 Reservoirs have been built for various purposes such as water supply, irrigation, and electricity generation in Brazil. There are over 700 large dams and associated reservoirs in the country's major rivers. These reservoirs bring economic benefits, but also cause significant alterations to the natural hydrologic regime of rivers, affecting habitat quality and biodynamics. The main impact of reservoirs is the change from Lodic to Lentic water, which affects aquatic fauna, including fish. Fish species richness usually increases immediately after reservoir formation, but decreases as reservoirs age. Dams promote discharge control, altering seasonal flood cycles and affecting fish composition and structure. The most affected species are reaphylics and long-distance migratory that require distinct habitats to complete their life cycles. Management actions taken to minimize dam impacts include fish passages, fishery control, and stocking. However, the results of these actions are questionable or have clear failures. This paper focuses on the Piranha River basin in Brazil, which is the most affected by dams. It describes patterns in the alteration and decline of fish diversity in areas influenced by dams and discusses negative consequences in the fishery and ecosystem functioning. The paper argues for the relevance and success of management actions taken and presents suggestions to improve conservation of Icthia fauna and South American basins influenced by dams. This article was authored by A. A. Augustino, F. M. Pelicis, and L. C. Gomez. We are article.tv, links in the description below.