Atmospheric rivers are one of the major causes of extreme precipitation and flooding in many regions around the world, and have been found to contribute substantially to global poleward moisture transport. However, the evaporative origin of the moisture in atmospheric rivers remains unclear, at least on climatological time-scales. Here we use the water tracer and water isotope-enabled CAM5 model to examine the moisture sources of atmospheric rivers that impact the West Coast of the United States. The climatological distribution of moisture sources is determined for both the modern-era and for 2100 under an RCP8.5 scenario. It is found that 33 to 53 % of the precipitable water over the West Coast of the United States originates from the Northeast Pacific, in particular the midlatitudes and subtropics, although in JJA more moisture is recycled from continental regions. It is also found that although atmospheric rivers are at least 70 % Northeast Pacific moisture, a significant amount of the moisture is derived from tropical latitudes (>15 % from south of 20° N). It is found that in the warmer 2100 climate there is a 39 % increase in the magnitude of ARs. In this future epoch, moisture is transported from more remote regions for all seasons and for both atmospheric rivers and the average climatology. To provide future observational evidence that this model result is robust, it is shown that water isotopes provide an observational constraint on the moisture transport pathways, and thus the possibility to observe changes in moisture source.