HR: 0830h
AN: H51D-1117    [PDF]
TI: Anchor Ice Formation and Fluvial Ice Rafting
AU: * Kempema, E W
EM: kempema@uwyo.edu
AF: Edward W. Kempema, Geology and Geophysics University of Wyoming, Laramie, WY 82071 United States
AB: The Laramie River in southeastern Wyoming is a small, low gradient, meandering, riffle and pool river with a winter discharge of about 1 m$^{3}$ s$^{-1}$. During late winter sections of the river surface are ice free. These river sections are subject to anchor ice formation during periods of cold weather. The diurnal formation of anchor ice at night, along with the subsequent release of anchor ice with entrained sediment from the streambed on following mornings results in downstream ice rafting of bed sediment. In February 2002, we placed 31 marked gravel clasts on the bed of the Laramie River in two areas where anchor ice was likely to form. These clasts, which were similar in size to the largest naturally occurring sedimentary particles on the riverbed, had masses ranging from 84 to 949 g. Over the next three weeks, we monitored anchor ice formation and ice rafting of the marked gravel. During this period 21 of the marked clasts moved. Four of the clasts that moved were never recovered. The remaining 17 clasts were ice rafted distances ranging from $\sim$20 cm to 85 m. An unexpected result of this study is that the lightest clast did not move, while the heaviest clast moved a total of 65 m downstream in three separate ice-rafting events. This result illustrates the apparently capricious nature of ice rafting: for the gravel used in this study there is no simple relationship between particle size and the probability that the particle will be ice rafted. This study also shows that coarse bed material is mobile during winter low-flow conditions. During anchor ice events, any size bed material may be buoyed to the surface by released anchor ice and ice rafted long distances downstream. Anchor ice events are sediment transport events in fluvial systems.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1827 Glaciology (1863)
SC: Hydrology [H]
MN: 2003 Fall Meeting