The purpose of this study was to characterize mountain river regimes in the Spanish Pyrenees and to assess the importance of snow accumulation and snowmelt on the timing of river flows.Daily streamflow data from 9 gauging stations in the Pyrenees were used to characterize river regimes.These data were analyzed by hydrological indices,with a focus on periods when snow accumulation and snowmelt occurred.These results were combined with data on Snow Water Equivalent (SWE) (from measurements of depth and density of snow in the main river basins and also simulated by a process-based hydrological model),snowmelting (simulated by a process-based hydrological model),precipitation (from observations),and temperature (from observations).Longitude and elevation gradients in the Pyrenees explain the transition or river regimes from those that mostly had low nival signals (in the west and at low elevations) to those that mostly had high nival signals (low winter runoff and late spring peakflow,in the east and at high elevations).Although trend analyses indicated no statistically significant changes,there was a trend of decreased nival signal over time in most of the analyzed rivers.Our results also demonstrated that snow processes cannot explain all of the interannual variability of river regimes,because the temporal distribution of liquid precipitation and temperature play key roles in hydrography.