• climate change;
  • paleohydrology;
  • resource management;
  • river discharge;
  • water resources;
  • western North America

[1] Shrinking glaciers and snowpacks are reducing discharge in rivers that drain the central Rocky Mountain region – water that supports downstream societies and ecosystems of western North America. However, a new 5200-year record of Lake Athabasca water-level variations, which serves as a sensitive gauge of past changes in alpine-sourced river discharge, reveals that western Canadian society has developed during a rare period of unusually abundant water ‘subsidized’ by prior glacier expansion. As the ‘alpine water tap’ closes, much drier times are ahead. Future water availability is likely to become similar to the mid-Holocene when Lake Athabasca dropped 2–4 m below the twentieth-century mean. Regions dependent on high-elevation runoff (i.e., western North America) must prepare to cope with impending water scarcity of magnitude not yet experienced since European settlement.