Holocene hydroclimate changes in the western US are poorly constrained, in part because the region's complex topography and seasonally distinct circulation patterns can generate heterogenous responses over small spatial scales. To better understand regional precipitation patterns over the Holocene, we measured the triple oxygen isotopes of goethite from high-elevation ferricrete deposits across a latitudinal gradient in the Rocky Mountains. Ferricretes form in stream water that reflects the isotopic composition of local precipitation, providing a record of past hydroclimate changes. We also used a 1-dimensional vapor transport model to simulate precipitation delta 18O and Delta ' 17O along a hypothetical storm track, given varying moisture recycling amounts and evaporation versus transpiration contributions. We find that both northern (central Montana) and southern (southwest Colorado) sites exhibit similar increases in goethite delta 18O over time, accompanied by declines in goethite Delta ' 17O. To interpret these trends, we considered three potential drivers: temperature, moisture recycling and evapotranspiration partitioning, and moisture source. Delta ' 17O is largely insensitive to temperature, and our model shows that changes in moisture recycling and evapotranspiration partitioning along the storm track cannot account for the observed Delta ' 17O decreases. Instead, we conclude that the isotopic trends primarily reflect an increased contribution of southerly-sourced precipitation, since moisture from the Gulf of Mexico-the dominant source of interior western US summer precipitation-is characterized by high delta 18O and low Delta ' 17O relative to Pacific moisture. More generally, this work demonstrates how triple oxygen isotopes can be used to disentangle changes in temperature, atmospheric moisture dynamics, and moisture source in semi-arid to arid regions.