ABSTRACT
Environmental disturbances that expose a community to sudden changes in resource levels may elicit insights about their structuring mechanisms, such as competition for food resources or predation (Piet, 1998). Because they change the physical and chemical characteristics of water and water speed, as well as microhabitats and food sources, river impoundments are among the most significant environmental disturbances (Julio Jr. et al., 1997). Human interference in natural watercourses-through the construction of dams for various purposes-has been a common practice since remote times in Brazil. For instance, the first hydropower plant was
built on the Paraibuna River in 1889 (Petrere Jr. et al., 2002). Construction of large reservoirs, especially aimed to energy generation, reached its maximum development in the 1960s and 1970s; a considerable number of large Brazilian rivers were then completely transformed into cascading dams (Tundisi, 1999). The Paran River, for instance, the second largest drainage basin in South America, has 70% of its Brazilian portion turned into reservoirs (Agostinho et al., 1994).
