The δ18O values of river waters in Late Cretaceous and Paleogene basins of Alberta, Montana, Wyoming, and Colorado are calculated from the isotopic composition of unaltered aragonite mollusk fossils. The isotopic composition of these river waters has significant implications for the uplift and climate history of the mountains surrounding the basins. Our data show that the δ18O values of river water varied dramatically, ranging from −23‰ to −5‰ relative to Vienna standard mean ocean water, with an uncertainty of ±2‰. Close stratigraphic association of river deposits with very different isotopic values indicates two different water sources: rainfall in the warm, low-elevation basins and snowfall in surrounding mountains. The δ18O difference between basinal and highland precipitation suggests that local relief was similar to today's, on the order of 2.5–3 km. Seasonal variation in shell δ18O values also indicates that there are times when large snow fields in the Maastrichtian and Paleocene mountains of Montana and Wyoming survived summer melting and were multiannual features.