Attribution of trends in streamflow is complex, but essential, in identifying optimal management options for water resources. Disagreement remains on the relative role of climate change and human factors, including water ions and land cover change, in driving change in annual streamflow. We construct a very dense network of gauging stations (n = 1,874) from Ireland, the United Kingdom, France, Spain, and Portugal for the period of 1961–2012 to detect and then attribute changes in annual streamflow. Using regression‐based techniques, we show that climate (precipitation and atmospheric evaporative demand) explains many of the observed trends in northwest Europe, while for southwest Europe human disturbances better explain both temporal and spatial trends. For the latter, large increases in irrigated areas, agricultural intensification, and natural revegetation of marginal lands are inferred to be the dominant drivers of decreases in streamflow. Plain Language Summary Reduced water resources availability is one of the most serious impacts of climate change since reductions in streamflow may cause noticeable ecological and socioeconomic impacts. However, attribution of streamflow trends to climate change is complex given the influence of other drivers of catchment change, including human and vegetation water uses, agriculture, and land use change. We show that for northwestern Europe most observed trends in annual streamflow are associated with climate change. However, in southwestern Europe there is a clear mismatch between observed trends in river flows and climate, with increasing vegetation and/or irrigated agriculture better explaining observed changes. Our results highlight the importance of human management in explaining large‐scale hydrological trends and the need to carefully evaluate both climate and land use changes to disentangle drivers of streamflow trends. Key Points Streamflow trends in countries bordering the northeast Atlantic show a north‐south latitudinal gradient, with strong decreasing trends in southern regions Climate trends largely explain the evolution of annual streamflow in northwestern Europe Climate trends cannot fully explain the large reductions in annual streamflow in southwest Europe, with land use changes and water demand from irrigation playing an important additional role