Sedimentary facies of the Eocene Green River Formation reflect a rapid increase in water supply to Lake Gosiute ca. 49 Ma, marked by a stratigraphic fill-to-spill surface. Deposits below this surface constitute repetitive lacustrine expansion-desiccation cycles, whereas those above consist of continuous profundal lacustrine mudstone, grading upward into volcaniclastic deltaic sandstone. Above the fill-to-spill surface, calcitic mudstone δ18O decreases from ~+26‰ to +20‰ over an interval representing ~100 k.y. We interpret this shift to have resulted from capture of a foreland river (or rivers) that drained higher topography north of Lake Gosiute, most likely in north-central Idaho. Accurate paleoelevation estimates derived from stable isotopic records in intermontane basins thus may require detailed knowledge of regional drainage systems.