Mauritania has four ecological zones: the Saharan Zone, the Sahelian Zone, the Senegal River Valley, and the Coastal Zone. Although the zones are markedly different from one another, no natural features clearly delineate the boundaries between them. Sand, varying in color and composition, covers 40 percent of the surface of the country, forming dunes that appear in all zones except the Senegal River Valley. Fixed sand dunes are composed of coarse, fawn-colored sand, while shifting ("mobile") dunes consist of fine, dustlike, reddish-colored sands that can be carried by the wind. Plateaus generally are covered with heavier blue, gray, and black sands that form a crusty surface over layers of soft, loose sand.

Saharan Zone

The Saharan Zone makes up the northern two-thirds of the country. Its southern boundary corresponds to the isohyet (a line on the earth's surface along which the rainfall is the same) that represents annual precipitation of 150 millimeters. Rain usually falls during the hivernage, which lasts from July to September. Often, isolated storms drop large amounts of water in short periods of time. A year, or even several years, may pass without any rain in some locations.

Diurnal variations in temperature in the Saharan Zone may be extreme, although annual variations are minimal. During December and January, temperatures range from an early morning low of 0°C to a midafternoon high of 38°C. During May, June, and July, temperatures range from 16°C in the morning to more than 49°C by afternoon. Throughout the year, the harmattan often causes blinding sandstorms.

The administrative regions (formerly called cercles) of Tiris Zemmour in the north, Adrar in the center, and northern Hodh ech Chargui in the east, which make up most of the Saharan Zone, are vast empty stretches of dunes alternating with granite outcroppings. After a rain, or in the presence of a well, these outcroppings may support vegetation. In the populated Adrar and Tagant plateaus, springs and wells provide water for pasturage and some agriculture. In the western portion of the Saharan Zone, extending toward Nouakchott, rows of sand dunes are aligned from northeast to southwest in ridges from two to twenty kilometers wide. Between these ridges are depressions filled with limestone and clayey sand capable of supporting vegetation after a rain. Dunes in the far north shift with the wind more than those in the south.

The Saharan Zone has little vegetation. Some mountainous areas with a water source support small-leafed and spiny plants and scrub grasses suitable for camels. Because seeds of desert plants can remain dormant for many years, dunes often sprout sparse vegetation after a rain. In depressions between dunes, where the water is nearer the surface, some flora--including acacias, soapberry trees, capers, and swallowwort--may be found. Saline areas have a particular kind of vegetation, mainly chenopods, which are adapted to high salt concentrations in the soil. Cultivation is limited to oases, where date palms are used to shade other crops from the sun.

Sahelian Zone

The Sahelian Zone extends south of the Saharan Zone to within approximately thirty kilometers of the Senegal River. It forms an east-west belt with its axis running from Boutilimit through 'Ayoûn el 'Atroûs to Néma, made up of steppes and savanna grasslands. Herds of cattle, sheep, and goats move across this zone in search of pasturage.

The hivernage begins earlier in the Sahelian Zone than in the Saharan Zone, often lasting from June until October. Because farmers and herders depend on annual rains, a delay of one month in the beginning of the rainy season can cause large losses and lead to mass migrations from Hodh ech Chargui and Hodh el Gharbi into Mali. Although temperature extremes are narrower than in the Saharan Zone, daily variations range from 16°C to 21°C. The harmattan is the prevailing wind.

In the northern Sahel, dunes are covered with scrub grasses and spiny acacia trees. Farther south, greater rainfall permits more dense vegetation. Sands begin to give way to clay. Large date palm plantations are found on the Tagant Plateau, and savanna grasses, brushwood, balsam, and spurge cover fixed dunes. Occasional baobab trees dot the flat savanna grasslands of the southern Sahel. Forest areas contain palm trees and baobabs. Vast forests of gum-bearing acacia grow in Trarza and Brakna regions. Farther south, particularly in Assâba and the northern portion of Guidimaka regions, rainfall is high enough to support forms of sedentary agriculture.

Senegal River Valley

The Senegal River Valley, sometimes known as the Chemama or the pre-Sahel, is a narrow belt of land that extends north of the Senegal River. Before the droughts of the 1960s, 1970s, and 1980s, the belt ranged from sixteen to thirty kilometers north of the river. By the late 1980s, desertification had reached the northern bank of the river in some parts of the valley. The valley is wider in Guidimaka Region and is completely dominated by the seasonal cycle of the river. Almost all of the valley's economically active population engages in sedentary agriculture or fishing along the Senegal River and its main tributaries--the Karakoro, the Gorgol, and the Garfa. This area supplies most of the country's agricultural production.

The climate of the Senegal River Valley contrasts with that of the Saharan and Sahelian zones. Rainfall is higher than in other regions, ranging from 400 millimeters to 600 millimeters annually, usually between May and September. This rainfall, combined with annual flooding of the river, provides the basis for agriculture. Temperatures are cooler and subject to less annual and diurnal variation than in other regions.

The Senegal is the only permanent river between southern Morocco and central Senegal. From its source in Guinea, it flows north and west 2,500 kilometers, reaching the Atlantic Ocean at Saint Louis, Senegal. From its mouth, the river is navigable as far as Kayes, Mali, during the rainy season and Podor, Senegal, during the rest of the year. Heavy rains, beginning in April in Guinea and May and June in Senegal and Mali, bring annual floods. These floods cover the entire valley up to a width of twenty-five to thirty-five kilometers, filling numerous lakes and sloughs (marigots) that empty back into the river during the dry season. When the waters recede from the bottomlands, planting begins.

The Senegal River Valley, with its rich alluvial and clayey soil, is comparatively abundant in flora. Moreover, higher rainfall, irrigation, and abundant side channels and sloughs tend to produce a lush, near-tropical vegetation, with baobab and gonakie trees and abundant rich grasses. Ddounm and barussus palms are also found here. Much of the flood plain is cultivated.

Coastal Zone

The Coastal Zone, or Sub-Canarian Zone, extends the length of the approximately 754-kilometer-long Atlantic coast. Prevailing oceanic trade winds from the Canary Islands modify the influence of the harmattan, producing a humid but temperate climate. Rainfall here is minimal; in Nouadhibou it averages less than three centimeters annually and occurs between July and September. Temperatures are moderate, varying from mean maximums of 28°C and 32°C for Nouadhibou and Nouakchott, respectively, to mean minimums of 16°C and 19°C.

Battering surf and shifting sand banks characterize the entire length of the shoreline. The Ras Nouadhibou (formerly Cap Blanc) peninsula, which forms Dakhlet Nouadhibou (formerly Lévrier Bay) to the east, is fifty kilometers long and up to thirteen kilometers wide. The peninsula is administratively divided between Western Sahara and Mauritania, with the Mauritanian port and railhead of Nouadhibou located on the eastern shore. Dakhlet Nouadhibou, one of the largest natural harbors on the west coast of Africa, is fortythree kilometers long and thirty-two kilometers wide at its broadest point. Fifty kilometers southeast of Ras Nouadhibou is Arguin. In 1455 the first Portuguese installation south of Cape Bojador (in the present-day Western Sahara) was established at Arguin. Farther south is the coastline's only significant promontory, seven-meter-high Cape Timiris. From this cape to the marshy area around the mouth of the Senegal River, the coast is regular and marked only by an occasional high dune.

On coastal dunes vegetation is rare. At the foot of ridges, however, large tamarisk bushes, dwarf acacias, and swallowworts may be found. Some high grass, mixed with balsam, spurge, and spiny shrubs, grows in the central region. The north has little vegetation.

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Source: U.S. Library of Congress