11.7: Deltas

Rivers flowing toward the sea carry particles as suspended sediment. When they reach the flat coastal plain, river current speed decreases because of diminishing slope and widening of the channel. Consequently, the larger size fractions of suspended sediment are deposited (CC4) before reaching the ocean, slowly filling the river valley. Many rivers flow through valleys cut by glaciers or rivers when sea level was lower during the most recent ice age, and the lower ends of these valleys have not yet filled with sediment. Because most rivers on the East Coast of the U.S. are in this category, they deliver little sediment to the Atlantic Ocean. Some rivers, such as the Mississippi, create a delta as they flow into the ocean because they carry a large amount of sediment and slow down abruptly when they flow into the ocean. These sediments build up over time creating a roughly triangular delta at the river’s mouth as the river flows in distributary channels through the deposited sediment. In some places, such as the Mississippi Delta, the channels pass through long extensions or lobes of these deposits (Fig. 11-5).

Deltas are built and supplied with sediment by the same basic processes that occur in wetlands. During normal river flow, relatively small quantities of suspended sediment are transported down the river channels and deposited in the channel or carried out to sea. When the river floods, it picks up large quantities of suspended sediments and can overflow its banks and inundate the flat land (flood plain) around it where it is slowed and deposits some of this suspended sediment including fine-grained, organic-rich particles. Deltas form when the river valley is filled by sediments at its lower end so that the river’s flood plain extends to the ocean. As the flood plain is created, the river cuts a narrower channel, or channels, and its speed increases accordingly. The faster-moving river then transports larger particles to the oceans where, since the current speed decreases, larger grained suspended sediments are deposited to form the base of the delta. The delta is built seaward as larger grained sediment is deposited along the edges of the narrow river mouth where river speed decreases as it enters the ocean. Once delta channels are formed, flooding deposits finer-grained sediment to form wetlands across the delta. Sediment distribution across the delta is aided by the river occasionally abandoning of channels as they fill up with sediment and creating new ones (Fig. 11-5). In this way, surface soils of the entire delta are periodically enriched by the addition of organic matter and nutrients from the river.

In some deltas, the deposition rate has been high enough that large quantities of organic-rich muds have been continuously buried by newly deposited sediments over long periods of geological time. These buried sediments are compacted by the overlying layers, water is squeezed out, and in many such areas, oil and gas deposits have been formed as the organic matter has decomposed. Many oil and gas deposits are located in deltas or offshore in areas where deltas existed when sea level was lower. Major deltas of the world include the Mississippi delta, the San Francisco Bay Delta, the Copper River Delta in Alaska, the Mackenzie Delta in northwest Canada, the Nile Delta in Egypt, the Niger Delta in Nigeria, and the Ganges–Brahmaputra Delta in Bangladesh and India.

Muddy delta soils are slowly compacted as water is squeezed out of them by the weight of overlying sediment. This process can be accelerated by the weight of structures built on the delta. In addition, the organic matter in the delta soils decomposes if it is not buried by additional river-borne sediments, and the soils themselves are further eroded by winds and rainfall. Therefore, periodic influxes of new sediment are needed to maintain the delta land surface slightly above sea level.

Because they are particularly rich in nutrients and organic matter, delta soils are among the best for agriculture. Consequently, many deltas, such as the Mississippi Delta and Nile Delta, are populated and intensively farmed. Unfortunately, deltas are also very easily flooded. Almost the entire population of Bangladesh lives on a delta that is regularly inundated by swollen rivers and hurricane storm surges (Chap. 9), often with great loss of life.

To control the frequent flooding of two major deltas in the U.S.—the Mississippi Delta and the San Francisco Bay Delta—extensive engineering projects have been carried out in which banks called levees have been built along each side of the river channels. Levees have worked extremely well by some measures because they have dramatically reduced flooding, but they also have unintended consequences. Without the periodic influx of nutrient-rich suspended sediment to the delta during floods, the fertility of the soils steadily declines. In addition, without the periodic inputs of new soil, the land slowly sinks as soil compacts and erodes. Sinking of the land surface is actually accelerated by agriculture through increased soil erosion and decomposition of soil organic matter. Worse still, withdrawal of freshwater from aquifers beneath the delta further accelerates sinking, particularly in the Mississippi Delta.

As the land sinks in the Mississippi and San Francisco Bay Deltas, more and higher levees are needed to prevent flooding. In the San Francisco Bay Delta, most of the land is now meters below the normal river water level. Any breach of the levee floods the entire area, even when the river is not swollen by flood rainwater. For example, a 100-m-wide breach occurred in a levee in the San Francisco Bay Delta in the summer of 2004. The breach flooded nearly 50 km² of farmland, destroying crops, homes, and other structures. It took about 200,000 tonnes of rock to seal the levee breach. The total cost of the crops and structures destroyed, repairing the levee, and pumping water out of the flooded land was nearly $100 million. Significant as this event was, it was just a hint of the devastation, chaos, and loss of life and property that was caused by the breaks in the protective levees of New Orleans that flooded most of that city to a depth of 10 m or more in 2005 following the passage of Hurricane Katrina.

Higher and stronger levees could be built, but they would be very expensive and difficult to construct because they would need to be capable of restraining increasing hydrostatic pressure as the delta floor sinks and sea level rises. In addition, levees constrain the river to flow very swiftly through a narrow channel, so erosion of existing levees is a major problem and would remain so even if levees were enhanced.

What is the future of the Mississippi and San Francisco Bay Deltas, particularly if global climate change causes sea level to rise more quickly? If we continue on our present course, these delta lands will continue to sink, and levees will continue to be built higher until holding back the water becomes impossible. Meanwhile, soils will become impoverished and the land’s agricultural value will decline. One alternative is to tear down the levees. Within a few years or decades, the deltas would return to their normal functioning, and valuable agricultural lands would again be available for the future. If we are to retain the highly productive deltas, the price we must pay is periodic flooding.

In the Mississippi Delta and the San Francisco Bay Delta, restoration programs have begun to establish breaches in the levees to replenish delta sediments in key areas, converting these areas back to wetlands. So far, on the Mississippi Delta, the program has focused mainly on areas that had become shallow coastal lagoons as a result of the levee diversions of sediments. Strong opposition to the program from fishers who used the lagoons to collect oysters had to be overcome. Even greater opposition is, and will continue to be, encountered when the proposed replenishments involve land that has been drained for agriculture. It remains to be seen whether these replenishment programs will ultimately succeed and can be applied to other deltas.