Sources of alluvium in a coastal plain stream based on radionuclide signatures from the ²³⁸U and ²³²Th decay series

Discerning alluvial sources and their change over time or distance is a fundamental question in hydrology and geology, often critical in identifying impacts of human and natural perturbations on fluvial systems. Surfaces of upland interfluves and subsoils, sources of alluvium in the lower Loco Bayou basin, Texas, were distinguished using the isotope ratios 226 Ra/ 232 Th, 226 Ra/ 230 Th, and 230 Th/ 232 Th. Channel alluvium indicates a transition from interfluve surface to subsoil sources during flood (subsoil ∼34% to ∼91%, over about 8 km) and bank‐full stages (subsoil ∼9% to ∼74%, over about 12 km), with distance downstream. These results indicate strong coupling between hillslope and channel processes, reflecting land use change from forested to agricultural, concentrated in lower Loco Bayou. This methodology shows that sediment sources can be differentiated based upon landscape placement where lithologic contrast is absent. The geochemistry, long half‐lives, and fractionation of 238 U and 232 Th decay series radionuclides during pedogenic and fluvial processes in humid climates suggest that these methods are applicable in a wide variety of fluvial systems.