Geometric constraints on tributary fluvial network junction angles

IF 2.8 2区 地球科学 Q2 GEOGRAPHY, PHYSICAL Earth Surface Dynamics Pub Date : 2024-04-25 DOI:10.5194/egusphere-2024-1153
Jon D. Pelletier, Robert G. Hayes, Olivia Hoch, Brendan Fenerty, Luke A. McGuire
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Abstract

Abstract. The intersection of two non-parallel planes is a line. Howard (1990), following Horton (1932), proposed that the orientation and slope of a fluvial valley within a tributary network are geometrically constrained by the orientation and slope of the line formed by the intersection of planar approximations to the topography upslope from the tributary junction along the two tributary directions. Previously published analyses of junction-angle data support this geometric model, yet junction angles have also been proposed to be controlled by climate and/or optimality principles (e.g., minimum-power expenditure). In this paper, we document a test of the Howard (1990) model using ~107 fluvial network junctions in the conterminous U.S. and a portion of the Loess Plateau, China. Junction angles are consistent with the predictions of the Howard (1990) model when the orientations and slopes are computed using drainage basins rather than in the traditional way using valley-bottom segments near tributary junctions. When computed in the traditional way, junction angles are a function of slope ratios (as the Howard (1990) model) predicts, but data deviate from the Howard (1990) model in a manner that we propose is the result of valley-bottom meandering/tortuosity. We map the mean junction angles computed along valley bottoms within each 2.5 km x 2.5 km pixel of the conterminous U.S.A. and document lower mean junction angles in incised late-Cenozoic alluvial piedmont deposits compared to those of incised bedrock/older deposits. To understand how this finding relates to the geometric model of Howard (1990), we demonstrate that, for an idealized model of an initially unincised landform, i.e., a tilted plane with random microtopography, lower ratios of the mean microtopographic slope to the large-scale slope/tilt are associated with lower mean junction angles compared to landforms with higher such ratios. Using modern analogs, we demonstrate that unincised late-Cenozoic alluvial piedmonts likely had ratios of mean microtopographic slope to large-scale slope/tilt that were lower (i.e., ~1) prior to tributary drainage network development than the same ratios of bedrock/older deposits (≫1). This finding provides a means of understanding how the geometric model of Howard (1990) results in incised late Cenozoic alluvial piedmont deposits with lower mean tributary fluvial network junction angles, on average, compared to those of incised bedrock/older deposits. This work demonstrates that the topography of a landscape prior to fluvial incision exerts a key constraint on tributary fluvial network junction angles via a fundamental geometric principle that is independent of any climate- or optimality-based principle.
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支流河网交角的几何制约因素
摘要两个不平行平面的交点是一条直线。霍华德(Howard,1990 年)继霍顿(Horton,1932 年)之后提出,支流网络中河谷的走向和坡度在几何上受支流交汇处沿两个支流方向上坡地形的平面近似值的交点所形成的直线的走向和坡度的制约。以前发表的交界角数据分析支持这一几何模型,但也有人提出交界角受气候和/或优化原则(如最小功率消耗)的控制。在本文中,我们利用美国本土和中国黄土高原部分地区的约 107 个河网交汇点对霍华德(1990 年)模型进行了测试。当使用流域而不是传统方法使用支流交汇处附近的谷底断面来计算方向和坡度时,交汇角度与霍华德(1990 年)模型的预测结果一致。按传统方法计算时,交界角是坡度比的函数(正如霍华德(1990 年)模型所预测的那样),但数据却偏离了霍华德(1990 年)模型,我们认为这是谷底蜿蜒/迂回的结果。我们绘制了美国大陆每 2.5 千米 x 2.5 千米像素范围内谷底计算出的平均交角图,并记录了与基岩/较老沉积物相比,切入晚新生代冲积山麓沉积物的平均交角较低。为了了解这一发现与霍华德(Howard,1990 年)的几何模型之间的关系,我们证明,对于初始未切入地貌的理想化模型(即具有随机微地形的倾斜平面),与具有较高比率的地貌相比,较低的平均微地形坡度与大尺度坡度/倾斜度比率与较低的平均交角有关。我们利用现代类似物证明,在支流排水管网形成之前,无切口的晚新生代冲积岩层的平均微地形坡度与大尺度坡度/倾斜度之比可能低于(即 ~1)基岩/较老沉积物的相同比率(≫1)。这一发现为理解霍华德(Howard,1990 年)的几何模型如何导致切入的晚新生代冲积山麓沉积平均支流河网交角低于切入的基岩/较老沉积的平均支流河网交角提供了一种方法。这项研究表明,河流切入前的地形通过基本的几何原理对支流河网交角产生了关键的制约作用,而这一原理与任何基于气候或优化的原理无关。
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来源期刊
Earth Surface Dynamics
Earth Surface Dynamics GEOGRAPHY, PHYSICALGEOSCIENCES, MULTIDISCI-GEOSCIENCES, MULTIDISCIPLINARY
CiteScore
5.40
自引率
5.90%
发文量
56
审稿时长
20 weeks
期刊介绍: Earth Surface Dynamics (ESurf) is an international scientific journal dedicated to the publication and discussion of high-quality research on the physical, chemical, and biological processes shaping Earth''s surface and their interactions on all scales.
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