Paleotemperature and salinity profiles for the Western Interior Seaway are reconstructed by use of oxygen and carbon isotopic signatures of shell material. Data have been gathered on different groups of molluscs distributed in three distinct habitat types: infaunal benthonic, epifaunal benthonic, and nektonic. Isotopic variations and inferred environmental tolerances of the organisms were used to determine upper and lower temperature boundaries for each habitat group and therefore water-depth zones. Water density for each temperature-salinity combination within a habitat range was calculated and used to develop a physically stable stratification model. The results clearly show that the water column consisted of denser, more saline, warm bottom layers that were overlain by, and decoupled from, cooler, less saline intermediate layers. It is proposed that this profile originated due to a surface current transporting runoff water from the western highlands toward the eastern shallow border of the seaway where evaporation and heating of surface water in a warm, humid atmosphere caused increased density and sinking, followed by westward return flow of the warm saline bottom water. This model is compatible with known climatic and oceanographic information for the Cretaceous.