Landscape evolution, valley excavation, and terrace development following abrupt postglacial base-level fall
Landscape evolution, valley excavation, and terrace development following abrupt postglacial base-level fall
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冰后基准面突然下降后的景观演变、山谷开挖和阶地开发
DOI：
10.1130/b30772.1
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发表时间：
2013
影响因子：
4.9
通讯作者：
Rittenour, T.
中科院分区：
文献类型：
--
作者：
Gran, K. B.;Finnegan, N.;Johnson, A. L.;Belmont, P.;Wittkop, C.;Rittenour, T.
自末次冰期以来，许多高纬度河流系统正在适应基准面的变化。经历过基准面下降的河道可能仍然在切割，通常是通过冰河斜面（tills）。这些耕耘物的性能非常好，其表现更像是软弱的基岩而不是松散的沉积物，并且侵蚀速度很快，从而为测试瞬态系统中的河道切口和拐点迁移模型提供了独特的机会。在这里，我们整合了光探测和测距 (LiDAR) 地形、地层阶地年代学和数值建模，以确定美国明尼苏达州中部勒苏尔河的小点迁移和切割历史。结果表明，Le Sueur 河最适合建模为分离限制河道，与来自耕地的滞后碎屑相关的下游粗化在纵向剖面发展中发挥着关键因素。Le Sueur 河在切割时蜿蜒，因此我们将最适合的切割模型与曲流模型相结合，以确定山谷开挖历史。挖掘历史用于确定 1800 年代中期与定居和农业扩张相关的土地利用变化之前的自然背景侵蚀率。我们将背景细沉积物（淤泥和粘土）侵蚀率与历史十年平均年度悬浮荷载进行了比较。结果表明，来自与山谷开挖相关的来源的现代细泥沙贡献比模拟的预沉降荷载高出三倍。最近与土地利用和气候变化相关的水文变化增加了河流的流量，导致沉积物负荷增加，这不仅来自于田野侵蚀，还来自于深切山谷中河岸和断崖侵蚀的增加。
Many high-latitude fluvial systems are adjusting to base-level changes since the last glaciation. Channels that experienced base-level fall may still be incising, often through glacial diamictons (tills). These tills can be quite competent, behaving more like weak bedrock than unconsolidated sediment, and erode at a fast pace, thus providing a unique opportunity to test models of channel incision and knickpoint migration in transient systems. Here, we integrate light detection and ranging (LiDAR) topography, strath terrace chronology, and numerical modeling to determine knickpoint migration and incision history of the Le Sueur River in central Minnesota, USA. Results indicate that the Le Sueur River is best modeled as a detachment-limited channel, with downstream coarsening related to lag clasts from tills playing a critical factor in longitudinal profile development.The Le Sueur River meanders as it incises, so we coupled the best-fit incision model to a meander model to determine valley excavation history. The excavation history was used to determine a natural background erosion rate, prior to land-use changes associated with settlement and agricultural expansion in the mid-1800s. We compared background fine sediment (silt and clay) erosion rates with historic decadal-average annual suspended loads. Results show that modern fine sediment contributions from sources associated with valley excavation are three times higher than modeled presettlement loads. Recent changes in hydrology associated with land use and climate change have increased flows in rivers, leading to higher sediment loads, not just from field erosion, but from increased bank and bluff erosion in the deeply incised valleys.
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