Geomorphic responses to sequential experimental floods in the lower Spöl River, Switzerland

Abstract. Experimental floods are increasingly used to restore geomorphic processes in regulated rivers, yet their effectiveness is often evaluated using only ecological indicators and mostly after individual flood events, limiting understanding of cumulative channel adjustment and the key controls on geomorphic response. This study evaluates the geomorphic effectiveness of six sequential experimental floods between 2018 and 2024 in the lower Spöl River, Switzerland, using a multi-metric assessment of channel change derived from repeat high-resolution topographic surveys. Geomorphic responses were quantified using erosion, deposition areas and volumes, sediment budgets, channel widening, vertical reworking, bed surface grain size, roughness, and slope, and were examined in relation to flood hydrograph characteristics and the initial state of the channel. All floods reactivated geomorphic processes through sediment mobilisation, erosion, deposition and channel adjustment, but the magnitude and form of the response varied markedly among events. Total flood volume was a better, although not exclusive, predictor of sediment mobilization than peak discharge or flood duration, while antecedent channel conditions, including sediment availability, bed surface grain size, channel morphology, roughness, slope and the legacy of earlier events, strongly influenced the magnitude and spatial distribution of channel adjustment. Moreover, event-scale effectiveness did not determine the river's longer-term trajectory: despite repeated sediment mobilisation, continued sediment delivery from an unregulated tributary, the Ova da Cluozza, promoted multi-year sediment storage and aggradation. These findings show that geomorphic effectiveness emerges from the interaction between hydraulic forcing, sediment availability and evolving channel conditions, and depends on the temporal scale of assessment. The multi-event, multi-metric approach provides a transferable framework for designing and evaluating sediment-informed experimental floods in regulated gravel-bed rivers.