Preprints
https://doi.org/10.5194/egusphere-2025-4283
https://doi.org/10.5194/egusphere-2025-4283
30 Sep 2025
 | 30 Sep 2025
Status: this preprint is open for discussion and under review for Earth Surface Dynamics (ESurf).

Limited influence of bedrock strength on river profiles: the dominant role of sediment dynamics

Nanako Yamanishi and Hajime Naruse

Abstract. Bedrock river incision is a fundamental process driving the evolution of mountainous landscapes. Bedrock strength is often considered a primary control on incision rates and river profile morphology, with laboratory experiments showing a strong correlation between erosion rate and tensile strength. However, in natural settings, lithological boundaries frequently do not correspond to changes in the channel gradient. This study addresses this apparent paradox by integrating field observations with numerical experiments in the tributaries of the Abukuma River basin, northeastern Japan. Field surveys were conducted to measure bedrock tensile strength, riverbed gravel grain size, and the spatial distribution of lithologies. Despite more than an order-of-magnitude variation in bedrock tensile strength across the study area, the channel slopes remained nearly uniform. Numerical experiments were performed using three models of bedrock river erosion to investigate the underlying mechanisms. Among them, the sediment-flux-dependent model, which explicitly incorporates sediment cover and tool effects, most accurately reproduced the observed longitudinal profiles. The results reveal that local lithology does not directly influence channel slope due to a negative feedback between sediment cover and river gradient. Increased erodibility reduces slope, which enhances sediment cover and suppresses further erosion, thereby offsetting the impact of bedrock strength. These findings highlight the limited role of bedrock strength in controlling channel gradients and underscore the importance of sediment dynamics, particularly sediment supply and grain size, in shaping fluvial topography. Future research should explore how lithology-dependent variations in sediment characteristics influence river profile development.

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Nanako Yamanishi and Hajime Naruse

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Nanako Yamanishi and Hajime Naruse
Nanako Yamanishi and Hajime Naruse

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Short summary
Bedrock strength in bedrock river is often seen as controlling incision rates and river profiles, natural changes in rock type do not always match slope changes. In the Abukuma River basin, Japan, we measured bedrock strength and despite large strength differences, slopes were nearly uniform. Numerical tests showed that the model, which includes sediment cover and erosion effects, best explained river profiles. Thus, sediment plays a greater role than bedrock strength in shaping river profiles.
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