Timing of river capture in major Yangtze River tributaries: Insights from sediment provenance and morphometric indices
Introduction
River networks are the skeleton of most landscapes on Earth. They transport water and sediment downstream, but also transmit signals of tectonic, climatic and base-level changes as kinematic waves upstream (Whipple and Tucker, 1999; Hergarten and Neugebauer, 2001; Harkins et al., 2007; Kirby and Whipple, 2012; Goren et al., 2014; Gallen, 2018; Malatesta et al., 2018; Li et al., 2020; Schwanghart and Scherler, 2020). With the pace of such a kinematic wave traveling from the trunk stream towards all its tributaries, the landscape adjusts towards a new topographic steady state, where uplift and erosion rates are in balance (Weissel and Seidl, 1997; Whipple et al., 2017). However, if the signal of change reaches the drainage divide then this divide may become asymmetric with higher erosion rates at the steeper side, which impedes the establishment of morphological equilibrium. Consequently, divides start migrating and cause catchments to grow at the expense of neighbouring catchments (e.g. Stokes et al., 2002; Goren et al., 2014; Willett et al., 2014; Whipple et al., 2017; Robl et al., 2017a, Robl et al., 2017b; Struth et al., 2020). While this process is in general slow and continuous, in rare cases this leads to an abrupt change in the river network, where one river captures another (Bishop, 1995; Fan et al., 2018a).
As river capture events can rapidly change drainage network properties, they play an important role in the evolution of river networks, and the landscapes shaped by it. River capture events are reported not only for low order channels (Mather et al., 2002; Fan et al., 2018a; Ren et al., 2014; Zhang et al., 2014; Ma et al., 2020) but also for continental scale rivers (Stüwe et al., 2008; Craddock et al., 2010; Antón et al., 2014; Hu et al., 2016; Bender et al., 2018; Xie et al., 2020). A river capture abruptly changes the ability of the river to incise into its bedrock as this event is accompanied with changes in base-level, discharge and sediment supply (Yanites et al., 2013; Fan et al., 2018a). However, such events do not only change the geometry of the drainage system but also affect processes of the adjacent hillslope systems. Local relief and hence slope gradient is increased by amplified incision leading to a higher landslide frequency (Griffiths et al., 2005) so that hillslopes remain close to their threshold slopes (Ouimet et al., 2009; Larsen and Montgomery, 2012; Bennett et al., 2016). Hence, river captures have significant impacts on landscape evolution (Prince et al., 2011; Willett et al., 2014; Robl et al., 2017a, Robl et al., 2017b).
Mobile drainage divides and river capture events occur frequently in regions of active tectonics (e.g. India-Asia collision zone) where gradients in the crustal velocity field change the size and shape of catchments and advect rivers (Hallet and Molnar, 2001; Castelltort et al., 2012). However, drainage systems are not passive strain markers but respond dynamically to tectonically induced changes in flow length or changes in catchment size. The progressive reorganization of the drainage network counteracts the tectonic distortion by re-establishing the balance between flow length and catchment size (Clark et al., 2004; Stüwe et al., 2008; Castelltort et al., 2012; Goren et al., 2015; Yang et al., 2015; Robl et al., 2017a). As both crustal deformation and the adjustment of the drainage system to it are recorded by the fluvially-conditioned topography over a time scale of millions of years, analysing and deciphering landscape metrics provide unique and important clues concerning the regional tectonic history (Clark et al., 2004; Clift et al., 2006; Zhang et al., 2019; Zheng et al., 2021). Besides the geoscience significance, river captures may have affected human societies if the capture event occurred in the recent past (Clift et al., 2012).
As a river capture occurs, both morphological and geological signals are left within the landscape, which can be used to determine the location and, in rare cases, the timing of capture (Bishop, 1995). In plan view, abrupt changes in flow direction denoted as ‘boat-hook’ or ‘elbow’-shaped bends may indicate that one river (aggressor) has captured the headwaters of another (victim) (Bishop, 1995; Robl et al., 2008a; Fan et al., 2018a). A drainage divide (i.e. wind gap) forms in the original valley of the beheaded river (victim). However, the sudden increase in discharge and the related higher erosion rate in the aggressor river causes base-level lowering and the migration of the wind gap towards the victim river by progressively reversing the flow direction. With time, a T-shaped river junction evolves, where two rivers flow in a common valley but in opposite direction. At the confluence, the river leaves the common valley in an approximately 90° bend (e.g. Robl et al., 2008a). Indications of capture events can also be found in profile view. Due to amplified incision and the accompanied base-level drop in the aggressor river a knickpoint (i.e. break in river longitudinal profile) originates at the capture point and propagates upstream the trunk and its tributaries (e.g. Antón et al., 2014).
In the past decade, both χ mapping / profiling, where χ represents the drainage area weighted river length (Willett et al., 2014) and Gilbert metrics (Forte and Whipple, 2018) were developed to quantify across divide differences in topography and hence erosion rate. Deviations from drainage divide symmetry (i.e. one side of the divide is steeper than the other) are interpreted in terms of drainage divide mobility, where river capture events were suggested as a trigger for such an asymmetry (e.g. Fan et al., 2018a; Trost et al., 2020). For equilibrium conditions (stable divides), both the χ values and the steepness of hillslopes across the divide are equal. However, a river capture event disturbs the morphological equilibrium, and as soon as the signal of the river capture reaches the drainage divides, they start to move from the aggressor to the victim side. In χ maps, this is indicated by across divide gradients in χ with lower χ values at the aggressor side tantamount to steeper streams on average (Willett et al., 2014).
In addition to morphological evidence, river capture events can also leave traces in the sedimentary records of rivers. In rare cases, the reversal of flow direction is recorded by the orientation of imbricated pebbles in river terraces (Fan et al., 2018a). A sudden decrease (victim river) or increase (aggressor river) in discharge can cause changes in particle size (Maher et al., 2007) and even more important in bedload composition due to changing contributing drainage areas. Hence, provenance studies are frequently applied since the source area and hence the composition of fluvial deposits in terraces are altered due to river capture. Fluvial provenance is reflected by mineral composition (Xie et al., 2020), detrital zircons and their age distribution (Zheng et al., 2013a; Chen et al., 2017), element isotopes (Clift et al., 2006; Xie et al., 2020), or directly by the lithology of the coarse particle composition (Maher et al., 2007). Hence, river sediments carry a characteristic composition and/or age signature representing the lithological assemblage of the catchment, which changes with a river capture event.
However, clear evidence for river capture events are often difficult to find and their interpretation might be ambiguous. Even documented capture events may be questionable, especially in tectonically active regions such as the margin of Tibetan Plateau (Clark et al., 2004; Clift et al., 2006; Zheng et al., 2013a; Wei et al., 2016; Zhang et al., 2016; Zhang et al., 2017; Zhang et al., 2019).
Here we report on two river systems, the Jialing River and the Hanjiang River, which are located at the eastern margin of the Tibetan Plateau and represent major tributaries of the Yangtze River (Fig. 1A). It has been proposed for decades that the Jialing River captured the headwaters of the Hanjiang River (Shen, 1956). This would have resulted in an abrupt change in catchment size by 26, 000 km2, which is about one sixth of the current Jialing or Hanjiang catchment size (Fig. 2A and B). The impact on the affected environment and ecosystem, and in the long term on the evolution of the landscape, would have to be significant. Following Shen (1956), who initially proposed and described the river capture event without any dating control, Zhou (2010) suggested that the capture happened as recently as 2 ka BP by analysing historical literature sources, proposing that the capture event heavily affected human societies (e.g. flooding, drought, transport routes).
We test the long standing and widely accepted hypothesis that the Jialing River integrated the headwaters of the Hanjian River by a river capture event in the recent past. We do this by combining (1) an extensive provenance study of fluvial deposits with (2) a morphometric analysis of the region supported by a time-depended numerical model. In the provenance study, we analyse catchment-specific age distributions of detrital zircons, which were sampled from modern river sands and terrace deposits of the investigated rivers. If the capture event occurred after the deposition of the oldest river terraces (~ 1.2 Ma before present), a distinct change in the age distribution of detrital zircons will indicate the river capture and furthermore will allow constraining the date of capture. However, if the capture event occurred earlier, evidence for the event may be recorded in the topography. The morphometric analysis focuses on landscape metrics sensitive to a capture event allowing a look into the further past. This includes the position of major knickpoints in rivers, steepness of channel segments above and below such knickpoints, and drainage divide asymmetry at catchment (χ mapping) and hillslope scale (Gilbert metrics). As capture related landscape metrics evolve with time, we support our morphometric analysis with a time-dependent 1-d numerical model. By integrating the outcomes of both independent approaches, we show whether the river capture happened and if so, we provide an estimate in the timing.
Section snippets
Regional setting
Both the Hanjiang and Jialing Rivers are major tributaries of the Yangtze River (Fig. 1A), which is a continental-scale drainage system originating on the Tibetan Plateau and flowing into the Pacific Ocean. The Hanjiang River has a length of 1, 580 km and a catchment size of 1.59 × 105 km2, which is the largest tributary of the Yangtze River in terms of length. The Jialing River has a length of 1, 340 km and a catchment size of 1.60 × 105 km2, which is the largest tributary of the Yangtze River
Methods
To investigate whether the current headwaters of the Jialing River were originally part of the Hanjiang drainage system, recent fluvial deposits from both the Jialing and Hanjiang rivers were sampled for provenance analysis. Furthermore, we sampled fluvial deposits from the Hanjiang terraces to explore whether the provenance changed with time. Independent from the sedimentary records, we also analyzed the stability of the Jialing-Hanjiang drainage divide based on the modern topography.
Fluvial deposits provenance
Six terraces were developed in the Liangshan reaches of the Hanjiang River, as shown in Fig. 5. Fluvial deposits in T3 are gravels interbedded with sand, whilst T4-T6 comprise mostly sand.
At Maidiwan, the Hanjiang River flows in mountainous areas and terraces are not well developed compared to those at Liangshan. Nonetheless, we found the third terrace (T3) with fluvial deposits positioned 18 m above the modern Hanjiang River, with fresh exposures due to house construction (Fig. 6). The
No evidence for river capture based on zircon provenance
The sedimentary record and its zircon age distributions up to the highest terrace at Liangshan (T6) are clearly distinguishable from that of the Jialing River (Fig. 7). This reveals that the capture of the Hanjiang River by the Jialing River as proposed by Zhou (2010) did not happen in the recent past. We suggest that if this capture event actually occurred then it must have taken place prior to the formation of the uppermost terrace (T6) preserved at Liangshan.
The paleomagnetic age of T5 at
Conclusion
Based on field surveys, a provenance study of fluvial deposits and a morphometric analysis of the Hanjiang and Jialing drainage systems, we come to the following conclusions.
The provenance study from the age distribution of zircon grains sampled at terraces seaming the Hanjiang River and the Jialing River are clearly distinguishable throughout the entire sedimentary record up to the topmost terrace. This implies that there were no large-scale changes in the headwaters of the investigated rivers
Declaration of competing interest
The authors declare no competing interests.
Acknowledgments
This study was supported by the Second Tibetan Plateau Scientific Expedition and Research Program (2019QZKK0204, 2019QZKK0707), the National Natural Science Foundation of China (51979179), the Open Funding of State Key Laboratory of Loess and Quaternary Geology (SKLLQGZR1801) and Sichuan University (2018SCUH0050). We thank the editor Martin Stokes and two anonymous reviewers for very constructive comments and suggestions, Yi Chen, Xiaohui Shi and Zongmeng Li for helpful discussions. We are
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