# Different geomorphic processes control suspended sediment and bedload export from glaciers

## TL;DR

Glaciers export sediment in two forms—suspended and bedload—controlled by different processes, with bedload transport being less efficient due to subglacial hydraulic conditions.

## Contribution

The study identifies distinct hydrogeomorphic controls on suspended and bedload sediment export from glaciers using a physically-based numerical model.

## Key findings

- Suspended and bedload sediment export quantities are comparable, but both depend on sediment availability.
- Bedload transport efficiency is lower due to subglacial channel shape and hydraulic roughness.
- Subglacial hydraulics must be considered to understand bedload export, and non-fluvial mechanisms may be needed for glacial erosion to continue.

## Abstract

Ongoing cryospheric change has modified sediment export from glacierized catchments substantially, with significant implications for ecosystems and downstream users, notably hydropower companies. Sediment is exported either as finer sediment in suspension or as coarser bedload with intermittent contact between sediment and the bed. To date, the difficulty in observing subglacial bedload transport limits the understanding of the physical processes associated with evacuating bedload compared with suspended load. We elucidate the factors controlling sediment export by inverting a physically-based numerical model of subglacial sediment production and sediment transport with suspended sediment and continuous bedload discharge records from an Alpine glacier. Comparable quantities of suspended sediment and bedload are exported, and model results suggest that both rely on the availability of sediment for transport. Yet, bedload export in subglacial channels also depends on particular hydraulic conditions, notably channel shape and hydraulic roughness. This makes exporting bedload-sized particles inefficient compared to fine-grained sediment. As a result, subglacial hydraulics should be explicitly considered when examining bedload export processes, and suspended and bedload transport should be considered separately. Inefficient bedload evacuation by melt water implies that glacial erosion may only continue when non-fluvial mechanisms evacuate sediment, such as sediment entrainment into the ice.

Suspended and bedload sediment export from glaciers responds to different hydrogeomorphic processes. Understanding both processes is key for evaluating glaciers’ impacts on landscape evolution and sediment export as the climate warms.