Aquifers in the Upper Colorado River Basin (UCRB) exhibit persistent uranium (U) groundwater contamination plumes originating from former ore processing operations. Previous observations at Rifle, Colorado, have shown that fine grained, sulfidic, organic-enriched sediments accumulate U in its reduced form, U(IV), which is less mobile than oxidized U(VI). These reduced sediment bodies can subsequently act as secondary sources, releasing U back to the aquifer. There is a need to understand if U(IV) accumulation in reduced sediments is a common process at contaminated sites basin-wide, to constrain accumulated U(IV) speciation, and to define the biogeochemical factors controlling its reactivity. We have investigated U(IV) accumulation in organic-enriched reduced sediments at three UCRB floodplains. Noncrystalline U(IV) is the dominant form of accumulated U, but crystalline U(IV) comprises up to ca. 30% of total U at some locations. Differing susceptibilities of these species to oxidative remobilization can explain this variability. Particle size, organic carbon content, and pore saturation, control the exposure of U(IV) to oxidants, moderating its oxidative release. Further, our data suggest that U(IV) can be mobilized under deeply reducing conditions, which may contribute to maintenance and seasonal variability of U in groundwater plumes in the UCRB.