Little is known about biogeochemical cycling prior to and immediately following the Cretaceous-Paleogene (K-Pg) mass extinction event that ended the realm of the dinosaurs. Possible environmental stressors during this time may support the notion of other contributing factors to the extinction in addition to the widely-accepted proposal of a large meteor impact. Here we present ranges of dissolved inorganic carbon isotopes (δ13C-DIC) in streams draining the landscape near the Western Interior Seaway (now Montana, USA) calculated from freshwater bivalve shell δ13C values. These data provide an indication of the nature of carbon cycling in streams feeding into the seaway. Metabolic carbon does obscure these data however, as it naturally incorporates into bivalve shells in varying amounts and thus hinders the calculation of δ13C-DIC values. However, we present a method of carbon isotope analysis within bivalve shells, which sets metabolic carbon values to zero, allowing for the calculation of the lowest possible δ13C-DIC value within a given sampled shell. Using this method for sampled shells, Cretaceous stream δ13C-DIC was calculated to be for the most part no lower than -10 ‰, though higher values of 2 ‰ were calculated as well. Given these ranges, it was determined that carbon in Cretaceous streams was not dominated by biotic respiration. Higher δ13C-DIC values in the stream may have been caused by a variety of factors likely including weathering or highly productive waters. These values do not indicate stressed or unusual conditions.