Large anabranching rivers form channels in sediments of varying strength, resulting from erosional and depositional processes that act over geological time scales. Although bank strength variability is known to affect channel morphodynamics, its impact on the migration of large sand-bed rivers remains poorly understood. We report the first in situ measurements of bank strength from an ~100-km-long reach of the Solimões River, the Brazilian Amazon River upstream of Manaus. These show that cohesive muds in Pleistocene terraces along the river's right margin have bank strengths as much as three times greater than Holocene floodplain deposits composing the left bank. Image analysis suggests these resistant outcrops determine channel-bar dynamics: channel widening and bar deposition are inhibited, which lowers planform curvature and reduces erosion of the opposing bank. Planform analysis of the 1600-km-long Solimões River between 1984 and 2021 shows that where the channel is associated with Pleistocene terraces, lower rates of bank erosion and bar deposition are evident. Heterogeneity in bank strength is thus a first-order control on the large-scale morphodynamics of the world's largest lowland river.

中文翻译：

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巴西亚马逊河上游河岸强度变异及其对系统尺度形态动力学的影响


大型分叉河流在不同强度的沉积物中形成河道，这是由于地质时间尺度上的侵蚀和沉积过程造成的。尽管已知河岸强度变化会影响河道形态动力学，但其对大型沙床河流迁移的影响仍知之甚少。我们报告了索利莫斯河（马瑙斯上游的巴西亚马逊河）约 100 公里长河段的河岸强度的首次现场测量。这些表明，河流右缘更新世阶地中的粘性泥浆的河岸强度是构成左岸的全新世洪泛区沉积物的三倍。图像分析表明，这些坚固的露头决定了河道坝的动态：河道加宽和坝沉积受到抑制，从而降低了平面曲率并减少了对岸的侵蚀。对 1984 年至 2021 年间全长 1600 公里的索利莫斯河进行的平面分析表明，在该河道与更新世阶地相关的地方，河岸侵蚀和沙坝沉积率明显较低。因此，河岸强度的异质性是对世界上最大的低地河流的大规模地貌动力学的一级控制。