“…Indeed, rivers are highly variable environments and are periodically subjected to extreme and often unpredictable fluctuations in their physical and chemical characteristics (e.g. flow, temperature, dissolved oxygen, pH, conductivity) and these fluctuations have been shown to affect the richness and structure of river fish assemblages (Horwitz, 1978;Grossman, 1982;Grossman et al, 1982Grossman et al, , 1985Grossman et al, , 1990Grossman et al, , 1998Schlosser, 1985;Merona, 1986;Schlosser and Ebel, 1989;Henderson and Walker, 1990;Poff and Allan, 1995;Landivar, 1995;Agostinho and Zalewski, 1995;Tito de Morais et al, 1995;Carrel and Rivier, 1996;Danehy et al, 1998;Lamouroux et al, 1999;Oberdorff et al, 2001b;TejerinaGarro, 2001;Fialho 2002) by leading to local population extinctions, individual immigration and emigration in response to current conditions and through recruitment success (Freeman et al, 1988;Carrel and Rivier, 1996). Furthermore, the importance of intradrainage immigration in shaping assemblage richness and structure has been emphasized by Osborne and Wiley (1992).…”