Deltas

by

Edward J. Anthony

• LAST REVIEWED: 27 July 2016
• LAST MODIFIED: 27 July 2016
• DOI: 10.1093/obo/9780199363445-0057

Introduction

River deltas are subaqueous and subaerial coastal accumulations of river-derived sediments adjacent to, or close to, the source river. The word “delta,” however, is used in a more general sense to describe any feature resulting from this type of marginal accumulation, including in lakes, lagoons, ponds, mining tails, and reservoirs. Most river deltas are formed on the margins of marine basins. River deltas vary considerably in size, and some are the largest coastal landforms in the world. In addition to fluvial sediments, delta deposits sometimes include marine or along-shore derived sediments transported by waves and currents. Deltas form where the hydrodynamic conditions in the receiving basin are not energetic enough to disperse all or the bulk of the sediment brought in by rivers. Sediment transported through deltas contributes to deposition on adjacent coasts, continental shelves, and marine basins. Much of the early research on modern deltas focused on their oil- and gas-bearing potential and how they are analogs for ancient deltas in the rock record. There has been a shift, however, toward increasingly more diverse and cross-disciplinary research on deltas. Deltas are complex landforms. Recent research has shown that deltas also act as filters, sinks, and reactors for continental materials, including carbon, in transit to the ocean. Deltas are home to nearly six hundred million people. They commonly have highly productive soils, rich and biodiverse ecosystems, and offer a wide range of ecosystem services such as coastal defense, drinking water supply, recreation, green tourism, and nature conservation. Many deltas support intense agriculture and fisheries and are food baskets for many nations. Industry and transport in some deltas are also very important, leading to the development of major urban centers, ports, and harbors. Deltas are characterized by low topography and thus particularly vulnerable to catastrophic river floods, tsunami, cyclones, subsidence, and global sea-level rise. This vulnerability is increasing as a result of reduced sediment flux from rivers and various other modifications caused by human interventions. Although deltas may develop resilience and adapt to changes in sediment supply and sea level, commonly by reorganizing their channels and their patterns of sedimentation, human impacts coupled with the effects of climate change are rendering many deltas economic and environmental hotspots. A better understanding of delta dynamics and vulnerability, and a lot of political goodwill, are needed to implement adaptive delta management, and delta restoration and rehabilitation strategies.

General Overviews

River deltas are described in most geoscience textbooks on coastal landforms. A number of studies have provided more detailed overviews, synthesizing various aspects of deltas ranging from definition, setting, and environmental conditions, through classification, environments, facies, functional dynamics, and vulnerability. A clear thread identifiable in these overviews is a strong diversity of emphasis in the latter, post-2000 publications, toward considerations on the increasing vulnerability of river deltas compared to an earlier, overarching focus on geological, morphodynamic, and oceanographic characterization. This can be seen as one goes through the overviews Anthony 2013, Coleman and Huh 2004, Elliott 1986, Evans 2012, Syvitski 2008, Woodroffe and Saito 2011, and Wright 1985. Wright 1985 proposes a detailed synthesis on deltas, especially on their morphodynamics, which is the coupling of hydrodynamic and oceanographic processes with delta morphology. There have, however, been significant new developments in delta studies since this work was published, especially on the twin aspects of delta analysis using remote sensing techniques and morphosedimentary modeling (see Modern Delta Geoscience Studies: Remote Sensing and Morphosedimentary Modeling). The book chapter Elliott 1986 is a thorough presentation of deltas based essentially on sedimentological and stratigraphic aspects, reflecting much of the early emphasis on modern deltas as analogs for ancient deltas in the rock record and the use of stratigraphy in determining their oil- and gas-bearing potential. Coleman and Huh 2004 is a web-based synthesis on fifty of the world’s deltas from remote sensing and miscellaneous data, including many poorly studied and inaccessible deltas. Hori and Saito 2008 is a well-balanced overview on deltas associated with large rivers, while Woodroffe and Saito 2011 proposes a fine and easily accessible synthesis on river-dominated coasts that brings out the overarching role of river deltas in coastal sediment supply. Many of the examples discussed by these authors concern Asian deltas, which merit a special section in this bibliographic review (see Large Asian Deltas). Syvitski 2008 is an accessible presentation of deltas that especially highlights their increasing vulnerability due to human activities. Evans 2012 provides a good overview of deltas, especially in terms of their geological framework, but this paper also largely insists on the vulnerability of deltas, hence its title. A further recent overview, particularly accessible to undergraduates and nonspecialists, is proposed in Anthony 2013, which includes a detailed bibliographic synthesis of recent advances in delta studies, in addition to a review of the basic concepts underlying deltas.

• Anthony, E. J. 2013. Deltas. In Coastal environments and global change. Edited by G. Masselink and R. Gehrels, 299–337. Oxford: Wiley.

  This chapter is part of a book on coasts, climate change, and management. It provides an easily accessible synthesis on various aspects of deltas, including delta vulnerability as a result of human activities. It is useful for both nonspecialists and undergraduates.

• Coleman, J. M., and O. K. Huh. 2004. Major deltas of the world: A perspective from space. Baton Rouge: Coastal Studies Institute, Louisiana State Univ.

  This web-based contribution provides miscellaneous data on the world’s largest deltas, including many poorly documented deltas. See also Modern Delta Geoscience Studies: Remote Sensing and Morphosedimentary Modeling.

• Elliott, T. 1986. Deltas. In Sedimentary environments and facies. Edited by H. G. Reading, 113–154. Oxford: Blackwell Scientific.

  This is a finely balanced overview chapter on the sedimentary and stratigraphic aspects of deltas. See also Why Do Some Rivers Have Deltas?, The Mississippi in Early and Modern Studies on Deltas, and Delta Development Framework, Environments, and Facies

• Evans, G. 2012. Deltas: The fertile dustbins of the world. Proceedings of the Geologists’ Association 123:397–418.

  DOI: 10.1016/j.pgeola.2011.11.001

  This is a fine and thorough review on deltas, spanning a large range of aspects from geological to the effects of human activities. Much of this article is accessible to nonspecialists. Also see Climate Change and Increasing Vulnerability of Deltas.

• Hori, K., and Y. Saito. 2008. Classification, architecture and evolution of large river deltas. In Large rivers: Geomorphology and management. Edited by A. Gupta, 214–231. Oxford: Wiley.

  This book chapter contains a balanced and thorough presentation of large deltas and their environments. See also Why Do Some Rivers Have Deltas? and Delta Classification.

• Syvitski, J. P. M. 2008. Deltas at risk. Sustainability Science 3.1: 23–32

  DOI: 10.1007/s11625-008-0043-3

  This is a useful paper that provides an accessible presentation of river deltas and their increasing vulnerability. Also see Climate Change and Increasing Vulnerability of Deltas.

• Woodroffe, C. D., and Y. Saito. 2011. River-dominated coasts. In Treatise on estuarine and coastal science. Vol. 3. Edited by E. Wolanski and D. S. McLusky, 117–135. Waltham, UK: Academic Press.

  Notwithstanding its title evoking river-dominated coasts, this book contribution provides a fine, thorough and accessible presentation of deltas, especially those of Asia. Also see Why Do Some Rivers Have Deltas?, Delta Development Framework, Environments, and Facies, and Large Asian Deltas.

• Wright, L. D. 1985. River deltas. In Coastal sedimentary environments. Edited by R. A. Davis, 1–76. New York: Springer-Verlag.

  DOI: 10.1007/978-1-4612-5078-4_1

  This book chapter summarizes many of the basic concepts behind deltas, their environments, and development framework. Also see Delta Development Framework, Environments, and Facies