Argentine water resources are located in three hydrologically contrasting regions: the so-called Del Plata basin, the Arid Central Andean area, and Patagonia. About 150 dams and impounding reservoirs that exceed a capacity of 150 x 103 m3, have been built. Most limnological studies of regulated rivers have been conducted recently. In the Del Plata basin, Argentine dams are scarce, although many have been built or are projected upstream, mainly in Brazil. The ecological problems of the reservoirs are related to decomposition of inundated terrestrial vegetation; algae blooms, dissolved oxygen depletion phenomena, eutrophication, impaired fish migrations, overproduction of water hyacinth Eichhornia spp ., and schistosomiasis. The importance of examining the influence of the projected middle Parank dams on downstream sediment regimes, especially as it affects the delta, is emphasized. In the Arid or Central Andean region, the main development of the Grande-Tercero rivers includes dams, reservoirs, and the operation of a nuclear power plant. One of the most important effects will be a slight increase in the water temperature of the Rio Tercero reservoir as a consequence of water use in the cooling system of the nuclear power plant. In the Patagonian region, the main dams are those built and projected for the Colorado and Negro basins. In the Negro basin studies are being carried out mainly in the Ramos Mexia Reservoir, on different aspects of zooplankton and phytoplankton, benthos, and fishes. Commercial fishing is not permitted and fishing is based mainly on the introduced salmonids which show symptoms of undernourishment because of a lack of forage fishes. Indigenous fishes have been decimated by the introduced salmonids.Although studies have been conducted in several areas, much work remains to be done in relation to the environmental effects of dams on streams in Argentina. Results from ecological studies are still insufficient to solve the multiple problems associated with them, such as climatic influences, retention of suspended solids, deforestation, schistosomiasis and other aquatic diseases, pollution, eutrophication, fish-ways and fish migrations, accumulations of the water-hyacinth, salt and nutrient regimes, and siltation. Even partial solutions to these problems will require an ensemble of urgent measures to assure the maximum benefits with minor risks and ecological costs. KEY WORDS Argentina Impounding reservoirs Interbasin transfers Ecological problems INTRODUCTION Contrary to what is commonly believed, the water resources of the Argentine Republic are quite poor overall. Soldano (1 947) distinguished three hydrological regions (Figure 1): 1. The Del Plata Basin with a discharge of around 20,000 m3 s-l from an area of 830,000 km'; 0886-9375/87/020129-15$07.50 0