Emergent in the last three decades has been a spatially coherent trend in the variance and synchroneity of stream flow across the four major river basins in western North American region–Fraser, Columbia, Sacramento-San Joaquin, and Upper Colorado. This has occurred at a time when freshwater (and energy) supplied by these river basins has been relied upon to sustain the pace of development and growth in the West. The increase in the variance of stream flow has been unprecedented in the 20th century, and has been accompanied by an increased simultaneous incidence of wet and dry years across all four river basins. It is discovered that these changes in stream flow have covarying analogues in the Indo-Pacific ocean temperatures, thus raising important new questions on the detection, attribution, and projection of regional hydrologic change. Joint analysis of observations and climate model simulations explore the role of changing ocean temperatures (and changing statistics of ENSO) in forcing the observed incidence of expansive regional water resources stresses. The observed trends in the variance of stream flow point to uncertain water supplies, that coupled with recent eprsistent drought, demographic and socio-economic trends are likely to exacerbate water resources problems into the future.