RECONSTRUCTING THE LATE HOLOCENE (LAST 3000 YEARS) ICE-JAM FLOOD HISTORY OF THE MIDDLE YUKON RIVER - DAWSON CITY, YUKON TO CIRCLE, ALASKA
LIVINGSTON, JOANNE M. University of Calgary.
Froese, Duane G. Simon Fraser University.
Smith, Derald G. University of Calgary.
Along northern interior continental rivers, the formation of ice-jams and their associated flooding is a common phenomenon. Analyses of extreme stage jams at Dawson City, Yukon, for the last century indicates that all overbank flood events have been ice-jam related (Figure 1). Summer storms and spring freshet events have no oral or historic record of overbank floods. By augmenting the historic hydrometric data with pre-historic evidence from ice-jam floods, it is possible to extend the flood history of this river to at least 3000 years B.P.
This study presents a review of historic ice-jam flooding of the Yukon River (Dawson City and Fortymile, Yukon and Slavens Roadhouse, Alaska - Figure 2) and a new methodology to reconstruct the pre-historic flood record from sedimentary evidence (Figure 3) preserved in channel overbank floodplain deposits. This method is based on the assumption that identifiable sand-silt couplets separated by organic layers represent large magnitude-duration, low frequency ice-jam floods. Sequences of up to 100 flood beds have been recorded in overbank deposits at twenty-nine sections within the study area; three key sites were selected for detailed analysis. Conventional and AMS radiocarbon dating of organic material between flood deposits are used to obtain a more accurate flood frequency from which to assess large flood recurrence intervals. In addition, the late Holocene White River Ash forms a prominent stratigraphic marker in seventeen of the sections. At one exceptional site, tree collars of a mature white spruce bracket the eruption within about 30 years from counting tree rings separating the tree growth collars. An AMS age on the upper tree collar is dated to 1700+/-40 B.P., providing a minimum age for the northern tephra lobe. Finally, the possibility exists to examine the influence of known climate changes (e.g. Little Ice Age, Medieval Warm Period) on late Holocene ice-jam flood frequency.
Figure 1. Figure 1: Plot showing that the maximum flood events of the last 50 years have been ice-jam related. Consequently the major natural hazard along the river is from ice-jam events.
Figure 2. Figure 2: Study sites along the Yukon River
Figure 3. Figure 3: Photograph and sketch of flood couplets preserved in the overbank environment. The sand-silt units arguably represent rapid deposition from ice-jam flood events while the organic cap indicates a relatively long period of quiescence and stability between each event.
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