Abstract: Nowadays, alteration of the natural flow regime is considered one the most widespread and damaging impacts for river ecosystems. Hence, increasing our understanding of large-scale hydrological alteration patterns would help us design more effective water use policies. The present study aims to establish general patterns of hydrological alteration caused by dams on a national level, with Spain as a case study. First, we developed a classification of the natural flow regime of the Spanish river network, which served as the reference to assess the degree of hydrological alteration of 139 altered-river gauges. In addition, using the flow series of the altered-river gauges we defined a set of 7 types of altered regimes (TARs), which allowed the stratification of the analyses. The results revealed that the magnitude and direction of hydrological alteration depended on the natural flow class of the altered rivers. In this regard, major effects of dams on Spanish rivers were related to the modification of the intra-annual variability of daily flow, the magnitude of seasonal maximum and minimum flows and the patterns of high flow events. Our results also showed that the distribution of the TARs partially followed a geographic order, but associations between TARs and natural flow classes were not straightforward. In addition, we highlighted that the nature of the hydrological alteration was independent of the registered dam uses.

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