The Santa Cruz main stem river is located in the extra-Andean biozone, a scarcely vegetated plateau with semi-arid grass and low shrubs. It originates in two oligotrophic to ultra-oligotrophic large glacial lakes, Viedma and Argentino, and flows for 382 km across the Patagonian plateau to drain into the Atlantic Ocean. River discharge is derived primarily from snow and glacial melt, which is in turn governed by the complex interaction between climatic conditions and the dynamics of the South Patagonian Ice Field. The two dams projected for the Santa Cruz River are located at river km 132 (Cóndor Cliff, 50.206° S, 70.785° W) and at river km 197 (Barrancosa, 50.185° S, 70.177° W). Together they will dam up 197 km of river, leaving only a lower stretch of 49% of current length of regulated river. Typical hydrologic impacts of dams around the world consist of reducing the amount and variation of downstream flow and the frequency of bed scouring flows. The result of such manipulation of river discharge is a simplification of channel morphology and an impoverishment of the mosaic of river habitats, with negative impacts on community composition and species richness. The most significant and obvious impact of the two dams to be built in the Santa Cruz will be the obliteration of 51% of the current lotic environment. Meanwhile, downstream impacts of dams on river biota through changes in thermal and hydrological regimes, and in nutrient and sediment loads are harder to predict. In general, only few species capable to withstand sudden changes in flow are expected to survive below the dams. In the end, the outcome will depend on specific aspects of the project design and operation, which have not been publicly revealed yet.