During summer 200 1, zebra mussels (Dreissena polymorpha Pallas) were found (500 adults m(-2)) in the coupled reservoir system composed by Mequinenza, Riba-Roja and Flix (Ebro River, Northeast Spain). Two years later, mussels achieved densities of 4000 adults m(-2), representing the first record of successful colonization by zebra mussels in the Iberian Peninsula. An August 2003 study investigated the environmental conditions that support the successful settlement of the zebra mussel populations during the stratification period. Flow patterns in the whole system and mussel populations near the dam were characterized. Chemical characteristics of water inputs, not the usual thermal stratification, determine the deep circulation of the Ebro River along the Riba-Roja reservoir, whereas water input from one tributary into Riba-Roja flows along the top of the water. Physico-chemical stratification of the water column seems to control the observed vertical distribution of zebra mussel biomass. Larger biomass and mean body size were found in the epilimnion, while meta- and hypolimnetic individuals were smaller in size and composed a lower biomass. Moreover, to know the influence of the water drawn off for electricity production in the settlement of mussel larvae, an artificial substratum (a rope) was placed in the influence area of the dam. Results indicate that flow conditions prevailing in the drawn-water layer may increase the attachment success of mussel larvae. Results show that during the stratification period the epilimnion of Riba-Roj a was functioning as a "biological reactor" where Dreissena populations thrived.