To estimate the threshold value triggering off destabilization of dominant habitats of the Drzewiczka, a river affected by a highly fluctuating discharge regime, an “experiment” was carried out in the river in three seasons: autumn, spring, and winter. It consisted in two-three week periods of maintaining the discharge on its natural level over a 24 hour period and then releasing from a dam reservoir a large water volume, which exceeded the natural discharge by several to over 16 times, reaching, respectively, 8.4 m3s-1 in September 2000, 12.0 m3s-1 in March 2001 and 41.8 m3s-1 in February 2002, the last discharge occurring during the complete emptying of the reservoir, before its dredging. In each habitat morphometric, hydraulic and biotic parameters were measured before and during water releases.
A threefold increase in discharge (in September) caused a destabilization of inorganic substrate, organic sediment and periphyton, only in a habitat located closest to the dam and the white/wild water canoe slalom track, while a fivefold increase in the discharge (in March) determined similar changes of parameters in this habitat and in another, of coarse substrate and very fast current. The highest discharge (February) was a cause of substrate destabilization, the least changes being recorded in a stagnant off-shore habitat, overgrown by submerged macrophytes; thus, this habitat may be considered a refuge for benthofauna.