The runoff washes fine clay particles and organic matter into the wetland depressions, which makes the wetland soils more productive than the surrounding sandy soils. A closer view of the Gâat Mahmoudé, the largest wetland in eastern Mauritania, illustrates this dynamic (see adjacent insets). Because they rely on annual precipitation, the duration, depth, and size of these wetlands vary widely from year to year. On average, this region receives 280 mm of rainfall per year (Shahin, 2007). These wetlands are fed by rainfall and runoff from their catchment areas and reach their maximum water level after the rainy season in October or November. Gâats are open wetlands present in a more shallow topography and typically cultivated during the dry season (Shine, 2011). Tamourts are contained in deeper depressions, characterized by stands of Acacia nilotica, and have the longest mean water duration. Using local terms, Mauritanian wetlands include tamourts (forested closed basins), gâats (herbaceous closed basins or flats) and oueds (seasonal watercourses). In the middle of the sandy zones covered by Sahelian short grass savanna, localized eroded areas around boreholes and villages appear as bright aureoles - they are especially visible on the 2013 image. These wetlands stand out in imagery from the nearby strips of sandy and bare soils.

Landsat images of the department of Néma in eastern Mauritania from 19 show many wetlands that occupy channels and depressions. However, because rainfall varies so much in eastern Mauritania, the water levels and vegetation in ephemeral wetlands fluctuate from year to year, and can even change significantly within a single year. As islands of resources, the ephemeral wetlands of eastern Mauritania fulfill key ecological and economic functions. In the arid northern Sahel, where people, plants, and animals all live on the edge of what is ecologically sustainable, wetlands stand out from their surroundings by the rich and diverse flora and fauna they support.

Ephemeral wetlands