Terrestrial ecosystems occupying the coastal floodplains of western North America during the Late Cretaceous were comprised of diverse assemblages of plants (e.g. gingkos, conifers and angiosperms) and herbivorous vertebrates (e.g. hadrosaurid and ceratopsid dinosaurs), even over relatively small spatial scales. Such taxonomic diversity is unusual, and begs the question of how these organisms were distributed over, and were influenced by, the landscapes they occupied. In an effort to answer this question, stable isotope isotope data have been collected from hadrosaurid tooth enamel & dentine, gar fish scales, paleosol alkanes and carbonate nodules, and micritic pond carbonates of the Campanian-aged Kaiparowits Formation in southern Utah. Results suggest the existence of fluvial systems akin to those of present-day Cambodia, with interfluves impacted by episodic flooding and the resultant mixing of water from low and high elevation sources. Carbon cycling in interfluve soils took place via oxidation and/or methanogenesis, depending on soil drainage/the degree of seasonal flooding. Vegetation appears to track landscape-scale variations in hydrology and carbon cycling, with closed-canopy and open–canopy forests associated with wetter and drier environments, respectively. In turn, the movement of dinosaurs appears to track vegetation, with different groups eating vegetation from only one type of forest or the other. Taken together, these results suggest that differences in soil drainage over a landscape allowed for the occurrence of many unique floral assemblages in close proximity, each of which supported its own faunal assemblage.