Page 513 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 15 River Systems 477
(a)
(b)
has been building for at least 500 years and is a classic example of a bird’s-foot delta—a long channel with many distributaries, and sediments carried beyond the tip of the delta into the Gulf of Mexico (Figure 15.31).
The history of the Mississippi River delta shows a dynamic system with inputs and outputs of sedi- ment and shifting distributaries. The 3.25-million-km2
▲Figure 15.27 Mackenzie River delta. (a) More than 25 000 small lakes dot the river’s marshy delta. Distributaries lace through the delta carrying a heavy sediment load to the Beaufort Sea, the visible plume interacting with currents and sea ice. The delta is approximately 200 km in length and 60 km wide. (b) The surface
of the delta is a complex maze of lakes, wetlands, and sediment- laden distributary channels. Sediment settles after being deposited outside the channels during floods or avulsions. [(a) nASA/gSFC.
(b) J.A. Kraulis/All Canada Photos/getty Images.]
Mississippi drainage basin produces 550 million met- ric tons of sediment annually—enough to extend the Louisiana coast 90 m a year. However, several factors are causing losses to the area of the delta each year.
Compaction and the tremendous weight of the sedi- ment in the Mississippi River create isostatic adjust- ments in Earth’s crust, and cause the entire delta region to subside. In the past, subsidence was balanced by additions of sediment that caused areal growth of the delta. With the onset of human activities upstream, the supply of alluvial sediment has decreased. The delta is now subsiding without sediment replenishment.
Georeport 15.1 The Disappearing nile River Delta
Over several centuries, more than 9000 km of canals were built in the nile River delta in northern Egypt to augment the natural river distributary system carrying water and sediment to the sea. As river discharge enters the network of canals,
flow velocity is reduced, stream competence and capacity are decreased, and sediment load is deposited far short of where the delta touches the Mediterranean Sea. In 1964, the completion of the Aswân High Dam blocked sediment movement downstream, decreas- ing the supply of sediment to the delta. As a result, the delta coastline is actively receding at an alarming 50 to 100 m per year. Sea- water is intruding farther inland into both surface water and groundwater. The delta, which provides the fertile soils that produce
60% of the country’s food, is also threatened by rising sea level; a 1-m rise, which experts predict is likely during the next 100 years, would inundate about 20% of the delta.