Influence of Lateral Input of Sediments on Longitudinal Luminescence Signal in a River (Rakaia River, Aotearoa New Zealand)

Abstract Rivers transport sediments from mountains to oceans, shaping landscapes and transmitting environmental signals. This transfer is repeatedly interrupted by storage and remobilization, complicating efforts to reconstruct erosion histories. Luminescence‐based methods, which exploit the progressive zeroing of the luminescence signals (bleaching) during transport, offer a new way to quantify sediment transit. Existing approaches, however, assume negligible lateral inputs, predicting a simple downstream decrease in luminescence signal intensity. We present new data from the Rakaia River, New Zealand Aotearoa, where observed downstream patterns deviate from such predictions and suggest an influence of local sediment supply. By using a stochastic transport‐storage model that incorporates continuous and localized inputs, we show that downstream signals reflect the balance between supply and bleaching efficiency, producing bleaching‐ or supply dominated regimes. These results demonstrate that luminescence‐based sediment tracing not only capture sediment transport dynamics in rivers but also their supply from hillslope erosion.