Does bed relief index inform about sediment supply in proglacial multi-channel systems? The Rutor case study (Italian Alps)

This study examines links between proglacial river-reach morphology and glacier dynamics at Rutor Glacier (Italian Alps). High-resolution photogrammetric DSMs (2021−2023) were used to compute the Bed Relief Index (BRI*) across six proglacial multi-channel systems. BRI* distinguishes sediment-starved from transport-limited morphologies; here we test its value for diagnosing spatial and temporal contrasts in sediment availability and transport regime across proglacial reaches. BRI* shows strong relationships with reach slope and active channel width, emphasizing their role in shaping proglacial morphodynamics. To improve cross-reach comparability, we develop a normalization approach that removes slope and width effects, producing a normalized BRI* suited to contrasting systems with different gradients and channel dimensions. Results reveal a spatial mosaic of transport-limited and sediment-supply-limited conditions, indicating strong variability in sediment availability. Planform type (fan-shaped versus linear) does not systematically control reach response, whereas delta reaches differ because their dynamics are primarily constrained by downstream base level. Several reaches exhibit elevated BRI* values consistent with modest sediment starvation linked to reduced glacial input and disconnection from upstream lakes. Normalized BRI* enables direct comparison with two braided reaches in France and yields consistent discrimination of transport regimes. Overall, BRI*—especially when normalized—provides a practical diagnostic of sediment-transport conditions in proglacial multi-channel systems, supporting continued monitoring to capture longer-term adjustments under changing boundary conditions.