The
Depositional Web on the Fly River Floodplain, Papua New
Guinea
Dietrich, William E.1, Geoff
Day2, Joel C. Rowland1 (1) University of California,
Berkeley, Berkeley, CA (2) Rio Tinto, London, United Kingdom
Floodplain deposition on lowland
meandering rivers is usually distinguished as either occurring by lateral
accretion during channel migration or by overbank deposition. Here we report
the results of an intensive field study that documents the discharge of
mainstem water from the Fly River, Papua New Guinea onto its floodplain and
maps the spatial pattern of sediment deposition on the floodplain (using
elevated particulate copper introduced into the system due to upstream mining
as a tracer). Sediment laden waters are injected across the floodplain by flow
reversal up low gradient tributaries and via smaller channels that link the
mainstem to off river water bodies (tie channels). While sediment deposition is
confined to less than 1 km either side of these channels, sediment is
distributed ten's of kilometers across the plain via the tributary and tie
channels. Deposition declines exponentially from the nearest channel. About 40%
of the total sediment load was deposited on the floodplain, with half of that
being conveyed by the floodplain channels. Levee topography along the mainstem
and floodplain channels are similar, but cannot be explained by the observed
exponential functions. Shear flow during extended periods of flooding my give
rise to the localized levee deposition. Our study demonstrates that floodplain
channels can inject large volumes of sediment-laden mainstem waters great
distances across the floodplain where they spill overbank, forming a narrow
band of deposition, and thereby creating a depositional web.