Bedload component of glacially discharged sediment: Insights from the Matanuska Glacier, Alaska

Abstract

The flux of glacially derived bedload and the proportions of the suspended and bedload components carried by proglacial streams are highly debated. Published data indicate a large range—from ,30% to .75%—in the bedload percentage of the total load. Two ''vents,'' where supercooled subglacial meltwater and sediment are discharged, were sam- pled over the course of an entire melt season in order to quantify the flux of glacially delivered bedload at the Matanuska Glacier, Alaska. The bedload component contributed by these vents, for the one melt season monitored, is negligible. Furthermore, the bedload fluxes appear to be strongly supply limited, as shown by the poorly correlated discharge, bedload-flux magnitude, and grain-size caliber. Thus, in this case, any attempt to employ a predictive quantitative expression for coarse-sediment production based on discharge alone would be inaccurate. A nonglaciated basin proximal to the Matanuska Glacier ter- minus yielded higher bedload sediment fluxes and larger clast sizes than delivered by the two monitored vents. Such nonglaciated basins should not be overlooked as potentially major sources of coarse bedload that is reworked and incorporated into valley outwash. usually subequal, although bedload may range from ,30% to .75% of the total clastic load. Establishing portable relationships between suspended load, bedload, and discharge would enhance our understanding of how and when glaciers erode. For the melt season studied, we show that bedload is ,1% of the total clastic load and does not have any obvious correla- tion to discharge at two of the main vents at the terminus of the Matanuska Glacier, Alas- ka. These observations, coupled with high measured bedload component of adjacent non- glaciated valleys, inspire several working hy- potheses for the observed origin of coarse bedload in the Matanuska River downstream of the ice margin.