ISS012-E-20456
The four main environments typical of salt (alkali) lakes in East Africa are illustrated well in the image. The delta of one of two dominant streams that flow into the lake (top left) shows where fresh water enters the basin. Open water, salt flats with salt crusts, and mud flats succeed each other in a west-to-east progression towards the shallow side of the lake. Vast numbers of the pink Lesser Flamingo (2.5 million by one calculation) rely on Lake Natron as their only breeding ground in the Rift Valley. The flamingoes feed on the nutrient-rich cyanobacteria. As salinity increases, so do the number of cyanobacteria, and the lake can support more nests. Threats to the salinity balance from increased fresh water influxes will come from projected logging in Natron watersheds and a planned hydroelectric power plant. Although development plans include construction of a dike at the north end of the lake to contain the fresh water, the threat of dilution to this breeding ground may still be serious. The government of Tanzania recognized both the threat and the uniqueness of the habitat in 2001, when it placed Lake Natron on the list of Wetlands of International Importance as part of the Ramsar Convention, an intergovernmental treaty meant to protect wetlands.
Images of Lake Natron from four other dates can be seen on the Earth Observatory and associated links. The area and shape of the open water and salt-raft zones depend on lake levels, controlled mainly by local rainfall and evaporation. Patterns of these sub-environments therefore appear different across the span of a few years.
ISS012-E-18774
This astronaut photograph of the southwestern edge of the Zagros mountain belt includes another common feature of the region—a salt dome (Kuh-e-Namak or “mountain of salt” in Farsi). Thick layers of minerals such as halite (common table salt) typically accumulate in closed basins during alternating wet and dry climatic conditions. Over geologic time, these layers of salt are buried under younger layers of rock. The pressure from overlying rock layers causes the lower-density salt to flow upwards, bending the overlying rock layers and creating a dome-like structure. Erosion has spectacularly revealed the uplifted tan and brown rock layers surrounding the white Kuh-e-Namak to the northwest and southeast (center of image). Radial drainage patterns indicate another salt dome is located to the southwest (image left center). If the rising plug of salt (called a salt diapir) breaches the surface, it can become a flowing salt glacier. Salt domes are an important target for oil exploration, as the impermeable salt frequently traps petroleum beneath other rock layers.
ISS012-E-6468
These photographs were taken in November 2005 (whole lake) and March 2006 (detail) by the Expedition 12 crew of the International Space Station. In November, water levels had dropped, exposing large tracts of salt and mud flats. A wet and cool period between December 2005 and the end of February 2006 resulted in flooding of Poopó with muddy waters from the Desaguadero River. The area of the March 9 photograph is indicated on the November 3 mosaic by a white polygon. Comparison of the photographs shows the extent of flooding of the western salt flats—sufficient to create an ephemeral island. The ISS crew is tasked to track such changes, which are related to regional weather patterns. Lake Poopó’s sensitivity to precipitation in the high Andes (possibly reflecting larger climate cycles) provides an excellent visual indicator of weather and climate trends.
ISS012-E-23057
Two images taken about nine months apart document an unusually wet summer in southern Africa. The upper view (March 2006) shows the point where the Oshigambo River flows into the salt lake; the lower regional image (June 2005) shows the same inlet—but dry—on the north shore of Etosha Pan. The Oshigambo River is almost never seen with water, but in early 2006, rainfall twice the average amount in the river’s catchment generated flow. Greens and browns show vegetation and algae growing in different depths of water where the river enters the dry lake (upper image, center). Typically, little river water or sediment reaches the dry lake because water seeps into the riverbed along its 250-kilometer (55-mile) course, reducing discharge along the way. In this image, there was enough surface flow to reach the Etosha Pan, but too little water reached the mouth of the river to flow beyond the inlet bay. The unusual levels of precipitation also filled several small, usually dry lakes to the north (upper image, right).
ISS012-E-21250
Separated from the smog mass by a band of puffy, white cumulus clouds is a light brown plume of dust. The line of white clouds has developed along the steep slope that separates the heavily populated North China Plain—the location of the largest population concentration on Earth—and the sparsely populated semi-desert plains of Inner Mongolia. Most Northern Hemisphere deserts saw dust storms in the spring of 2006, and the Gobi and Taklimakan Deserts of western China were no exception. Dust plumes originating in these deserts typically extend hundreds of kilometers eastward, regularly depositing dust on Beijing, the Korean Peninsula, and Japan. Some plumes even extend over the Pacific Ocean. In extreme cases, visible masses of Gobi-derived dust have reached North America.
An astronaut handheld-camera image taken in 1996 shows a broad corridor of smog moving off the mainland out into the Pacific Ocean from China’s more southerly population center near Taiwan.
