Although projected near-future precipitation and temperature changes over western North America are subject to much debate, they are generally projected to cause widespread reductions in snowpack accumulation and changes in the timing and intensity of snowmelt-derived streamflow. These changes would have important consequences for water resources management, especially for the western United States where largest contributions to annual streamflow from mid- and high-altitude basins is from spring snowmelt runoff. Regionally coherent fluctuations and trends in the timings of both (1) the start of the spring snowmelt runoff pulse and (2) the "center of mass" of streamflow (center time) towards earlier in the water year are found in much of western North America during the past 50 years, using a new data set combining US (HCDN) and Canadian streamflow records. The great regional coherency of the observed timing variations suggests continental- and regional-scale climatic forcings, that are superimposed upon basin-scale processes. Changes in streamflow timing show strong correlations with seasonal temperature anomalies syncronized with the streamflow center time, in the sense that warmer temperatures produce earlier flow, and vice versa. Also, the changes in timing are correlated with annual precipitation, in the sense that higher annual precipitation associates with later streamflow timing, and vice versa. There are significant links to global climate variability, with widespread negative correlations between center timing and both the PNA and PDO indices throughout the study area, especially in the interior northwestern parts of the contiguous US, in Canada, and in Alaska. Notably, though, observed trends in runoff timing are most closely associated with recent warming trends across western North America.