HR: 16:00h
AN: OS44A-01    [Abstracts]
TI: Rates and spatial patterns of sediment dispersal across the lower Strickland River floodplain, Papua New Guinea
AU: * Swanson, K M
EM: KSwanson@berkeley.edu
AF: Department of Earth and Planetary Sciences, University of California Berkeley, Berkeley, CA 94720-4767 United States
AU: Apte, S
EM: Simon.Apte@csiro.au
AF: CSIRO Energy Technology, Private Mail Bag 7, Bangor, NSW 2234 Austria
AU: Aalto, R
EM: aalto@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195 United States
AU: Marshall, A
EM: amara@ozemail.com.au
AF: Andrew Marshall Pty. Ltd, Woronora Heights, Sydney, NSW 2233 Austria
AU: Dietrich, W
EM: Bill@eps.berkeley.edu
AF: Department of Earth and Planetary Sciences, University of California Berkeley, Berkeley, CA 94720-4767 United States
AB: As part of a NSF Margins Source to sink study, we are exploring the rates and patterns of floodplain sediment vertical accretion on the Middle Fly River and the lower Strickland River, Papua New Guinea. The rivers join at 6 m above sea level where tidal influences are currently weak (except at exceptionally low flow). In response to post-glacial sea level rise both rivers aggraded, but the Strickland, carrying 7 times the sediment load, twice the discharge, and transiting through a much shorter lowland reach, has built a roughly 10 times steeper slope and a much coarser bed than the Fly. We have hypothesized that as a consequence of this difference the rate of sediment loss to the floodplain will be much lower than on the Fly (which is currently about 40% for the Middle Fly Reach). Near the gravel-sand transition, the Strickland floodplain rapidly widens downstream to an average width of approximately 10 km, and the channel has built an elevated meander belt across the roughly 100 km long plain. Recently, sediment laden flood flows have occasionally spilled out of the Strickland through an oxbow and traveled along a small channel into Lake Murray. Lake Murray is directly connected to the Strickland via the Herbert River, a large, deep channel that occasionally reverses and directs Strickland waters to the Lake. To document the spatial pattern of sediment deposition, sediment cores were collected at 11 transects across the floodplain. In addition, samples were collected along the outflow channel, in Lake Murray, and in a separate oxbow fed only by a tie channel. Duplicates and, in places, triplicate core samples were taken to enable independent analysis of sedimentation rates using two methods. Mine tailings introduced into the headwaters of the Strickland since 1990, although of relatively minor importance to the total load of the lower Strickland; provide a distinct elevated Pb and Ag signature in the recently deposited sediment. In over 30 cores, 210Pb analyses have been performed to document sedimentation rates over longer periods. Analyses are still underway, but findings to date show that the two methods of estimating sedimentation rates give roughly similar rates. Across the approximately 2 km active portion of the floodplain, vertical accretion rates on average are on the order of 1 cm/yr, with rates being highest near the channel. Applying this rate uniformly across the entire active floodplain gives an annual sedimentation rate of about 2 million tonnes, which is less than 3% of the estimated annual load of 70 Mt/yr. It is unlikely that further refinement of our analyses will increase this percent to a value similar to the 40% found on the Fly. Although the current sediment loss rate to the Strickland floodplain appears to be relatively low, sediment laden flood pulses from the uplands are substantially damped out by transient flow storage in the main channel and floodplain.
DE: 1824 Geomorphology (1625)
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting