Understanding the uranium (U) cycle-reservoirs and processes-at the watershed scale is key to manage contaminated areas, to elucidate ore formation processes, as well as to implement paleoenvironmental research. Here, we investigated the different steps of the U cycle, from sources to sinks, and the relative roles of redox processes and organic matter in the control of U mobility in the naturally U-rich small mountainous watershed of Lake Negre (France). We interpret the U repartition in U reservoirs through chemical, isotopic (delta U-238 and (U-234/U-238)), and speciation analyses, in light of anterior studies of the site. We show that U(VI) originates from the leaching of U-rich rock fractures and is transported in dissolved forms. Wetlands and meadow soils then act as intermediary sinks where U(VI) is complexed by organic matter (up to >5000 mu g/g) and subsequently partly reduced to U(IV). Dissolved U is also supplied to the lake, in addition to particulate and colloidal U resulting from soil physical erosion. After entering the lake, most U(VI)-bearing organic particles settle in the sediments and U(VI) is reduced to U(IV), resulting in high sedimentary U concentrations (up to >1000 mu g/g), while a fraction of U is potentially desorbed from particles. Remaining dissolved U is exported from the watershed through the lake outlet stream. In this high-mountain lake catchment, the U cycle is mainly controlled by organic matter complexation and particulate transport, though U reduction in the lake sediments may help its long-term immobilization.