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Summary: This paper analyzes streamflow trends in 187 sub-basins in the Iberian Peninsula for the period 1945–2005. A database of monthly river discharges for the entire Iberian Peninsula including natural and regulated river regimes enabled assessment of the magnitude and spatial patterns and mechanisms of the hydrological trends. Annual and seasonal trend analyses were conducted. The results showed a marked decrease in annual, winter, and spring streamflows in most of the Iberian sub-basins, especially those in the south. In addition, changes in the seasonality of river regimes have occurred, most of them as consequence of dam regulation and water management strategies. We showed how river regulation by dams does not affect the sign of the trends, but its magnitude, by decreasing the releases during winter to meet the demand of water in summer creating important seasonal differences. The decrease of streamflows during the second half of the Twentieth Century in the Iberian Peninsula may accelerate in coming decades, as future climate projections show a generalized decrease in precipitation and more evapotranspiration induced by higher temperatures. [ABSTRACT FROM AUTHOR]