Riparian vegetation and the late Holocene development of an anabranching river: Magela Creek, northern Australia
Riparian vegetation and the late Holocene development of an anabranching river: Magela Creek, northern Australia
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DOI：
10.1130/b26165.1
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发表时间：
2008-07
影响因子：
4.9
通讯作者：
S. Tooth;J. Jansen;G. Nanson;T. Coulthard;T. Pietsch
中科院分区：
文献类型：
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作者：
S. Tooth;J. Jansen;G. Nanson;T. Coulthard;T. Pietsch
许多再分支河流的特点是河流过程与河岸植被之间的动态相互作用，但在再分支发展的过程和时间尺度上存在不确定性。我们利用地貌调查和光学激发发光（OSL）测年技术来确定澳大利亚北部季节性热带地区6.5公里长的Magela Creek支流发展的时空趋势。许多树木和灌木在雨季的洪水中幸存下来，在沙床和较低的河岸上生长，这样，在许多山脊和岛屿周围，树枝分开并重新连接起来，这些山脊和岛屿主要是由河道内植被的背风处的增生形成的，而较少出现的情况是由以前连续的岛屿或洪泛平原表面的切除形成的。一旦山脊和岛屿形成，外来的植被通过增加沉积物的凝聚力和减少水流的侵蚀力来保持它们的稳定性。全新世以来，马吉拉河经三角洲进积，纵向聚集并延伸至Madjinbardi Billabong，形成了由老（上游）到新（下游）的分支及相关山脊和岛屿的时间序列。上游和中游岛屿的OSL年龄约为1.6 ka或更老，窄而深的支流（宽度/深度[w/d]通常为10-30）很少有通道内障碍物。再往下游，岛屿的OSL年龄约为0.7 ka，更年轻，支流更宽更浅（w/d bb30），障碍物更多，扇形和局部冲刷的岛屿和洪泛平原表面更常见。基于这些发现、之前的流量和泥沙输运测量以及理论分析，我们假设分支效率从质量通量平衡的上游平衡系统下降到以河床淤积和局部岛屿和洪泛平原侵蚀为特征的下游不平衡系统。在下游，由于河道内植被生长与山脊和岛屿增生或局部切除的相互作用，低效率（高w/d和受阻）的分支不会持续存在，因为它们要么退化并被遗弃，要么被细分为更有效（低w/d和较少受阻）的分支。因此，一个更高效的分支系统逐渐发展，其特征与上游相似。这加强了下游沉积物的转移，从而使三角洲不断淤积，并为植被的殖民和形成新的分支提供了新的沉积物表面。Magela Creek的OSL年龄表明，一个可识别但相对低效的分支系统可以在几个世纪内发展，而向更有效的系统的调整需要几千年的时间。
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 ~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.