Climate and tectonics impact water and sediment fluxes to fluvial systems. These boundary conditions set river form and can be recorded by fluvial deposits. Reconstructions of boundary conditions from these deposits, however, is complicated by complex channel–network interactions and associated sediment storage and release through the fluvial system. To address this challenge, we used a physical experiment to study the interplay between a main channel and a tributary under different forcing conditions. In particular, we investigated the impact of a single tributary junction, where sediment supply from the tributary can produce an alluvial fan, on channel geometries and associated sediment-transfer dynamics. We found that the presence of an alluvial fan may either promote or prevent the movement of sediment within the fluvial system, creating different coupling conditions. By analyzing different environmental scenarios, our results reveal the contribution of both the main channel and the tributary to fluvial deposits upstream and downstream from the tributary junction. We summarize all findings in a new conceptual framework that illustrates the possible interactions between tributary alluvial fans and a main channel under different environmental conditions. This framework provides a better understanding of the composition and architecture of fluvial sedimentary deposits found at confluence zones, which can facilitate the reconstruction of the climatic or tectonic history of a basin.
The geometry of channels and the downstream transport of sediment and water
in rivers are determined by climatic and tectonic boundary conditions
(Allen, 2008, and references therein). Fluvial deposits and landforms such
as conglomeratic fill terraces or alluvial fans may record phases of
aggradation and erosion that are linked to changes in sediment or water
discharge and thus provide important archives of past environmental
conditions (Armitage et al., 2011; Castelltort and Van Den Driessche, 2003;
Densmore et al., 2007; Mather et al., 2017; Rohais et al., 2012; Tofelde et
al., 2017). Tributaries are an important component of fluvial networks, but their contribution to the sediment supply of a river channel can vary substantially (Bull, 1964; Hooke, 1967; Lane, 1955; Leopold and Maddock, 1953; Mackin, 1948; Miller, 1958). Their impact on the receiving river (referred to as
By modulating the sediment supplied to the main channel, tributaries may influence the distribution of sediment within the fluvial system, the duration of sediment transport from source areas to depositional basins (Simpson and Castelltort, 2012), and the origin and amount of sediment stored within fluvial deposits and at confluence zones. Additionally, complex feedbacks between tributaries and main channels (e.g., Schumm, 1973; Schumm and Parker, 1973) may enhance or reduce the effects of external forcing on the fluvial system, thus complicating attempts to reconstruct past environmental changes from these sedimentary deposits.
The dynamics of alluvial fans can introduce an additional level of complication to the relationship between tributaries and main channels. Fans retain sediment from the tributary and influence the response of the connected fluvial system to environmental perturbations (Ferguson and Hoey, 2008; Mather et al., 2017). Despite the widespread use of alluvial fans to decipher past environmental conditions (Bull, 1964; Colombo et al., 2000; D'Arcy et al., 2017; Densmore et al., 2007; Gao et al., 2018; Harvey, 1996; Savi et al., 2014, 2016; Schildgen et al., 2016), we lack a clear understanding of the interactions between alluvial fans and main channels under the influence of different environmental forcing mechanisms. This knowledge gap limits our understanding of (1) how channels respond to changes in water and sediment supply at confluence zones and (2) how sediment moves within fluvial systems (Mather et al., 2017; Simpson and Castelltort, 2012), with potential consequences for sediment-transport dynamics as well as for the composition and architecture of fluvial sedimentary deposits.
In this study, we analyze the interplay between a main channel and a tributary under different environmental forcing conditions in an experimental setting, with particular attention to tributaries that generate an alluvial fan. Physical experiments have the advantage of providing a simplified setting with controlled boundary conditions that may include water and sediment discharge and the uplift rate or base-level changes. These models may thus capture many components of complex natural behaviors (Hooke, 1967; Paola et al., 2009; Schumm and Parker, 1973), and they provide an opportunity to analyze processes at higher spatial and temporal resolution than is generally possible in nature (e.g., De Haas et al., 2016; Parker, 1999; Reitz et al., 2010) and to directly observe connections between external perturbations (e.g., tectonic or climatic variations) and surface processes impacting landscapes.
We present results from two groups of experiments in which we separately
imposed a perturbation either in the tributary only (Group 1, Fig. 1a and b) or solely in the main channel (Group 2, Fig. 1c). Group 1 can be further
subdivided into cases in which the tributary has (a) an aggrading alluvial
fan (Fig. 1a) or (b) an incising alluvial fan (Fig. 1b). In this context, we
distinguish between two modes of fan construction:
Schematic representation of the three scenarios analyzed in this study.
By analyzing how a tributary may affect the main channel under these different forcing conditions, we aim to build a conceptual framework that lends insight into the interplay between alluvial fans and main channels. Toward this goal, we provide a schematic representation of how the downstream delivery of sediment changes under different environmental conditions. Through this representation, we hope to contribute to a better understanding and interpretation of fluvial morphologies and sedimentary records, which may hold important information about regional climatic and tectonic history (Allen, 2008; Armitage et al., 2011; Castelltort and Van Den Driessche, 2003; Densmore et al., 2007; Mather et al., 2017; Rohais et al., 2012).
An alluvial river is considered to be in steady state when its water discharge provides sufficient power, or sediment-transport capacity, to transport the sediment load supplied from the upstream contributing area at a given channel slope (Bull, 1979; Gilbert, 1877; Lane, 1955; Mackin, 1948). When a perturbation occurs in the system, the river must transiently adjust one or more of its geometric features (e.g., slope, width, depth, or grain-size distribution) to re-establish equilibrium (Mackin, 1948; Meyer-Peter and Müller, 1948). Slope adjustments are not uniform along the channel. If the perturbation occurs in the headwater of the basin (e.g., a change in water or sediment supply), slope adjustments propagate downstream from the channel head (Simpson and Castelltort, 2012; Tofelde et al., 2019; Van den Berg Van Saparoea and Potsma, 2008; Wickert and Schildgen, 2019). In contrast, slope adjustments propagate upstream if a perturbation occurs toward the downstream end of the channel (e.g., a change in base level) (Parker et al., 1998; Tofelde et al., 2019; Van den Berg Van Saparoea and Potsma, 2008; Whipple et al., 1998). The sediment transport rate of the river also depends on the direction of the change, as an increase or a decrease in precipitation or uplift rates trigger opposite responses (i.e., increase or decrease in sediment transport rate; Bonnet and Crave, 2003).
At confluence zones, the main channel is expected to adapt its width, slope, sediment transport rate, and sediment-size distribution according to the combined water and sediment supply from the main channel and the tributary (Benda et al., 2003, 2004b; Benda, 2008; Best, 1986; Ferguson et al., 2006; Lane, 1955; Miller, 1958; Rice and Church, 2001; Rice et al., 2008). Consequently, a perturbation occurring in the tributary will also affect the main channel. In their numerical model, Ferguson et al. (2006) explored the effects that changes in sediment supplied from a tributary have on the main-channel slope. They found that when tributaries cause aggradation at the junction with the main channel, the main-channel slope adjustments extend approximately twice as far upstream as they do downstream. They additionally found that variations in grain size of the tributary influence the grain-size distribution in the main channel, both upstream and downstream of the tributary junction. Because we used a homogeneous grain size in our experiments, the work of Ferguson et al. (2006) complements our analyses.
Whether the tributary is aggrading, incising, or in equilibrium may also
have important consequences for
The main channel influences a tributary primarily by setting its local base
level. Therefore, a change in the main-channel bed elevation through
aggradation or incision represents a downstream perturbation for the
tributary, and tributary-channel adjustments will follow a
When a non-incising main channel (
We conducted physical experiments at the Saint Anthony Falls Laboratory
(Minneapolis, USA). The experimental setup consisted of a wooden box with
dimensions of 4 m
Experimental setup.
We performed six experiments with different settings and boundary conditions
to simulate different tributary–main-channel interactions (Table 1). As a
reference, we included one experiment without a tributary and with a
constant
Overview of input parameters.
Each group includes one experiment with no change (NC) in
Every 30 min we stopped the experiments to perform a scan with a laser scanner mounted on the railing of the basin that surrounded the wooden box. Digital elevation models (DEMs) created from the scans have a resolution of 1 mm (Fig. 2b). We extracted long profiles and valley cross sections from these DEMs (i.e., elevation profiles perpendicular to the main flow direction) for the main channel and the tributary. Long profiles for the main channel were calculated by extracting the lowest elevation point along each cross section in the flow direction. Long profiles for the tributary were calculated with a similar procedure using outputs from Topotoolbox's SWATH profile algorithm (Schwanghart and Scherler, 2014) at 1 mm spatial resolution along the line of the average flow direction (Fig. 2b). By plotting elevation against down-valley or down-fan distance, rather than along the evolving path of the channels, the resulting slopes are slightly overestimated due to the low sinuosity of the channels. Cross sections were extracted at fixed positions, perpendicular to the main flow direction, for both the main channel and the tributary (Fig. 2b).
For the main channel, spatially averaged slopes were additionally calculated by manually measuring the bed elevation at the inlet and at the outlet of the wooden box at 10 min intervals during the experiments. This procedure yielded real-time estimates of channel slope. For comparison, spatially averaged slopes were subsequently calculated also for the tributary channel using the maximum and minimum elevation of the tributary long profile calculated within the SWATH grid. Slope data are reported in the supplementary material.
We defined the width of the active valley floor as the area along the main
channel that was occupied at least once by flowing water. It was measured
along the main channel both upstream and downstream of the tributary
junction (Fig. 3a, upper panel). The active valley floor was isolated by
extracting all DEM values with an elevation of
The sediment discharge at the outlet of the basin (
The volumes are normalized to the
All experiments included an initial adjustment phase characterized by high
Following the spin-up phase, channel-slope adjustments in our experiments matched the theoretical models described above (Sect. 2.1). The main-channel slope decreased in all experiments through incision at the upstream end, except for T_NC2 and the initial phase of T_IWMC, in which the boundary conditions favored aggradation (Fig. 4, Table 1). The slope of the tributary increased during periods of fan aggradation (e.g., IS phase of the T_ISDS run and DW phase of the T_DWIW run) and decreased during periods of fan incision (DS phase of the T_ISDS run and IW phase of the T_DWIW run) (Fig. 4). Slope adjustments did not occur uniformly but rather followed a top-down or bottom-up direction depending on the origin of the perturbation (e.g., changes in headwater conditions or base-level fall at the tributary outlet).
Long profiles of the main channel (left panels) and of the tributary channel (right panels) for all runs. Profiles represent the experiments between 300 and 570 min for the MC_Ctrl2, T_NC1, T_ISDS, and T_DWIW runs (legend values to the left of the slashes) and between 180 and 450 min for the T_NC2, and T_IWMC runs (legend values to the right of the slashes). For both the main and the tributary channel, left panels show the topographic evolution of the channels with time, whereas right panels show a single profile (i.e., at a specific time) compared to the average slope of the first plotted profile. Along the main-channel profiles, horizontal arrows indicate the position and extent of the tributary channel/alluvial fan, whereas colored arrows indicate the position of the channels in particular run times discussed in the text.
Valley width in both the main channel (Fig. 5) and the tributary (Fig. S1 of
the Supplement) increased during the experiments through bank erosion and bank collapses, until reaching relatively steady values (Fig. 6). The experiments with the tributary (Fig. 6b–f) developed a much wider main-channel valley, especially downstream of the tributary, due to higher
total
Left panels: cross sections obtained from the DEMs at three different locations along the main channel (p1, p2, and p3 respectively). The color code represents successive DEMs as illustrated in Fig. 4 (i.e., same colors for the same run times). All cross sections are drawn from left to right looking in the downstream direction. Right panels: DEM maps expressed in meters; color code represents the elevation with respect to the channel floor (also in meters).
Variations in the geometry of the active valley floor for all
experiments. For each experiment the upper panel shows the measured slope
(measured every 10 min during each experimental run). The middle panel shows the calculated average position of the right and left valley margins with respect to the central line, respectively for the main channel upstream and downstream of the tributary junction (as indicated in Fig. 3a). Gray areas represent the spin-up phase of each experiment (based on the break-in-slope registered through the manual slope measurements;
Our experiments offered an opportunity to evaluate the impacts of sediment supply from the tributary to the main channel through space and time. In general, sediment moved in pulses, and areas of deposition and incision commonly coexisted (Fig. 7a).
Volumes of sediment mobilized within the system. Black line: net
mobilized volume of sediment measured using the DoD. For comparison, black
dots represent the
To analyze the effects of the tributary on the mobility of sediment within
the coupled tributary–main-channel system, we monitored the volumes of
sediment mobilized ( In all experiments, including the one without a tributary (MC_NC), sediment moved in pulses through the system (Fig. 8). As such, the mobilized volumes ( The sediment mobilized in the middle and lower sections of the T_NC1 run showed a decrease in In the T_ISDS run, the middle section showed, as expected, a strong reduction in Patterns similar to those described for the T_ISDS can be seen for the T_DWIW run. However, due to the type of change in the tributary (i.e., decrease in The T_NC2 experiment is dominated by aggradation, and In the T_IWMC experiment, as expected,
Volume (
Schematic representation of the average sediment mobilized in each
section of the main channel. Solid black line represents the idealized
equilibrium profile of the main channel, whereas dashed lines represent the
volumes mobilized from the main channel and from the tributary.
Our six experiments provide a conceptual framework for better understanding how tributaries interact with main channels under different environmental forcing conditions (Fig. 1). We particularly considered geometric variations in the two subsystems (i.e., tributaries and main channels) and the effects of tributaries on the downstream delivery of sediment within the fluvial system.
In our experiments, the aggrading alluvial fans strongly impacted the width
of the main-channel valley both upstream and downstream of the tributary
junction. By forcing the main channel to flow against the valley wall
opposite the tributary, bank erosion was enhanced (Tables S3–S8 and Fig. S8), thus widening the main-channel valley floor (Figs. 4, 6, and S4). Bank erosion and valley widening in the main channel also occurred during periods of fan incision (Figs. S4b, S5, and S8). We hypothesize that this widening was related to pulses of sediment eroded from the fan, which periodically increased the sediment load to the main channel and helped to push the river to the side opposite the tributary (Grimaud et al., 2017; Leeder and Mack, 2001). Once there, the river undercut the banks, causing instability and collapse. As such, periods of fan incision triggered a positive feedback between increased load in the main channel and valley widening, which occurred through bank erosion and bank collapses. In these scenarios, bank contribution (
Our analysis of sediment mobility within the different sections of the main
channel highlighted that the presence of the alluvial fan affects the time
needed to reach equilibrium in the different reaches of the main river; in
the T_NC1 run, for example, due to the sediment input from the tributary, the middle and lower sections have a higher
In our experiments, fans were built under conditions that caused deposition
at the tributary junction (e.g., an increase in
Given the relative size of the tributary and main channel in our experiments
(
The main-channel bed elevation dictates the local base level of the tributary, such that variations in the main-channel long profile may cause
aggradation or incision in the tributary (Cohen and Brierly, 2000; Leeder
and Mack, 2001; Mather et al., 2017). In our experiments, lowering of the
main-channel bed triggered tributary incision that started at the fan toe
and propagated upstream (insets in Fig. 4). Because tributary incision
increases the volume of sediment supplied to the main channel, a phase of
fan progradation would be expected, similar to the cases described above
(and in the
We did not observe the complex response described by Schumm (1973), characterized by tributary aggradation following incision along the main channel. The complex response in Schumm's experiments likely occurred because the main river had insufficient power to remove the sediment supplied by the tributaries, as opposed to what occurred in our experiments. When aggradation occurs at the tributary junction, one may expect to temporarily see an evolution similar to that proposed in the “aggrading alluvial fan” scenario, with the development on an alluvial fan that may alter the sediment dynamics of the main channel, modulating the sediment mobilized in the upper and lower sections of the river and delaying main-channel adjustments. In our experiment, instead, a prolonged erosional regime within the main channel may have led to fan entrenchment and fan-surface abandonment (Clarke et al., 2008; Nicholas and Quine, 2007; Pepin et al., 2010; Van Dijk et al., 2012). Despite the lack of fan progradation, an increase in bank contribution following incision of the main channel did occur (Figs. 7f and S9) and could be explained by (1) higher and more unstable banks and (2) an increased capacity of the main channel to laterally rework sediment volumes under higher water discharges (Bufe et al., 2019).
Understanding the interactions between tributaries and the main channel and the contribution of these two subsystems to the sediment moved (either eroded or deposited) in the fluvial system is extremely important for a correct interpretation of fluvial deposits (e.g., cut-and-fill terraces or alluvial fans), which are often used to reconstruct the climatic or tectonic history of a certain region (e.g., Armitage et al., 2011; Densmore et al., 2007; Rohais et al., 2012; Simpson and Castelltort, 2012).
In their conceptual model, Mather et al. (2017) indicated that an alluvial
fan may act as a
Conceptual framework for the coupling conditions of an
alluvial-fan–main-channel (MC) system under different environmental forcings.
For
If the tributary has perennial water discharge, a During fan incision, large volumes of sediment are eroded from the fan and transported into the main channel as healing wedges, allowing the fan to prograde into the main channel (Figs. S4c and 10c). This process creates a During times of fan progradation, the fan creates an obstacle to the transfer of sediment down the main channel, creating a
In summary, downstream fluvial deposits record the competition between the
main channel and the tributary; the alluvial fan pushes the main channel
towards the opposite side of the valley to adjust its length, whereas the
main channel tries to maintain a straight course by removing the material
deposited from the fan. If the main channel dominates, it cuts the fan toe
and permits sediment from upstream of the junction to be more easily moved
downstream. If the tributary dominates, the main channel will be displaced,
and the transfer of sediment through the junction will be disrupted. An
autogenic alternation of these two situations is possible, whereby fan-toe
cutting may trigger fan incision and progradation, increasing the influence
of the fan on the main channel. The composition of the sediment downstream
thus reflects the competition between main channel and alluvial fan, with
contributions from both subcatchments. In addition, bank erosion may make
important contributions to sediment supply and transport, particularly
during periods of fan incision (Fig. S8). From these results, we therefore distinguish between the following: (1)
Lowering of the main-channel bed triggers incision into the alluvial fan, thereby promoting a An increase in main-channel water discharge increases the transport capacity of the mainstem so that it persistently “wins” the competition with the alluvial fan. In this case, despite the incision triggered in the alluvial fan, which increases the sediment supplied by the tributary, the main channel efficiently removes the additional sediment load, thereby reducing the influence of the alluvial fan on downstream sediment transport within the main channel (Fig. S7). The consequence is a
Physical experiments have the advantage of simulating many of the complexities of natural systems in a simplified setting (Paola et al., 2009). Because of the simplifications, however, a number of limitations arise when attempting to compare experimental results to natural environments. One limitation of our study concerns the small number of experiments that we have performed compared to the full variability in natural river systems and the lack of repetition of experiments. This limitation prevents us, for example, from fully distinguishing significant trends in sediment mobility from stochastic or autogenic processes that are inherent of alluvial systems. In Sect. 2.2, we described how fan-toe cutting may create the same response in the tributary as incision along the main channel. However, we are not able to quantify the relative contribution of these two processes on the changes occurring in the tributary. One way to distinguish between fan-toe cutting and main-channel incision is to study the whole fluvial system, thus including all tributaries. Main-channel variations will affect all tributaries with a timing that is diachronous in the direction of the change (Mather et al., 2017, and references therein). Fan-toe cutting, on the other hand, will be specific of single tributaries with random timings.
Another limitation of our experiments relates to the scaling. Our experiments were not scaled to any particular environment. Instead we used the principle of
Despite these shortcomings, the analysis presented here provides insights into how channels respond to changes in water and sediment discharge at confluence zones and how sediment moves through branched fluvial systems. In particular, the dynamics that govern the movement of sediment can have important repercussions for field studies, particularly for interpretations of alluvial-channel long profiles, dating of material within stratigraphic sequences, and interpretations of their geochemical composition (e.g., Tofelde et al., 2019, and references therein). Additionally, by partially decoupling the upper and lower sections of the main channel, fan progradation may lead to pulses of sediment movement from the upper to the lower sections of the main channel, therefore disrupting environmental signals that could be transmitted downstream (e.g., Simpson and Castelltort, 2012). Indeed, the stratigraphy of the downstream section of the main channel may record periods of high sedimentation rates, erroneously pointing to periods of high sediment supply, when in reality the fast accumulation may be related to a pulse of sediment being eroded from the upstream section of the main channel.
These complexities highlight the need for further research on these topics and the importance of studying the coupled tributary–main-channel system to fully understand the dynamics acting in the river network and correctly interpret both geochemical and stratigraphic signals.
We performed six experiments to analyze the interactions of a tributary–main-channel system when a tributary produces an alluvial fan. We found that differing degrees of coupling may be responsible for substantial changes in the geometry of the main channel and the sediment transfer dynamics of the system. In general, we found that the channel geometry (i.e., channel slope and valley width) adjusts to changes in sediment and water discharge in accordance with theoretical models (e.g., Ferguson and Hoey, 2008; Parker et al., 1998; Whipple et al., 1998; Wickert and Schildgen, 2019). Additionally, by analyzing the effects of the tributary–main-channel interactions on the downstream delivery of sediment, we have shown that the fluvial deposits within the main channel above and below the tributary junction may record perturbations to the environmental conditions that govern the fluvial system.
Our main results can be summarized as follows (Fig. 10):
Fan aggradation leads to a partial coupling between the fan and the main channel, which permits a complete coupling between the main channel reaches upstream and downstream of the tributary junction. As such, the provenance of downstream sediment reflects the dynamics of both subcatchments (e.g., tributary and main river), and remobilized material from older deposits will be minimal. Fan incision favors a complete coupling between the fan and the main channel and remobilizes material previously stored in the fan. Fan progradation (either during prolonged aggradation or fan incision) strongly influences the main channel. As a result, the connectivity of the main river across the tributary junction is reduced, and the deposits of the fluvial system above and below the junction may record different processes. Incision along the main channel triggers incision in the alluvial fan that, despite an increased sediment supply to the main river, reduces its influence on the dynamics of the main channel. The result is a fully connected fluvial system in which the deposits record sediment-transfer dynamics and the interactions between both the alluvial fan and the main river, including a large component of material remobilized from older deposits.
The theoretical framework proposed in this study aims to illustrate the
dynamics acting within a tributary junction. It provides a first-order
analysis of how tributaries affect the sediment delivered to the main
channels and of how sediment is moved through the system under different
environmental forcing conditions. The (dis)connectivity within the fluvial
system has important consequences for the stratigraphy and architecture of
depositional sinks, as it may be responsible for the continuity of the
sedimentary record or for the disruption of the environmental signals
carried through the main channel (Simpson and Castelltort, 2012). Our findings may be used to improve the understanding of the interactions between tributaries and main channels, providing essential information for the reconstruction of the climatic or tectonic histories of a basin.
Data, DEMs, and videos are available through the Sediment Experimentalists Network Project Space to the SEAD Internal Repository (
Time-lapse videos of the experiment are available
through the Sediment Experimentalists Network Project Space
to the SEAD Internal Repository (
Supplement tables and figures can be found in the Supplement. The supplement related to this article is available online at:
SS, ST, and ADW designed and built the experimental setup. SS and ST performed the experiments. SS analyzed the data with the help of ST, ADW, and AB. All authors discussed the data, designed the paper, and commented on it. SS designed the artwork.
The authors declare that they have no conflict of interest.
We thank Ben Erickson, Richard Christopher, Chris Ellis, Jim Mullin, and Eric Steen for their help in building the experimental setup and installing equipment. We are also thankful to Jean-Louis Grimaud and Chris Paola for fruitful discussions and suggestions.
This research has been supported by the Deutsche Forschungsgemeinschaft (grant nos. SCHI 1241/1-1 and SA 3360/2-1), the Alexander von Humboldt-Stiftung (grant no. ITA 1154030 STP), and the University of Minnesota.
This paper was edited by Greg Hancock and reviewed by Lucy Clarke and Luca C. Malatesta.