Riparian vegetation and the late Holocene development of an anabranching river: Magela Creek, northern Australia
Many anabranching rivers are characterized by dynamic interactions between fl uvial 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 fl oods 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 fl oodplain surfaces. Once ridges and islands form, colonizing vegetation maintains their stability by increasing sediment cohesion and decreasing fl ow 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 ~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 fl oodplain surfaces are more common. Based on these fi ndings, previous fl ow and sediment-transport measurements, and theoretical analyses, we posit that there is a decline in anabranch effi ciency from an upstream equilibrium system in mass-fl ux balance to a downstream disequilibrium system characterized by bed aggradation and localized island and fl oodplain erosion. In the downstream reaches, ineffi cient (high w/d and obstructed) anabranches do not persist because they either aggrade and are abandoned, or they are subdivided into more effi cient (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 effi cient 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 ineffi cient anabranching system can develop within a few centuries, while adjustment to a more effi cient system occurs over a few millennia.
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