A long-term study (1985-1999) conducted at backwater and tailwater sites of a large reservoir located in the middle course of the 808 km long Warta River (Poland), showed that not only obligatory riverine species suffer from the effect of the dam. Facultative riverine eel, burbot, wels and pike considerably decreased in number and standing crop, including their apparent absence in some years following the damming. Nevertheless, median densities of perch and roach, two generalists thriving in European regulated rivers, increased from tens and hundreds, respectively, to thousands per hectare. Significant increases in density were also recorded for other four species: zander, ruffe, bream and silver bream. Thus, the reaction of facultative riverine fish, often supposed to be resistant to aquatic habitat deterioration, is diverse with respect to impoundment. Apart from the migratory eel, which have not been caught at the backwater site for the last 8 years, all the remaining significant temporal changes were recorded at the tailwater site. Out of habitat flexibility coefficients relating to refuge, feeding and spawning, only the latter considerably differed between the successful and unsuccessful species after impoundment, which testifies to the critical role of spawning flexibility in a human-modified environment.