Timescales of tree‐covered island dynamics on the mixed bedrock‐alluvial anabranching Vaal River, South Africa

IF 1.7 4区 环境科学与生态学 Q4 ENVIRONMENTAL SCIENCES River Research and Applications Pub Date : 2024-05-07 DOI:10.1002/rra.4296
S. Tooth, A. Keen‐Zebert, M. C. Grenfell, G. Addison
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Abstract

Previous research on mixed bedrock‐alluvial anabranching rivers has documented how alluvial islands commonly grow under vegetation influences atop slowly eroding bedrock templates, but timescales of island dynamics remain poorly constrained. We focus on the Vaal River near Parys, South Africa, and combine field investigations, aerial image analyses and optically stimulated luminescence (OSL) dating to establish timescales of initiation, growth and erosion for nine bedrock‐cored, tree‐covered, alluvial islands. For each island, two OSL samples were collected in vertical succession from sand‐rich exposures up to 4 m in thickness to establish minimum ages for island initiation (~1802 to 243 years) and to estimate local vertical aggradation rates (~0.20 to 1.8 cm year−1). The diachronous lower ages and lack of systematic upstream‐downstream trend in island age support an interpretation of patchwork initiation, growth, and erosion of islands throughout the late Holocene. Following island initiation, vertical island aggradation occurs in association with establishment of reeds, shrubs and trees, but erosion of island margins or dissection by cross‐cutting channels also can occur. Observations during and after recent large floods (peak discharges >3000 m3 s−1) provide further insights into island dynamics, including the influence of exotic trees (e.g., Eucalyptus spp.) that have colonised many islands in the postcolonial era (last ~150 years). Our findings extend previous conceptual models by constraining timescales of island dynamics and providing new insights into island stability and longevity in mixed bedrock‐alluvial anabranching rivers. Improved communication of findings regarding island geomorphology, ecology and stability can benefit local community engagement, geo/eco‐tourism and education activities, and land use planning.
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南非瓦尔河基岩-冲积混合支流上树木覆盖岛屿动态的时间尺度
以前对基岩-冲积混合型无支流河流的研究记录了冲积岛屿是如何在植被影响下在缓慢侵蚀的基岩模板上生长的,但岛屿动态的时间尺度仍然难以确定。我们以南非帕里斯附近的瓦尔河为研究对象,结合实地调查、航空图像分析和光激发发光(OSL)测年,确定了九个基岩覆盖、树木覆盖的冲积岛屿的形成、生长和侵蚀的时间尺度。在每个岛屿上,从厚度达 4 米的富含沙土的暴露区垂直连续采集了两个 OSL 样本,以确定岛屿形成的最低年龄(约 1802 年至 243 年),并估算当地的垂直侵蚀速率(约 0.20 年至 1.8 厘米/年)。由于岛屿的年龄较低,且缺乏系统的上游-下游趋势,因此支持对整个全新世晚期岛屿成片生成、生长和侵蚀的解释。在岛屿形成之后,岛屿的纵向退化与芦苇、灌木和树木的生长有关,但岛屿边缘的侵蚀或横切水道的切割也可能发生。在最近的大洪水(峰值排水量达 3000 m3 s-1)期间和之后进行的观测进一步揭示了岛屿的动态变化,包括外来树木(如桉树属植物)的影响,这些树木在后殖民时代(过去约 150 年)已成为许多岛屿的殖民地。我们的研究结果扩展了以前的概念模型,限制了岛屿动力学的时间尺度,并为基岩-冲积混合无支流河流中岛屿的稳定性和寿命提供了新的见解。加强对岛屿地貌学、生态学和稳定性研究结果的宣传,有利于当地社区参与、地质/生态旅游和教育活动以及土地利用规划。
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来源期刊
River Research and Applications
River Research and Applications 环境科学-环境科学
CiteScore
4.60
自引率
9.10%
发文量
158
审稿时长
6 months
期刊介绍: River Research and Applications , previously published as Regulated Rivers: Research and Management (1987-2001), is an international journal dedicated to the promotion of basic and applied scientific research on rivers. The journal publishes original scientific and technical papers on biological, ecological, geomorphological, hydrological, engineering and geographical aspects related to rivers in both the developed and developing world. Papers showing how basic studies and new science can be of use in applied problems associated with river management, regulation and restoration are encouraged as is interdisciplinary research concerned directly or indirectly with river management problems.
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