Changes in the Hinterland and Floodplain

As human populations have increased on the planet, so have their effects on the natural landscape. When human-engineered changes in the movement of soils and rocks occur in the vast watersheds of major rivers, they can have dramatic consequences with respect to the amount of sediment needed to “feed” and support large river deltas at the coast. Many of the largest effects of human activity on the surface of the earth have occurred recently—in the past 200 or so years—and they have been so dramatic it has been argued it is time to create a new epoch in the Geologic Time Scale, one called the Anthropocene. That suggestion is being considered seriously. Nevertheless, the first alterations of the landscape began as early as the Paleolithic, approximately 400,000 to 500,000 years ago, when our human-like ancestors Homo erectus are believed to have begun altering the natural landscape with simple dwelling structures. As humans evolved, so did the tools they used, from sticks and animal antlers to wood and iron plows. Although modern humans (Homo sapiens sapiens) had developed in East Africa by about 200,000 years ago, their ability to extensively modify the landscape through agricultural activities did not likely happen for another 120,000 years. Incredibly, there was a rise in agricultural communities about five millennia ago that seems to have occurred simultaneously, yet independently, in six different regions of world (see Chapters 1 and 2 for linkages among human civilizations, deltas, and stabilization of climate in the Holocene). After the invention of the wheel in the middle Holocene, it became much easier to perform earth-moving activities. This was followed by the Iron Age, around 2,500 years ago, during which iron replaced earlier, less efficient copper and bronze tools for moving earth. Amazingly, the first man-made canal, connecting the Mediterranean and Red seas, was constructed before the Iron Age, around 3,600 years ago. Today, humans are the most effective animals on the planet with respect to altering Earth’s surface, and the use of machinery enables earth-moving activities, such as strip- mining, for extraction of valuable mineral resources like copper and silver.

Early Human Civilizations and River Deltas

For millennia, humans have been dependent upon rivers and their resources for food, transport, and irrigation, and by mid-Holocene times (about 5,000 years ago), humans harnessed hydraulic power that in part contributed to the rise of civilization. It is generally accepted that the earliest civilizations to develop such linkages with irrigation and cultivation of crops arose in the Old World, in Mesopotamia and the Levant, the Indus Valley, and the Central Kingdom, associated with, respectively, the Tigris, Jordan, Euphrates, and Nile; the Indus; and the Huang He (Yellow) and Changjiang (Yangtze) rivers—and, of course, their associated deltas. In this chapter, I examine the role of selected coastal deltas that were important in the development of these early Old World civiliza­tions, and how those people began to alter the shape and character of the highly productive and constantly changing deltaic environments. Before we begin, how­ever, I need to provide some basic definitions. First, I use the definition of civilization provided by Hassan, “a phenome­non of large societies with highly differentiated sectors of activities interrelated in a complex network of exchanges and obligations.” Second, I use the defini­tion of delta presented by Overeem, Syvitski, and Hutton, “a discrete shoreline protuberance formed where a river enters an ocean or lake, … a broadly lobate shape in plain view narrowing in the direction of the feeding river, and a sig­nificant proportion of the deposit … derived from the river”. Although I will at times discuss linkages between development of human settlements and river reaches upstream from the coastal delta, my primary focus in this chapter is on coastal deltaic regions, in particular those of the Nile, Indus, Yellow, and Yangtze rivers, which provide the best examples for link­ages between relatively recent early human populations and coastal deltas. I will address other deltas later in the book. My rationale for beginning this book with a discussion of the relationship between Old World civilizations and deltas is that this long- term interaction has been so dramatically altered over the past few millennia— essentially, it is a good relationship “gone bad.”

Exploring a Sustainable Future

In this chapter, I will explore the concept of sustainability, as viewed in the United States and around the world, and examine how we have arrived at our current thinking about conservation practices in a continually evolving, complex geopolitical sphere. I will do this to link delta restoration with the broader, global issues of providing food and clean water as described in the United Nations Millennium Development Goals (http:// www.un.org/millenniumgoals). Many people have written on global environmental sustainability, so I will only briefly summarize these views here and conclude with a brief statement about delta sustainability. During the short time that humans have been on this planet, we have altered nearly 50% of the land surface, and 50% of the wetlands in the world have been lost—a consequence of the unsustainable mindset of human civilizations. Sustainability embodies “stewardship” and “design with nature,” with well-defined goals and an agreed upon “carrying capacity,” that can be developed and modeled by scientists and planners. The most popular definition of sustainability can be traced to a 1987 United Nations conference, in which sustainable development programs were described as those that “meet present needs without compromising the ability of future generations to meet their needs.” Robert Gillman, editor of In Context magazine, extends this goal-oriented definition by stating “sustainability refers to a very old and simple concept (The Golden Rule) … do unto future generations as you would have them do unto you.” These well-established definitions set forth an ideal premise, but they do not specify the human and environmental parameters needed to model and measure sustainable development. So, here are some more specific definitions: “Sustainable means using methods, systems and materials that won’t deplete resources or harm natural cycles.” Sustainability “identifies a concept and attitude in development that looks at a site’s natural land, water, and energy resources as integral aspects of the development.”

Saving the Deltas

Coastal deltas occupy only 1% of Earth’s land surface, yet they are home to 500 million people; this translates to a population density that is 10 times the world average. More amazing, these people all live within only 5 m of sea level! The importance of deltas for global agriculture cannot be overstated, as they are the “rice bowls” of the world, providing one of the major food staples for human populations. Unfortunately, many of these systems are threatened by sea-level rise and flooding in the future. In fact, most coastal plains around the world less than 1 m above sea level are under the threat of being drowned within the next century, something that has not happened at this rate over the past 7,000 years. This means that major cities like Shanghai, Dhaka, and Bangkok are currently threatened and, in most cases, have no viable plan to deal with this threat. A concrete scientific plan is needed to manage and sustain these dynamic systems, or many will be lost. Although the effects of climate change on coastal regions and issues of management response have been topics of concern for at least the last 20 years, other human drivers of environmental change that are specific to certain regions, primarily linked with population expansion, have made it difficult to develop comprehensive plans for coastlines. The northern portion of the Nile River Delta, whose fertile soil allowed Egypt to become one of the cradles of civilization, is tilting and sinking toward the Mediterranean Sea at an alarming rate. In the northeastern part of the delta, near the Suez Canal, the land is sinking by as much as 0.5 cm/yr. Recent studies show that the weight of river sediments accumulated over the centuries has resulted in enhanced subsidence of deltaic sediments. Subsidence is an inherent problem in all delta systems because of the high accumulation of sediments, which over time continue to settle, resulting in compaction and dewatering of these thick mud layers.

Effects of Sea-Level Rise and Subsidence on Deltas

As I briefly mentioned in Chapter 3, the global mean sea level, as deduced from the accumulation of paleo-sea level, tide gauge, and satellite-altimeter data, rose by 0.19 m (range, 0.17–0.21 m) between 1901 and 2010 (see Figure 3.3). Global mean sea level represents the longer-term global changes in sea level, without the short-term variability, and is also commonly called eustatic sea-level change. On an annual basis, global mean sea-level change translates to around 1.5 to 2 mm. During the last century, global sea level rose by 10 to 25 cm. Projections of sea-level rise for the period from 2000 to 2081 indicate that global mean sea-level rise will likely be as high as 0.52 to 0.98 m, or 8 to 16 mm/ yr, depending on the greenhouse gas emission scenarios used in the models. Mean sea-level rise is primarily controlled by ocean thermal expansion. But there is also transfer of water from land to ocean via melting of land ice, primarily in Greenland and Antarctica. Model predictions indicate that thermal expansion will increase with global warming because the contribution from glaciers will decrease as their volume is lost over time. (Take a look at Figure 5.1 if you have doubts about glaciers melting.) And remember our discussion in Chapter 2 about the role of the oceans in absorbing carbon dioxide (CO2) and the resultant ocean acidification in recent years. The global ocean also absorbs about 90% of all the net energy increase from global warming as well, which is why the ocean temperature is increasing, which in turn results in thermal expansion and sea-level rise. To make things even more complicated, the expansion of water will vary with latitude because expansion of seawater is greater with increasing temperature. In any event, sea level is expected to rise by 1 to 3 m per degree of warming over the next few millennia.

The Ever-Changing Delta

After his visit to Egypt in the year 500 B.C.E., Herodotus compared the triangular shape of the lowland region, where the Nile and sea meet, to the Greek letter Δ, thereby introducing the term delta to the geographic literature. In Chapter 1, we defined a delta as “a discrete shoreline protuberance formed where a river enters an ocean or lake … a broadly lobate shape in plain view narrowing in the direction of the feeding river, and a significant proportion of the deposit … derived from the river.” Coastal deltas are geologic structures that are also subcomponents of an estuary, which is commonly defined as a semienclosed body of water, situated at the interface between the land and ocean, where seawater is measurably diluted by the inflow of fresh water. James Syvitski, a world-renowned expert on deltas, describes how a delta’s area can be defined as “1) the seaward prograding [building outward] land area that has accumulated since 6,000 years, when global sea level stabilized a few meters of present level, 2) the seaward area of a river valley after the main stem of a river splits into distributary channels, 3) the area of a river valley underlain by Holocene marine sediment, 4) accumulated river sedi¬ment that has variably been subjected to fluvial, wave, and tidal influences, 5) the area drained by river distributary channels that are under the influence of tide, or 6) any combination of these definitions.” These delta-front estuaries, hereafter referred to as deltas, are dynamic ecosystems that have some of the highest biotic diversity and production in the world. Consequently, an estimated 25% of the world’s population lives in environments that are coastal deltas and their associated estuaries/ wetlands. Deltas provide not only a direct resource for commercially important estuarine species of fishes and shellfish but also shelter and food resources for commercially important shelf species that spend some of their life stages in estuarine marshes. For example, high fish and shellfish production in the northern Gulf of Mexico is strongly linked with discharge from the Mississippi and Atchafalaya river delta complexes and their associated estuarine wetlands.

The Holocene and Global Climate Change

The Pleistocene Epoch, often referred to as the Ice Age, lasted from approximately 2.6 million to 11,700 years ago. The last major ice advance began about 110,000 years ago, and the most recent episode of maximum ice coverage, the Last Glacial Maximum, began about 26,500 years ago and ended approximately 19,000 years ago. Thereafter, glacier retreat began, largely ending by about 11,700 years ago. That marked the beginning of the Holocene interglacial geologic epoch, which continues to the present. During the last glacial period, sea level was much lower because so much water was locked up in ice sheets, largely at the poles. This lowering of the sea level exposed the margins of the continents (the continental shelves) around the world. When the Ice Age ended, sea level started to rise during the deglacial period, a process that continued into the Holocene. Deltaic regions received meltwaters from the thawing glaciers, along with glacier- derived sediments. Of particular note in the late Holocene is a climate episode called the Medieval Warm Period, originally identified by the English botanist Hubert Lamb. The Medieval Warm Period was a time of warm climate in the North Atlantic region and may have also impacted other areas around the world. It lasted from about the years 950 to 1250. Later in this chapter, I will discuss this climate anomaly, along with something called the “Hockey Stick” debate, which relates to exceptional warming during recent centuries of the Holocene (i.e., global warming). In any case, all modern and paleodeltas formed during periods of peak sea level in the Holocene. These new deltas had fertile soils that were constantly irrigated by the flow of fresh water, which promoted early settlement by humans. So, the Holocene started near the end of the retreat of the Pleistocene glaciers, and human civilizations arose entirely in the Holocene Epoch. To view the Holocene, simply look around you today. In this chapter, I will explore the natural and human-induced causes of global climate change and how they impact deltaic regions.