Low oxygen isotope values (∼−16‰, relative to Peedee belemnite standard) obtained from microbial carbonates of the Green River Formation have been interpreted as evidence for snowmelt and high elevations surrounding this early Eocene lake basin. However, low values from microbial precipitates could also represent a diagenetic overprint. We investigate these alternate hypotheses by measuring the oxygen isotope composition of altered and unaltered freshwater bivalves from the basin. We analyzed subannual samples from 3 individuals and bulk samples from 54 individuals. Subannual samples exhibit clear seasonal fluctuations in oxygen isotope values. These fluctuations are large enough to require seasonal shifts in temperature and/or the oxygen isotope value of the lake water. However, the lowest value of any unaltered sample (either bulk or high resolution) corresponds to lake-water δ18O of ∼−12‰ (relative to standard mean ocean water) and is not low enough to require the addition of snowmelt to the lake. A few bulk samples exhibit very low oxygen isotope values, which would seem to suggest snowmelt. However, these samples also show clear evidence of calcite in X-ray diffraction patterns, demonstrating the presence of diagenetic precipitates. Given that (1) diagenetic alteration is a plausible explanation of very low δ18O values, (2) alteration has not been examined in microbial carbonates, and (3) unaltered aragonitic bivalves provide no independent evidence for low δ18O values for lake waters, we conclude that robust isotopic evidence for high elevations surrounding the Green River Basin during the Eocene is currently lacking.