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Riparian vegetation and the late Holocene development of an anabranching river; Magela Creek, northern Australia

Stephen Tooth, John D. Jansen, Gerald C. Nanson, Tom J. Coulthard and Tim Pietsch
Riparian vegetation and the late Holocene development of an anabranching river; Magela Creek, northern Australia
Geological Society of America Bulletin (August 2008) 120 (7-8): 1021-1035

Abstract

Many anabranching rivers are characterized by dynamic interactions between fluvial processes and riparian vegetation, but uncertainties surround the processes and time scales of anabranch development. We use geomorphological investigations and optically stimulated luminescence (OSL) dating to determine spatial and temporal trends in the development of anabranching along a 6.5-km-long reach of Magela Creek in the seasonal tropics of northern Australia. Many trees and shrubs that survive the wet-season floods establish on the sandy beds and lower banks, such that anabranches divide and rejoin around numerous ridges and islands that are formed mainly by accretion in the lee of in-channel vegetation and, less commonly, by excision from formerly continuous island or flood plain surfaces. Once ridges and islands form, colonizing vegetation maintains their stability by increasing sediment cohesion and decreasing flow erosivity. Over the Holocene, Magela Creek has vertically aggraded and extended in length by delta progradation into Madjinbardi Billabong, resulting in a time sequence of anabranches and associated ridges and islands from older (upstream) to younger (downstream). OSL ages for islands in the upstream and middle reaches are ca. 1.6 ka and older, and the narrow, deep anabranches (width/depth [w/d] typically approximately 10-30) have few in-channel obstructions. Farther downstream, island OSL ages are ca. 0.7 ka and younger, anabranches tend to be wider and shallower (w/d >30) with more obstructions, and splays and locally scoured island and floodplain surfaces are more common. Based on these findings, previous flow and sediment-transport measurements, and theoretical analyses, we posit that there is a decline in anabranch efficiency from an upstream equilibrium system in mass-flux balance to a downstream disequilibrium system characterized by bed aggradation and localized island and floodplain erosion. In the downstream reaches, inefficient (high w/d and obstructed) anabranches do not persist because they either aggrade and are abandoned, or they are subdivided into more efficient (lower w/d and less obstructed) anabranches as a result of the interactions between in-channel vegetation growth and ridge and island accretion or local excision. Consequently, a more efficient anabranching system gradually develops with characteristics similar to those in the upstream reaches. This enhances downstream sediment transfer, which enables ongoing delta progradation and provides fresh sediment surfaces for vegetation to colonize and initiate new anabranches. The OSL ages from Magela Creek demonstrate that a recognizable but relatively inefficient anabranching system can develop within a few centuries, while adjustment to a more efficient system occurs over a few millennia.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 120
Serial Issue: 7-8
Title: Riparian vegetation and the late Holocene development of an anabranching river; Magela Creek, northern Australia
Affiliation: University of Wales, Institute of Geography and Earth Sciences, Aberystwyth, United Kingdom
Pages: 1021-1035
Published: 200808
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 95
Accession Number: 2008-104176
Categories: Geomorphology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sketch map
S12°34'60" - S12°30'00", E132°45'00" - E133°00'00"
Secondary Affiliation: University of Glasgow, GBR, United KingdomUniversity of Wollongong, AUS, AustraliaUniversity of Hull, GBR, United KingdomCSIRO Land and Water, AUS, Australia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 200836
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