Sediment evacuation and glacial erosion rates at a small alpine glacier

Glacial erosion is known to be driven by sliding, which occurs in brief episodes in the melt season. What is not known is how well this sliding history is reflected in the sediment output from a glacier and to what degree temporary sediment storage in the glacier smoothes the history of sediment production. We document sediment evacuation rates for three seasons at Bench Glacier, a small temperate alpine glacier in the Chugach Range of Alaska. Two lines of evidence suggest that subglacial sediment storage is small. Strong hysteresis in the relationship between sediment and water discharge argues for sediment exhaustion from the subglacial system on seasonal and flood event timescales. Uplift of the glacier surface during periods of enhanced basal motion reflects intimate contact of the glacier sole with up-glacier-inclined stoss slopes of bedrock bumps. Sediment evacuation rates at Bench Glacier should therefore closely track glacial erosion rates over annual timescales. Much of the sediment emerges in a major pulse that closely follows the termination of enhanced sliding each year, suggesting that the conduits and well-connected cavities that allowed removal of englacial water to terminate sliding also promoted efficient transport of sediment. This small glacier efficiently modifies the landscape, lowering its bed by 1-2 mm/yr while sliding only 1-2 m/yr. Our sparse measurements of instantaneous bed load discharge average 41% of concurrent suspended sediment discharge. Assuming that half of the suspended load derives from abrasion of subglacially quarried clasts, roughly two thirds of the sediment evacuated can be attributed to quarrying.