Distributary channels and their impact on sediment dispersal

doi:10.1016/j.margeo.2005.06.030

Marine Geology

Volumes 222–223, 15 November 2005, Pages 75-94

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Abstract

A global analysis of world deltas, with details from a natural and an anthropogenic-influenced delta, demonstrates how distributary channels control the flux of sediment into the coastal ocean. The study addresses the range in the number of distributary channels across world deltas using remote sensing techniques. A power law relationship is found between the number of distributary channels and the length of river and, separately, the delta gradient. These relationships hold for all types of deltas whether controlled or strongly influenced by waves, river discharge, tides or ice (permafrost or sea-ice). Nature-controlled deltas, such as the Klinaklini delta, have distributary channels that act as overflow conduits that become active during flooding events. Anthropogenic-controlled deltas, like the Po delta, have distributary channels that are controlled for flood mitigation or low flow maintenance. Anthropogenic influences greatly impact the natural rate of delta progradation through changes in sediment supply, controlling the position of distributary channels, and impacting subsidence from gas and groundwater extraction. Even with flood controls, the Po delta traps 16% of the sediment load in its channels that are becoming super-elevated at rates of 4 to 10 cm/yr, with respect to the delta plain. A new model is formulated and shown to predict accurately the sediment flux through each channel, along with their hydraulic properties. Deltas with high numbers of distributary channels produce hypopycnal plumes with reduced transport capacity. As a result, sediment diffuses out of the multi-channel deltas as a buoyantly driven plume, rather than as a momentum driven jet.

Introduction

Sediment delivered to the ocean is mostly by rivers, which accounts for approximately 95% of the global flux of sediment off the land (Syvitski, 2003). The transport is mostly as particles suspended in the flowing river water (72%), or sediment dissolved in the water (20%) with only a small amount transported as bed load (8%; Syvitski, 2003). There is a much wider range in these values at the level of individual rivers. Sediment deposited initially in the marine (prodelta) realm is often reincorporated into the terrestrial landmass as the shoreline progrades seaward. Terrestrial deposits include delta plains and flats, distributary channel deposits, mouth and tidal bars, lagoons and shoreface complexes. River deltas found along our present coastline appear to have formed extensively in the last 6 kyr (Amorosi and Milli, 2001), after sea level stabilized circa 6 kyr (Syvitski et al., 2005b).

The internal architecture of deltaic systems is controlled by the complex interaction between boundary conditions and forcing factors (Coleman and Wright, 1975, Orton and Reading, 1993, Postma, 1995, Overeem et al., in press): (1) sediment supply of bedload and suspended load: reflecting drainage basin characteristics, water discharge and sediment yield; (2) accommodation space: reflecting sea-level fluctuations, offshore bathymetry, tectonics, subsidence, compaction, and isostasy; (3) coastal energy: waves and tides, longshore and cross-shelf transport; and (4) density differences between effluent and receiving waters critical in defining the dynamics of plumes.

Within a single delta, forcing factors may vary both spatially and temporally. For example, the San Juan River delta is influenced differentially by river discharge, wave action and tides at its various outlets (Restrepo et al., 2002). Similarly, the southern part of the Danube delta is a wave-dominated system, whereas the northern part is discharge-dominated (Bhattacharya and Giosan, 2003). Arguably the most important factor controlling the three-dimensionality of deltas is the number and pattern of distributary channels (Gould, 1970, Kanes, 1970, Bondesan et al., 1995). The apex of the delta (the hinge point) is where the river leaves the confines of its river valley and divides into distributary channels that over time avulse across very low slopes of its delta plain. It has been suggested that the number of distributary channels found on a delta plain is limited by wave action and dispersal of the fluvial bedload in the form of longshore transport (Bhattacharya and Giosan, 2003). This study tests this suggestion using information on a large number of global deltas. The questions to be addressed are: (1) what is the range in the number of distributary channels across world deltas; and (2) how does the number of distributary channels scale with key environmental factors?

This study also examines the discharge dynamics of distributary channels both in the natural environment, using the Klinaklini River in Canada as an example, and under strong anthropogenic influence, using the Po River in Italy as an example. There are over one billion humans living on world deltas in coastal cities, including mega cities (Syvitski et al., 2005a). Few deltas remain in their pristine form—most are highly engineered to control flooding both from rivers and from the sea. A model is presented to predict the flux of sediment through distributary channels. The model is then compared to observations on the Po Delta. Finally the paper presents a short discussion on some of the impacts of distributary channels on the marine dispersal system.

Section snippets

Data and method

To examine the causal factors that control the number of distributary channels, a database was established that included the following factors (Appendix 1 includes a subset of this more extensive database): (1) drainage area feeding the delta; (2) maximum relief of the drainage basin; (3) water discharge across the delta; (4) sediment yield within the drainage basin; (5) length of the main stem of the river; (6) gradient of the delta plain; (7) number of distributary channels; (8) spring tidal

Natural distributary channels: e.g. Klinaklini delta

The Klinaklini delta, flowing into Knight Inlet in British Columbia, is one of the few remaining deltas that shows little influence by humans—limited logging is a minor anthropogenic influence. Fig. 3 provides a sequence of aerial photographs, collected across a 23-yr period, wherein the flow path of the main channel switches a number of times across the delta plain (after Syvitski and Farrow, 1983). The delta is influenced by seasonally strong river discharge that flows across tidal flats

Distributary channels in the Anthropocene: e.g. Po delta

The evolution of the Po delta is perhaps one of the best documented in the world, and provides valuable insight to the nature of distributary channel switching across human history. With over three millennia of human occupation, and a present hinterland population of 16 million, the delta evolution also tracks human perturbation through the Anthropocene. The 75,000 km² Po watershed is one of the most agriculturally productive areas in Europe and is highly irrigated in summer. The Po is bounded

Discharge dynamics through distributary channels: e.g. Po delta

There are almost no studies on the flux of water and sediment flowing through distributary branches of a delta. One reason is the complexity and cost of the task, with complexity/cost increasing concomitant with the number of branches. Each distributary branch would need at least two sampling stations, one near the main stem channel, and one near the outlet mouth. This would ensure that the flux of water and sediment entering the branch and leaving the branch would be ascertained. Sampling

Modeling the flux of sediment through distributary channels: e.g. Po delta

Given the importance of distributary channels, much of the determinism related to the physics of the problem remains speculative. There are no models that successfully predict where discharge within a distributary channel will break through its levee, and over time create a new channel. With large populations occupying deltas, engineers now limit natural avulsion, or at least control the timing and location of channel breaches. Topset aggradation is artificially reduced, and most of the

The role of distributary channels on sediment dispersal in the marine environment

The mean discharge velocity out of the Po distributary channels ranges from an average of 0.6 m/s for the Maestra and Tolle branches, to 0.9 m/s for the Pila (Table 2). The discharge-adjusted average velocity from all five distributary channels is 11% smaller than if the Po entered the ocean as a single channel. The mean hydraulic width of the distributary channels is 780 m, 160% larger than for the single main stem of the Po River (Table 2). Together these two hydraulic parameters (velocity

Conclusions

1)
A power law relationship is proposed between the number of distributary channels and either a river's length, or the gradient of a delta. These relationships hold for all types of deltas whether controlled by wave, river, tides or permafrost.
2)
No statistical relationship was found between the number of distributary channels and any of the following parameters (discharge, wave magnitude, tidal range, sediment load, basin relief, basin area).
3)
Nature-controlled deltas, such as the Klinaklini delta in

Acknowledgments

We respectfully acknowledge the reviews by Liviu Giosan, Patrick Friend, and Alessandro Amorosi that helped the presentation of this report. This report is supported by the U.S. Office of Naval Research, EuroSTRATAFORM program under the guidance of Dr. Tom Drake (code 321). We thank the efforts of program leaders Chuck Nittrouer (UW), Fabio Trincardi (CNR), Serge Berne (IFREMER), Phil Weaver (SOC). The senior author particularly thanks the Italian scientists at CNR-IGM who hosted his sabbatical

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Citation Excerpt :

We question findings that do not address the impacts of these driving factors. The simultaneous study of multiple deltas relies on global methods using change detection from orbital platforms (Syvitski and Saito, 2007; Syvitski et al., 2012; Brakenridge et al., 2013a, 2013b; Higgins et al., 2013, 2014; Kuenzer et al., 2019), or human-landscape interaction modelling (Syvitski et al., 2005a, 2005b; Brakenridge et al., 2013a, 2013b; Tessler et al., 2015, 2018; Cohen et al., 2022), or the application of findings from global database studies (Syvitski and Milliman, 2007; Syvitski and Saito, 2007; Syvitski, 2008; Syvitski et al., 2009; Besset et al., 2019; Wu et al., 2020a, 2020b; Anthony et al., 2021). Yet these advances rely on field observations as the gold standard required for critical and often costly environmental decisions (Asselman and Middelkoop, 1998; Correggiari et al., 2005; Blum and Roberts, 2012; Rogers et al., 2013; Brown and Nicholls, 2015; Wang et al., 2017; Minderhoud et al., 2018; Day et al., 2011, 2019).

Deltas are subaerial landforms that cap underlying deposits with subaqueous extensions that result from a river feeding sediment directly into a standing body of water at a rate that overwhelms any effective dispersal processes derived from the ambient basin. This definition encapsulates both the terrestrial surface expression and the geological focus on the entire sediment mass. Environmental studies also focus on the ecology of deltaic wetlands, their drowning history, and related sustainability issues including societal considerations, history, and culture. A mean 76 ± 16% drop in hydraulic energy occurs in all subaerial deltas regardless of size, given the break in gradients separating fluvial and deltaic surfaces, driving an ever-decreasing bed-material transport, shallowing of distributary channels and concomitant overbank flooding. A delta's sediment mass grows from the addition of new river loads but can also include aeolian and marine sediment derived from outside the delta domain, growth of peat and other biomass, and inputs from human action. Removal of sediment is via river plumes interacting with marine currents, wave-induced transport, sediment failures and gravity flows, high-tide inundation onto the delta plain, tidal channel widening and deepening, and human action (peat, clay, sand and gravel mining). A delta's trapping efficiency ranges from 0 for small-load rivers that discharge directly into an energetic ocean, to 80% for large deltas, and up to 100% for some semi-enclosed bayhead deltas, including fjords. The global (ensemble) subaerial delta aggradation rate is ∼1.6 mm/y if 70% of the global sediment load exits the river mouth(s), a reminder of how much sediment can be expected to be delivered to the surfaces of global deltas at a time when the 2022 CE sea level rise is ∼4 mm/y.

At the planetary scale, deltas are environmentally complex given Earth's range in climate, hydrodynamics, tectonic settings, relative sea-level provinces, sediment input, redistribution processes, and human actions. Under natural conditions, the subaerial portion of deltas adapt to change by advancing, retreating, switching, aggrading, and/or drowning, whereas many modern deltas are structurally constrained by societal needs. The 89 large and mud-rich coastal marine deltas (i.e. subaerial area > 1000 km²) account for 84.3% of Earth's total deltaic area that hosts >89% of all humans occupying deltas, many living within megacities. The 885 medium-size deltas (i.e. subaerial areas 10–1000 km²) account for 15.5% of the global delta area and 10.5% of humans living on deltas, with characteristics that fall between the small and large delta categories. The 1460 small and essentially sandy deltas (1–10 km²), including all fjord deltas, are impacted less from human action (with exceptions) and most are better able to withstand climate change. Recognizing the limits of big data in capturing delta complexity, field data remains a necessary gold standard for site investigators.

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Distributary channels and their impact on sediment dispersal

Abstract

Introduction

Section snippets

Data and method

Natural distributary channels: e.g. Klinaklini delta

Distributary channels in the Anthropocene: e.g. Po delta

Discharge dynamics through distributary channels: e.g. Po delta

Modeling the flux of sediment through distributary channels: e.g. Po delta

The role of distributary channels on sediment dispersal in the marine environment

Conclusions

Acknowledgments

Sediment. Geol.

Quat. Int.

Chemosphere

Glob. Planet. Change

Sediment. Geol.

Mar. Geol.

Sediment. Geol.

Mar. Geol.

Comput. Geosci.

Comput. Geosci.

Sediment. Geol.

Wave-influenced deltas: geomorphological implications for facies reconstruction

Sedimentology

L'area deltizia padana: caratteri geografici e geomorfologici

Dinamica e analisi morfologica statistica dei litorali del delta del Po e alle foci dell'Adige e del Brenta

Mem. Sci. Geol.

Indagine sulla, ripartizione della portata del Po tra i vari rami del delta e sulla loro attivita di deflusso

Geometric investigation of the subsidence in the Po Delta

Boll. Geofis. Teor. Appl.

Modern river deltas; variability of processes and sand bodies

A 3-D hydrodynamic model of river flow in a delta region

Hydrol. Process.