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Quantifying the time scales of sediment transport and storage through river systems is fundamental for understanding weathering processes, biogeochemical cycling, and improving watershed management, but measuring sediment transit time is challenging. Here we provide the first systematic test of measuring cosmogenic meteoric Beryllium-10 (10 Be m) in the sediment load of a large alluvial river to quantify sediment transit times. We take advantage of a natural experiment in the Rio Bermejo, a lowland alluvial river traversing the east Andean foreland basin in northern Argentina. This river has no tributaries along its trunk channel for nearly 1,300 km downstream from the mountain front. We sampled suspended sediment depth profiles along the channel and measured the concentrations of 10 Be m in the chemically extracted grain coatings. We calculated depth-integrated 10 Be m concentrations using sediment flux data and found that 10 Be m concentrations increase 230% from upstream to downstream, indicating a mean total sediment transit time of 8.4 ± 2.2 kyr. Bulk sediment budget-based estimates of channel belt and fan storage times suggest that the 10 Be m tracer records mixing of old and young sediment reservoirs. On a reach scale, 10 Be m transit times are shorter where the channel is braided and superelevated above the floodplain, and longer where the channel is incised and meandering, suggesting that transit time is controlled by channel morphodynamics. This is the first systematic application of 10 Be m as a sediment transit time tracer and highlights the method's potential for inferring sediment routing and storage dynamics in large river systems. Plain Language Summary Understanding how long sediment takes to travel downstream in rivers, also known as sediment "transit time," is crucial for responsible watershed management and constraining global biogeochemical cycles. We aim to measure transit times for large rivers and determine the processes regulating this time scale. We present a new transit time proxy based on beryllium-10 (10 Be), a rare isotope produced in the atmosphere and delivered to Earth by rain. If river sediment collected downstream has more 10 Be than sediment upstream, this indicates that sediment was trapped in the floodplain for many years before continuing to travel downstream. We collected river sediment at multiple locations from upstream to downstream along a large, undammed river, the Rio Bermejo in Argentina. We found that 10 Be increased 230% from upstream to downstream, translating to a transit time of~8,500 years. This long time scale implies that sediment and carbon delivered to rivers have enough time to be weathered or oxidized to CO 2 before they are buried in the ocean. We also show that transit time is controlled by the river's shape and lateral mobility, suggesting that human alteration of channel shape or sediment supply will reduce a river's ability to use its floodplain for natural flood and erosion control.

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