ANCHOR ICE RAFTING: OBSERVATIONS FROM THE LARAMIE RIVER

Anchor ice rafting (transport of sediment by released anchor ice) occurs regularly on many ice-affected rivers. However, rafting is poorly understood and there is scant quantitative information about it. This paper describes anchor ice formation and ice rafting observations made in the Laramie River, a small river in Wyoming. Anchor ice forms on cobble, gravel and sand substrates in the Laramie River. When released from the bed, anchor ice rafts sediment from all of these size classes. Observations suggest that anchor ice plays a substantial role transporting coarse fluvial sediment because anchor readily envelops large particles extending above the bed. A comparison of anchor-ice rafted sediment with peak-discharge bedload sediment shows that the largest sediment is transported downstream during the winter when flows are at a minimum. The largest ice-rafted cobble identified in this study weighed 9.5 kg; peak floods are capable of moving sediment only about a hundredth of the cobble's weight. There is overlap in ice-rafted and bedload sediment transport; both processes move significant amounts of sand-sized material. The average sediment concentration in released, floating anchor ice samples was 22 g l(-1). Copyright (C) 2010 John Wiley & Sons, Ltd.