Isotopic analysis of lacustrine carbonates from the Eocene Green River Formation suggests that lake waters were derived partly from snow melt. This evidence for cool climates is in marked contrast to paleontological and model evidence for mild temperatures in the continental interior. Oxygen isotope ratios of carbonates frequently reach −12‰ to nearly −16‰ (Peedee belemnite), which suggests that lake waters probably had δ18O of <−13‰ (standard mean ocean water). Consideration of the evaporative 18O enrichment that typically occurs in modern large saline lakes suggests that the source waters to the Green River basin had a δ18O of <−18‰. These ratios are consistent with snow melt and are too negative to be easily accounted for by distillation in the atmosphere during heavy rainfall. The Green River lakes formed in a closed basin encircled by large mountains; this suggests that the snow melt was locally produced. The mountains surrounding the lake must have been high enough to occasionally supply significant melt water to the much lower lake. Lapse rate calculations suggest minimum altitudes of >3000 m for the mountains encircling the Green River basin.