Rocky Mountain headwaters are a critical source of water for major watersheds in Alberta. For example, the combined contribution of mountain headwaters supplies 99% of the annual volume of the South Saskatchewan River. Annual yield in these Rocky Mountain headwaters has increased in southern Alberta (Oldman, Bow, Red Deer, and NSR Basins) over the 40-year period between 1976-2015 and largely decreased in the Athabasca and Peace River Basins. Later spring freshet timing and earlier onset of the low flow season were found for most watersheds, leading to more water over a shorter duration for watersheds with increasing water yield, and water losses in late summer and autumn for watersheds with decreasing water yield. Water yield and streamflow timing are strongly related to precipitation and moderately related to temperature, indicating that changes in precipitation drive changes in annual yield and streamflow timing.