The influence of vegetation upon bedload transport and channel morphodynamics is examined along a channel stability gradient ranging from meandering to anabranching to anabranching–braided to fully braided planform conditions along trunk and tributary reaches of the Upper Yellow River in western China. Although the regional geology and climate are relatively consistent across the study area, there is a distinct gradient in the presence and abundance of riparian vegetation for these reaches atop the Qinghai–Tibet Plateau (elevations in the study area range from 2800 to 3400 m a.s.l.). To date, the influence of vegetative impacts upon channel planform and bedload transport capacity of alluvial reaches of the Upper Yellow River remains unclear because of a lack of hydrological and field data. In this region, the types and pattern of riparian vegetation vary with planform type as follows: trees exert the strongest influence in the anabranching reach, the meandering reach flows through meadow vegetation, the anabranching–braided reach has a grass, herb, and sparse shrub cover, and the braided reach has no riparian vegetation. A non-linear relation between vegetative cover on the valley floor and bedload transport capacity is evident, wherein bedload transport capacity is the highest for the anabranching reach, roughly followed by the anabranching–braided, braided, and meandering reaches. The relationship between the bedload transport capacity of a reach and sediment supply from upstream exerts a significant influence upon channel stability. Bedload transport capacity during the flood season (June–September) in the braided reach is much less than the rate of sediment supply, inducing bed aggradation and dynamic channel adjustments. Rates of channel adjustment are less pronounced for the anabranching–braided and anabranching reaches, while the meandering reach is relatively stable (i.e., this is a passive meandering reach).
Transitions in river character and behavior are a key focal point of enquiry in fields such as geomorphology, hydrology, and sedimentology. Such concerns have significant management applications, especially relating to issues such as management of flood risk and sedimentation hazards. These issues are likely to become even more pronounced in the future, as rivers adjust in response to climate and land use changes, and management actions. Putting aside concerns for terminological issues associated with differentiation of river types and their morphological attributes (see Lewin and Ashworth, 2014; Carling et al., 2014; Tadaki et al., 2014), process-based understandings of morphodynamic adjustments are required to address concerns for prospective future river changes (Beechie et al., 2010). Here we evaluate the influence of riparian vegetation upon process interactions along relatively understudied reaches of the Upper Yellow River atop the Qinghai–Tibet Plateau in western China. Qualitative description and analysis of this complex influence on bedload transport capacity remains unclear to our knowledge.
Channel bars are products of instream deposition of bedload materials, whether at the channel margin (bank-attached forms) or mid-channel bars (Brierley and Fryirs, 2005). Typically, bars mutually adjust with channel geometry, such that they scale to the size of the channel in which they form (Task Force on Bed Forms in Alluvial Channels, 1966; Nicholas et al., 2013). If these features become vegetated and stabilized, they are referred to as islands (Fryirs and Brierley, 2012). Unit bars (migrating lobate bed forms with heights and lengths that scale with channel depth and width) are differentiated from larger, more complex compound bars (e.g., Bridge, 1993; Brierley, 1989, 1991; Brierley and Hickin, 1991; Smith, 1974). Compound bars are products of multiple phases of accretion and reworking, with stacked sequences of unit bar, dune, and smaller bed form deposits that are often trimmed at their margins by bank erosion processes or dissected by chute channels (Ashworth et al., 2011; Best et al., 2003; Bridge, 2003; Brierley and Fryirs, 2005; McGowen and Garner, 1970; Reesink et al., 2014; Sambrook et al., 2009). Various studies have characterized the main morphological elements of large bars and islands, while other studies have developed conceptual models of bar evolution (e.g., Ashworth et al., 2000; Gurnell et al., 2001; Latrubesse and Franzinelli, 2002; Mertes et al., 1996).
There is notable variability in the presence, form, and hydraulic/sedimentologic (morphodynamic) role of bars along the continuum of channel planform (Bridge, 1993; Brierley, 1996). By definition, as suspended-load rivers have limited bedload-calibre materials, they have very few, if any, bars. The prominence of fine-grained (silt-clay) deposits under low energy conditions (often very low channel gradient) promotes passive channel behavior, typically with a low sinuosity, passive meandering, or anabranching (anastomosing) planform (Eaton et al., 2010; Fryirs and Brierley, 2012; Makaske, 2001; Wang et al., 2005). Patterns of bar formation in mixed- and bedload-dominated rivers reflect the flow–sediment balance along any given reach, with a spectrum of planform types ranging from active meandering and wandering variants through to fully braided rivers (see Ashworth, 1996; Ashworth and Lewin, 2012; Burge, 2006; Church and Rice, 2009). Braiding results from the inability of flow to transport all sediments that are made available to the channel, such that mid-channel sedimentation occurs (i.e., competence and/or capacity limits are exceeded). Recurrent reworking of bedload materials via thalweg shift during flood events alters the number, shape, and location of bars. Bar dissection and avulsion create multi-thread channel systems with a disorderly river planform, extremely unstable bars, and inconstant flow paths (Ashmore, 1991; Ashworth et al., 2000; Jerolmack and Mohrig, 2007).
If channel boundary conditions induce sufficient bank strength, and flows are able to transport available bedload sediments, the river adopts a configuration with better-defined, less mobile channels with a much lower width–depth ratio, whether within a single-channel configuration (typically passive meandering) or a multi-channel anabranching configuration (Eaton et al., 2010; Song and Bai, 2015). Controversy abounds in our theoretical understanding of process controls upon anabranching river behavior (see Carling et al., 2014; Nicholas et al., 2013). While Huang and Nanson (2007) and Jansen and Nanson (2004, 2010) attributed an anabranching configuration to the least action principle, wherein channels adjust their form to transport available sediment in the most hydraulically efficient manner, Eaton and co-workers postulated an alternative theoretical framing in which anabranching channels adjust to minimize their capacity to transport materials (Eaton and Church, 2007; Eaton et al., 2004, 2010). It is not our concern here to address this issue directly. Rather, our focus lies with analysis of relationships between bedload transport capacity and channel morphodynamics along a continuum of channel planform types that is coincident with a gradation in riparian vegetation cover along the Upper Yellow River (Yu et al., 2014).
Bar stability is the key distinguishing attribute of braided and anabranching rivers. In some instances, vegetation may support the long-term development of stable sandbars within a stable multi-channel system – a variant of an anabranching river (Latrubesse, 2008; Nanson and Knighton, 1996; Murray and Paola, 2003; Tal and Paola, 2010). Vegetation increases flow resistance and stabilizes the channel bed and bank along anabranching rivers, thereby altering channel geometry, bedload transport rates, and the resulting rates and patterns of bed deposition or erosion. Once a particular morphology has been formed, the configuration of channels and associated distribution of bars and roughness elements fashion process responses to subsequent flood events (Hooke, 1986, 2015; Hooke and Yorke, 2011; Luchi et al., 2010). If critical threshold conditions are exceeded, alterations to the balance and patterns of erosion and deposition processes may bring about transitions to different planform types.
Characteristics of the four study reaches (flood season is June–September).
Mutual adjustments between patterns of vegetation types (size, spacing, and density) and flow–sediment dynamics (patterns and rates of erosion and deposition) vary at different positions on the valley floor. Vegetation encroachment by pioneer species and successional processes induce abiotic and biotic transitions in geomorphic processes from the unvegetated channel bed and bar surfaces to grassland, shrubs, and treed areas at the margins of bars/islands and on floodplains (Corenblit et al., 2007, 2011; Gurnell, 2014; Hickin, 1984; Hupp and Osterkamp, 1996; Millar, 2000; Tooth and Nanson, 2000). Vegetation attributes influence the pattern of roughness elements and the associated distribution of flow energy, thereby affecting the distribution of erosional and depositional processes, and resulting morphological attributes (including the grain size distribution of bed/bar materials). Hence, vegetative controls influence the stability and behavior of the alluvial bed and bars, and the influence of vegetation upon flow–sediment interactions vary for differing planform types (Gran and Paola, 2001; Gradzinski et al., 2003; Jang and Shimizu, 2007; McBride et al., 2007).
Although the prominence of seasonal low–flow stages and nutrient-rich fine sands may support the growth of annual or perennial herbs and shrubs on mid-channel and transverse bars in braided rivers, this sparse vegetation cover has a negligible impact upon sediment deposition and erosion patterns, and is removed easily at flood stage (Coulthard, 2005). This mutual interaction between vegetation and erosion-deposition can be viewed as a threshold condition: if sufficient vegetation establishment occurs, resistance may exceed the erosion-deposition capability of a normal flood such that stabilization ensues, prospectively altering sedimentation patterns, increasing bank strength, and reducing channel width–depth ratio (Gran and Paola, 2001; Coulthard, 2005; Eaton et al., 2010). In anabranching channels the vegetation cover on mid-channel bars inhibits lateral migration, inducing a stable branching channel condition. During lower frequency floods, when bars are partially or completely submerged by flow, vegetation increases flow resistance, traps sediment, and inhibits erosion.
This study builds upon previously reported exploratory analyses of river
diversity in the source zone of the Yellow River (Blue et al., 2013;
Brierley et al., 2016; Li et al., 2013; Yu et al., 2014). In this region,
herbs and sparse shrubs that grow on the sand/gravel bars of braided
rivers have a trivial influence upon channel morphodynamics, while
establishment of dense shrubs and sparse trees on sand/gravel bars promotes
the emergence of anabranching channel configurations. Building on these
previous observations, here we appraise process interactions along a
vegetative gradient of river morphologic adjustments for four reaches: the Dari
and Maqu reaches of the Yellow River main stream, and the Daheba and Lanmucuo
river tributaries of the Upper Yellow River (Table 1). The Dari reach has a
semi-stable braided channel, where sandbars are covered by herbaceous
vegetation and sparse shrubs. The Maqu reach has a very stable anabranching
channel with dense willows (
Upstream of Tangnaihai hydrological station the source zone of the Yellow
River drains an area of 132 000 km
For this study, field investigations of vegetative influences upon bed/bar geomorphic processes were conducted 4 times in the summers of 2011–2014. Particle size distributions of bed and bank materials size were analyzed using a laser particle size analyzer (Mastersizer 2000) and field sieves were used to test 10 samples of river bed and bank materials in each reach. As a supplement, photographs of gravel and cobbles on the bed/bar surface were taken to visually estimate bed particle size. To estimate bedload transport capacity, channel geometry was assessed in the field and from remote sensing images of the branching channel network (Google Earth images from 2005–2014, with a resolution of about 0.24 m).
The best available hydrological data that could be accessed for this study were daily stage–discharge data from Jimai (1964–1985), monthly stage–discharge data from the Maqu (1959–1970), monthly cross section elevation change data from Shangcun station along the Daheba River (1.8 km upstream from its confluence with the Yellow River, 2009–2011), and 2011–2014 field data for the Lanmucuo River (a tributary of the Yellow River in the Maqu–Tangnaihai section, at an elevation of 3400–4200 m a.s.l., for which upstream and mid-catchment reaches have a typically meandering channel, while the downstream reach has a confined bedrock channel). There are no intensive human activities in this area of the Yellow River source zone.
Characteristics and bed material of alluvial channels in the four study reaches.
Figure 1b shows the planform morphology of the four channel reaches. Figure 2 shows the channel morphology, pattern of bar types, and bed sediment. Basic channel characteristics of the study reaches are summarized in Table 2.
The Dari reach has a semi-braided and semi-anabranching channel in a wide valley (Fig. 1 – R1 and Fig. 2 – R1). This anabranching–braided transition zone is considered to be semi-stable, with an active channel zone that is around 1 km wide. The braided part of the channel is made up of many small longitudinal and transverse bars, with multiple connected branching channels. In the anabranching part, the large bars/islands have a dense grassland vegetation. Given the extensive width of the active channel zone, annual floods during June–September exert negligible impacts upon these relatively stable surfaces.
The Maqu reach is located in a wide alluvial valley (1.5–3.0 km wide). The dense tree cover of the vegetated islands is indicative of a stable channel configuration (see Fig. 1 – R2 and Fig. 2 – R2). During the flood season, tree trunks are partly submerged into water, but the trees are sufficiently dense to limit erosion. As a result, the anabranching system as a whole is quite stable with high bedload transport capacity.
The Lanmucuo River is a meadow meandering river with nearly 100 % vegetation
cover (see Fig. 1 – R3 and Fig. 2 – R3). The root system of riparian grasses
induces considerable protection from near-bank erosion. Field investigations
from 2011–2014 indicate that the lateral migration induced by cantilever
bank failure occurred at a rate of less than 0.2 m yr
R1: channel morphology and gravel bed of the Dari reach (photographs
taken on 2 July 2012, 33.7553
The Daheba River has incised into the Gonghe–Xinhai sedimentary basin. Severe gully erosion has incised river–lacustrine sediments to a depth of 50–100 m, supplying large volumes of gravel/cobble to the middle and lower Daheba channel, inducing significant bed aggradation and the formation of a braided planform. Alluvial fans at gully outlets not only supply additional sediment, but also push the channel to the opposite side of the valley floor (a big fan is shown near D point in Fig. 1 – R4 and Fig. 2 – R4). As a result, the main branching channels are subjected to frequent and recurrent avulsion. Flows erode new small branching channels during the flood season, but a main channel coexists with several branching channels in the non-flood season. Unstable mid-channel bars are unvegetated other than sparse vegetation coverage (grass and shrubs) on riparian banks. The gravel–cobble bed and high bedload transport rate restrict vegetation establishment and growth, resulting in a typically unstable braided river.
River bank of the study reaches
Bank strength induced by sediment material mix and vegetation root networks
exerts a critical influence upon the stability of alluvial channels (Eaton
and Giles, 2009). Reinforcement of bank strength reinforced by grass, shrub,
and tree roots is related to the density, depth, and spatial structure of
the root network (Abernethy and Rutherfurd, 2001). Figure 3 shows
representative photographs of river banks in the four study reaches. The
diverse bank material composition and vegetation cover affect the relative
strength of banks and their capacity to resist near-bank flow scour. The
river bank in the Dari reach has a two-layer structure, with a 20–30 cm deep
soil-root layer (
Estimation of hydraulic coefficients and bedload transport rates.
Given the lack of observed data of bedload transport rate, bedload
transport capacity has been estimated for a rectangular cross section using
the theoretical bedload formulae outlined below. Channel flow follows the
laws of flow continuity, flow resistance, and sediment transport with flow
continuity law taking the form:
This study adopts the Manning formula to embody the law of flow resistance
for uniform alluvial channel flow:
Among numerous bedload formulae, the Meyer-Peter and Müller equation has
been extensively and successfully applied (Meyer-Peter and Müller,
1948). The modification developed by Wong and Parker (2006) has been used in this study:
Cross section and water depth were measured based on field survey and remote sensing images (see Table 2). Estimated hydraulic parameters and bedload transport capacity for the four reaches, derived using Eqs. (1)–(5), are summarized in Table 3. Note that channel width is effective bankfull width in the flood season, not valley width. The adopted mean grain size is lower than bed sediment size. Results shown here are considered to be approximations, and are analyzed solely in relational rather than absolute terms. Results show that the bedload transport capacity of the four reaches from high to low is as follows: Maqu, Lanmucuo, Daheba and Dari reaches.
The Dari reach is a wide semi-braided and semi-anabranching channel, where the
channel width is up to 1600 m (Fig. 2 – R1a). Some large stable gravel bars or
islands have a dense grass and sparse shrub cover. Many unstable bars with
low vegetation cover are subjected to recurring erosion and channel
adjustment. Vegetation may inhibit erosion and enhance bar stability at
middle flood stage, but the resistance effect of vegetation at high flood
stage is very limited. As a result, the whole channel may be eroded at high
flow stage, resulting in disordered patterns of mid-channel gravel bars. The
estimated bedload transport capacity per unit channel width is 1.77 kg m
Monthly stage–discharge relationships for Jimai hydrological
station in the Dari reach
Figure 4a and b show monthly stage–discharge relationships for 1968 and 1984,
respectively. Since the Dari reach is a multi-thread channel system, the
stage–discharge relationship is not a single function relationship. In
non-flood months (December, January, February, March, and April) the river
bed is frozen. May and November are pivotal times in the stage–discharge
relationship (the former reflects ice melt, the latter freezing). In flood
months (June, July, August, and September) the stage–discharge relationship
adjusts due to strongly erosion and deposition within the channel. The
stage–discharge data (June, July, August, and September) in the 1968 and
1984 are to run non-linear regression (power function,
For instance, different discharges for the same flow stage in June and July 1968 are considered to reflect erosion of the channel (Fig. 4a). In the other instance shown here, the maximum discharge in 1984 occurred in July (Fig. 4b), probably marking the transition from erosion to deposition phases. The geomorphological significance of the two different trends is shown in Fig. 4a and b (i.e., the trend formed by the data in March and April against the trend formed). Also, there is a difference between the high scatter trend for low discharges (probably low flood stages) and regular trends for high discharges (probably high flood stages) because the water submerged the bars in high discharge and the multi-thread channels appears in low discharges. The Dari reach is defined in this study as semi-braided and semi-anabranching since vegetation (grass and some shrubs) partially develops on channel bars, and bars are relatively stable during low and middle flood stages, whereas they are prone to change during high flood stages.
Figure 5 shows the stage–discharge relationships of the Upper Yellow River at
the Dari from June to September in 1964–1984. Apparently, the stages of 1975 are
out of line with 1978, perhaps indicating that the elevation benchmark of
the station changed in 1976 or 1977. In the same month of different years,
the stage–discharge relationship does not have a simple corresponding
relation, especially in August and September. This may reflect (1) responses
of the channel bed to strong deposition in June and July, and thereafter the
high stage corresponds to low discharge such as August in 1978–1984 and
September in 1964–1975; (2) the channel bed strongly erodes in June and July,
and thereafter the high stage corresponds to high discharge such as August
in 1964–1975 and September in 1978–1984. Overall, Fig. 2 R1a and b
indicate that the channel of the Dari reach is quite unstable during the flood
season, with erosion and deposition changing the stage–discharge
relationship. A sketch showing how flow erosion divides bars and deposits to
form new bars is shown in Fig. 6. The stage–discharge data in July from
1964–1984 are to run power function regression (
Annual stage–discharge relationship (1964–1984) of the Dari reach in Jimai hydrological station.
Sketch of channel bed deposition and erosion in flood season in the Dari reach.
The Maqu reach in the wide Ruoergai basin is covered by a dense tress (
Monthly stage change of the Maqu hydrological station (1959–1970).
Water stage change at the Maqu station from 1959 to 1970 is shown in Fig. 7. The
stage peak occurs in July and September. The maximum difference of 2.43 m
occurred between June and September in 1963. If the water depth increases to
8.0 m from 4.0 m, bedload transport capacity increases to 18.52 kg s
The Lanmucuo River is a typical meandering river covered by dense meadow. Although typically characterized by large bends in a flat valley, mid-channel gravel bar covered by herbs sometimes form at the apex of bends (Fig. 2 R3a). The meandering channel and bars are very stable because of low sediment supply in the flood season and good vegetation coverage. The tight root-soil complex on concave banks inhibits flow scour. When cantilevered bank failures do occur, slump blocks restrict further erosion of the bank. Grass develops on the point bars of convex banks. When the overbank flow submerges the point bar, the herbaceous vegetation can increase flow resistance and promote fine sand deposition (Fig. 9), thereby maintaining channel geometry with a relatively low migration rate. Growth of herbs on mid-channel bars (Fig. 2 R3a) helps to increase the flow resistance and trap fine sediment, facilitating channel stability.
Sketch of branching channel deposition and stage increasing in flood season in the Maqu reach.
Sketch of submerged bend apex with a mid-channel bar in the Lanmucuo River.
The gravel bed of the Daheba River is characterized by deposition in the flood
season and erosion in the non-flood season. This makes it difficult for
vegetation to develop on bars and banks of the braided channels. Figure 10
shows morphological changes of the riverbed before and after the flood
season in 2005. The main branching and sub-branching channels of the channel
completely changed, with an initial phase of sediment deposition followed by
flood-induced division of bars and the re-emergence of a multi-thread
braided system. Table 3 shows derived estimates of the bedload transport
capacity per width,
Braided channel evolution of the middle Daheba River in 2005
Elevation change of cross section in Shangcun hydrological station (2009–2011) (left for left bank, right for right bank).
Adjustments to channel geometry as a result of erosion and deposition processes before, during, and after the flood season are shown in Fig. 11. The elevation of the riverbed on 29 July 2009 was 0.27 m higher than on 1 April 2009. Other than a slight erosion of the left bank, the subsequent phase was depositional, with up to 1.59 m of aggradation occurring by 23 October 2009. The elevation of the riverbed was increased by 0.27 m after the flood season in 2010. The elevation of the riverbed in 1 July 2011 was 0.26 m higher than on 29 April 2011. Trivial deposition occurred from 1 to 8 July, but 0.24 m of erosion occurred by 23 July, with subsequent deposition of 0.27 m by 23 October. As a result, the riverbed elevation was 0.24 m higher after flood season in 2011, but multiple phases of deposition and erosion has occurred. The deposition–erosion–deposition phases may reflect lower bedload transport capacity relative to sediment supply in the early flood season, but widespread deposition increases the local bed slope, thereby increasing bedload transport capacity. According to Eq. (3), a 10 % increase in bed slope increases the transport capacity by 85 % in Daheba reach, so bed erosion occurs again. Bed erosion decreases the bed slope until the transport capacity has adjusted to reduced sediment supply, thereby inducing riverbed deposition once more. Consequently, alterative deposition and erosion leads to the extreme instability in the middle and lower Daheba River.
This study has outlined the complex interplay between flow and sediment supply in the flood season, and the geomorphic/hydrodynamic role of vegetation cover on the valley floor, as determinants of channel morphodynamics/stability and bedload transport capacity for four alluvial reaches of the Yellow River source zone. Although the elevation of four reaches is different (Dari is 3960 m, Maqu is 3465 m, Lanmucuo River is 3604 m, and Daheba is 2832 m), the precipitation, temperature, and bed sediment size are basically similar (Yu et al., 2014). Nevertheless, vegetation coverage in the four reaches is quite different. The Dari reach (anabranching–braided) has a herb and shrub cover, the Maqu (anabranching) reach has trees, the Lanmucuo River (meandering) has meadow, and the Daheba River (braided) has no vegetation cover. As shown elsewhere, bar morphodynamics vary markedly for differing planform types, with key differences outlined here for braided, anabranching, and meandering channels (cf., Hooke, 1986; Kleinhans, 2010; Kleinhans and van den Berg, 2010; Church and Ferguson, 2015). Bar development and stability reflect the ability of vegetation to trap sediments and stabilize banks, which in turn is directly influenced by flow energy relationships (i.e., these are mutual adjustments; Corenblit et al., 2007; Gurnell et al., 2012; Gurnell, 2014; Osterkamp and Hupp, 2010; Pietsch and Nanson, 2011). In this study, riparian vegetation and its root network are considered to restrict channel width and increase hydraulic efficiency, inducing greater bedload transport capacity in multi-thread channels (Allmendinger et al., 2005; Huang and Nanson, 2007). Islands and floodplains are able to trap more fine-grained sediment in the flood season, enhancing the longer-term (decadal) stability of anabranching channels, as shown by the stable islands of the Maqu reach.
Relative to the passive (resisting) role of vegetation, bedload transport actively affects short-term patterns and rates of bed erosion and deposition. This, in turn, is affected by relationships between the flow regime (especially flood events and formative flows) and the influence of sediment supply upon bedload transport for differing river types (Church and Ferguson, 2015; Dunne et al., 2010). The supply of bed material sediment to an alluvial channel accelerates the growth of longitudinal, transverse, and point bars, thereby enhancing thalweg development and locally increasing flow velocity. Non-equilibrium between sediment supply and transport induces local channel instability, accentuating either bed erosion or deposition (Jansen and Nanson, 2010; Nanson and Huang, 2008). In this study, a channel stability gradient accords with both sediment movement and vegetation cover, wherein bedload transport capacity (a function of bed slope, hydraulic geometry, and sediment particle size) is related to the influence of riparian vegetation upon channel geometry/planform. We contend that the differing vegetation cover and planform response reflect the delicate balance between erosion and deposition on the channel bed and bank as influenced by bedload sediment supply in the flood season. Only when the bedload transport capacity is equivalent or greater than sediment supply does vegetation act as a key determinant of channel stability.
Zhi Wei Li and Guo An Yu designed and conducted the field investigations. Gary Brierley and Zhao Yin Wang supervised the research and helped to contextualize the findings. Zhi Wei Li prepared the manuscript with contributions from all co-authors.
This study is funded by the CRSRI Open Research Program (program CKWV2016369/KY), Alexander von Humboldt Foundation, the Natural Science Foundation of China (NSFC, 41571009; 41330751; 91547112; 91547113), and the Key Laboratory of Water Sediment Sciences and Water Disaster Prevention of Hunan Province (2016SS04). X. Z. Wang, C. D. Zhang, and X. D. Zhou are acknowledged for field assistance (2011–2014). We deeply appreciate the constructive comments and suggestions from two reviewers (Peng Gao and M. Coenders-Gerrits). Edited by: G. Blöschl Reviewed by: P. Gao and M. Coenders-Gerrits