Critical Zone controls on bedrock river grain sizes in the tropics and implications for mountain relief

Sediment grain size in bedrock rivers is critical in determining incision thresholds and energy expenditure on bedrock channels during floods, affecting fluvial relief over geological time. However, little work has explored controls on grain size distribution on mountainous bedrock river systems, particularly in the tropics and sub-tropics, where intense chemical weathering alters near-surface properties, producing thick critical zones. Here, we quantify grain size distributions using Wolman pebble counts in three rock types, serpentine (SP), volcaniclastic (VC), and granodiorite (GD), across topographic gradients on the tropical island of Puerto Rico. Our experimental design focuses on >150 low-order channels (draining ~2.5-100 km²) that are morphologically in a steady state, drain largely a single rock type, and span as large of topographic gradients as possible. This approach enables us to explore how differences in Critical Zone architecture – as dictated by rock type and topography – affect grain sizes. In aggregate, our results suggest minimal differences in median (D₅₀) and D₈₄ grain sizes among the three rock types, ~35 cm and ~180 cm, respectively. However, GD-draining rivers are ~36% sand, while VC and SP catchments have about 2.5 times less sand. The high sand content in GD-draining rivers is due to in-situ sand production within the Critical Zone, which emphasizes the role of rock type in determining grain sizes in Puerto Rican rivers. Across all rock types, grain size coarsens with increasing catchment average steepness (e.g., slope, normalized channel steepness indices), with significant and stronger correlations for coarser grain size fractions. We interpret that grain size coarsening with increased topographic steepness is due to faster erosion rates and shorter residence times in the critical zone, resulting in less weathering and larger grains delivered to river channels. These findings emphasize the role of rock type and erosion rate in determining grain size, which, in part, determines the magnitude of incision thresholds in river channels. We use a detachment-limited stream power model that includes incision thresholds to explore the implications of these findings on mountain relief. Considering models of sediment mobilization that consider the role of sand in reducing the critical shear stress of coarser grain size fractions, elevated sand content in GD-draining rivers could result in a ~20-40% reduction in fluvial relief relative to VC and SP draining rivers at comparable rock uplift rates. These models also suggest the progressively coarser sediment load in steeper topography might increase fluvial relief from ~20 to ~200% relative to standard models without an incision threshold. These findings demonstrate the fundamental role of the Critical Zone in modulating grain size distributions in topical bedrock rivers and imply a key role in determining mountain relief.