Observations have shown that the hydrological cycle of the western United States changed significantly over the last half of the 20th century. We present a regional, multivariable climatechange detection and attribution study, using a high-resolution hydrologic model forced by globalclimate models, focusing on the changes that have already affected this primarily arid regionwith a large and growing population. The results show that up to 60% of the climate-related trendsof river flow, winter air temperature, and snow pack between 1950 and 1999 are human-induced.These results are robust to perturbation of study variates and methods. They portend, in conjunction with previous work, a coming crisis in water supply for the western United States.