depth. From the Sobradinho dam to Petrolina/Juazeiro, with a length of 42 kilometres (26 mi) and an average depth of 2 metres (6 ft 7 in), sustained by a flow of 1,500 m3/s (53,000 cu ft/s). 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. In a short time, conditions were such that navigation became impossible for the large gaiolas, although still possible for smaller boats. The shells of those old riverboats can still be seen on the river at Pirapora. As of 2009, a single boat, the Benjamim Guimarães, remains in activity, making short-distance tourist cruises from Pirapora to São Romão and back