This data release includes geochemical and fallout radionuclide data for suspended sediments and potential sediment source types for the Loutsenhizer Arroyo and Sunflower Drain watersheds in western Colorado. Suspended sediments were collected by passive samplers deployed in the stream channel of the watershed outlets during four study periods between August 2018 and August 2019. Potential sediment source type samples were collected in the watersheds in duplicate using compositing techniques that targeted the top 5 centimeters of soil. The four potential sediment source-types were: agricultural fields, rangeland, arroyo walls, and streambanks. Source type samples were wet sieved to less than (<) 63 microns (µm) to target the size fraction likely to be entrained in streamflow as suspended sediments. Geochemical analyses were conducted by AGAT Laboratories (Canada) and data are reported for concentrations of 42 elements along with total carbon, carbonate, and organic carbon. Selenium concentrations were measured by hydride generation atomic absorption spectroscopy. Fallout radionuclide analyses were conducted at the USGS St. Petersburg Coastal and Marine Science Center by gamma-ray spectrometry methods. Activity ratios for total lead-210 to radium-226 and ratios of radium-226 to uranium-238 are reported along with propagated uncertainty for the ratios.