Science2014-06-11 6:34 PM

研究揭示了河流为什么会蜿蜒流动 Dynamic Reorganization of River Basins

论文摘要 

研究人员设计了一个可预测河流网络如何演变的模型;他们说,该模型可对过去、现在及未来地形的动态物理特征做出某些阐释。Sean Willett及其同事对江河流域及排水系统划分或分水线随着时间的推移因应侵蚀及地壳构造隆升而致的改变方式进行了研究。他们用其得出的公式来测量目前世界上3个河流体系的状态;他们提出,渠道、流域及分水线常常会迁移以达到一种平衡的状态,在平衡状态时,地形会停止变迁。他们说,例如,汇入黄河的支流现在似乎已经变得相对稳定。但是据研究人员披露,在台湾中部的年轻的地貌以及美国东南部古老的河流网络仍然处在不断的变化之中。这一新的观察过去、现在及未来河流体系的方式或能给来自不同背景的科学家探索由河流系统连接的地质学、化学及生物学系统提供方法。对许多生物来说,河流网络所起的或是实体的通道或是屏障的作用,因此更好地了解它们相对的稳定性和长期性可能会产生某些有关演化的生物物理驱动因素的有价值的并具有对未来的生态学及环境保护有意义的见解。

Abstract 

River networks evolve as migrating drainage divides reshape river basins and change network topology by capture of river channels. We demonstrate that a characteristic metric of river network geometry gauges the horizontal motion of drainage divides. Assessing this metric throughout a landscape maps the dynamic states of entire river networks, revealing diverse conditions: Drainage divides in the Loess Plateau of China appear stationary; the young topography of Taiwan has migrating divides driving adjustment of major basins; and rivers draining the ancient landscape of the southeastern United States are reorganizing in response to escarpment retreat and coastal advance. The ability to measure the dynamic reorganization of river basins presents opportunities to examine landscape-scale interactions among tectonics, erosion, and ecology.

Structured Abstract

Introduction
River networks, the backbone of most landscapes on Earth, collect and transport water, sediment, organic matter, and nutrients from upland mountain regions to the oceans. Dynamic aspects of these networks include channels that shift laterally or expand upstream, ridges that migrate across Earth’s surface, and river capture events whereby flow from one branch of the network is rerouted in a new direction. These processes result in a constantly changing map of the network with implications for mass transport and the geographic connectivity between species or ecosystems. Ultimately, this dynamic system strives to establish equilibrium between tectonic uplift and river erosion. Determining whether or not a river network is in equilibrium, and, if not, what changes are required to bring it to equilibrium, will help us understand the processes underlying landscape evolution and the implications for river ecosystems.

Methods
We developed the use of a proxy, referred to as χ, for steady-state river channel elevation. This proxy is based on the current geometry of the river network and provides a snapshot of the dynamic state of river basins. Geometric equilibrium in planform requires that a network map of χ exhibit equal values across all water divides (the ridges separating river basins). Disequilibrium river networks adjust their drainage area through divide migration (geometric change) or river capture (topologic change) until this condition is met. We constructed a numerical model to demonstrate that this is a fundamental characteristic of a stable river network. We applied this principle to natural landscapes using digital elevation models to calculate χ for three, very different, systems: the Loess Plateau in China, the eastern Central Range of Taiwan, and the southeastern United States.

Results
The Loess Plateau is close to geometric equilibrium, with χ exhibiting nearly equal values across water divides. By contrast, the young and tectonically active Taiwan mountain belt is not in equilibrium, with numerous examples of actively migrating water divides and river network reorganization. The southeastern United States also appears to be far from equilibrium, with the Blue Ridge escarpment migrating to the northwest and the coastal plain rivers reorganizing in response to this change in boundary geometry. Major reorganization events, such as the capture of the headwaters of the Apalachicola River by the Savannah River, are readily identifiable in our maps.

Discussion
Disequilibrium conditions in a river network imply greater variation of weathering, soil production, and erosion rates. Disequilibrium also implies more frequent river capture with implications for exchange of aquatic species and genetic diversification. Transient conditions in river basins are often interpreted in terms of tectonic perturbation, but our results show that river basin reorganization can occur even in tectonically quiescent regions such as the southeastern United States.

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