Wetlands in the Sahel: 12. a summary The wetlands in the Sahel have much in common, but they also by September. The Senegal Delta also has a short hinterland. differ in several important respects (Chapters 6-11). The differences Discharge of the Niger River normally reaches its maximum in mainly depend on flood dynamics and whether or not natural November, but peak flow is one month earlier with a low discharge, resources are exploited by local people. The timing and extent and two months later with a high inflow. Moreover, the Inner Niger of flooding have been substantially altered by the creation of Delta is so large that when the water in the southern part has already reservoirs in rivers responsible for the flooding of the Senegal Delta, receded, the northern delta still has to be flooded. Hadejia-Nguru, Logone and Inner Niger Delta (Fig. 102). Flooding and human exploitation have an impact on vegetation and fauna, Floodplain size, permanent and seasonal The surface area of the including birds. immersed floodplains is six to ten times as large as the surface area permanently covered by water (Fig. 103D), except for Lake Floodplain size The seven wetlands described in the previous chapters Chad and the Sudd, where the difference is much smaller. In the nicely illustrate the various fortunes of flooding, vegetation and bird Sudd, more than half of the wetlands are permanently flooded, life in the past decades (Fig. 103). A comparison of maximal flood independent of flood extent. In Lake Chad, the surface area of the extent (Fig. 103A) in the 1960s and 2000s shows that the Sudd became seasonally flooded fringe of the lake increased from 2000 to 5000 the largest wetland in Africa, after some years with an extremely high km2 after 1973, due to a drop of the lake’s level. Dikes and dams in discharge of the White Nile in the mid-1960s (when flood extent the Senegal River changed a large part of the Senegal Delta from an more than tripled in just four years). It took over 30 years before the estuarine floodplain into a lake and, beyond the dikes, into drylands Sudd was back again at its pre-1960s size of 10 000 km2. Lake Chad, or irrigated areas. The presence of permanent lakes in the Inner on the other hand, measured about 25 000 km2 before 1973, but has Niger Delta depends on flood level: with low floods the lakes in the been about twice as small since then. The flood extents of the Inner northern half of the area are not reached and dry out, but the lakes Niger Delta, Hadejia-Nguru, Logone and Fitri showed large annual are refilled during high floods. fluctuations in response to variations in rainfall and river flow, but – with the exception of Lake Fitri – were also permanently reduced in size due to the construction of hydrological infrastructures IORRGSODLQ²²!GU\ODQG upstream. The annual flood extent in the Senegal Delta varied even VHDVRQDOIORRGSODLQ IORRGSODLQ²²!SHUPDQHQWZHWODQG 2 more than in the other floodplains: between 500 and 3500 km before SHUPDQHQWZHWODQG the mid-1960s (still a natural river system) and between 250 and 500 km2 since 1990 (dikes and dams have gradually reduced floodplain size). Without the artificial flooding of the National Parks Djoudj and Diawling, flood extent would have been reduced by another 200 km2. All together, the Sahelian floodplains and lakes lost more than half of their surface area between the 1960s and 2000s, mainly due to reduced rainfall, but also exacerbated by the construction of IORRGHGDUHD dams and dikes. Floodplain loss in the Waza-Logone, resulting from the implementation of an irrigation scheme, was partly rectified by 2 ZLWKRXWUHVHUYRLU reopening the dike, which enabled reflooding of some 200 km . ZLWKUHVHUYRLU - $ 6 21' - )0$ 0- Water column The depth of maximum flooding is highest in the Inner Niger Delta, varying from 4-6 m. The Hadejia-Nguru held up to 3.5 m of water before upstream reservoirs started to reduce floodplain Fig. 102 Upstream reservoirs have an impact on floodplains in several size. The taming of the Senegal River substantially reduced the ways. When the reservoir is filled at the start of the rainy season, it annual fluctuation of water level in the Senegal Delta, from 3.5 m to retards flooding (1) and lowers the peak level (3). This also reduces the 0.5 m. In contrast, the seasonal variation in water level trebled in the flood extent (5). Some months later, when the reservoir is emptied, the southern basin of Lake Chad, but the northern basin fell dry. The low water level is raised (4), causing an extension of the permanent water level varied less in Fitri (up to 150 cm), Logone (100 cm) and marshes (6). These man-induced changes turn the higher floodplains the Sudd (50 cm). (Fig. 103B). into drylands (5) and the lower floodplains into permanent marshlands (6); the remaining seasonal floodplains contain less water for a shorter Seasonality of flooding The rainy period for all wetlands falls between period (2). The irregular water releases from the reservoir cause erratic June and October, but the timing of flooding and deflooding differs short-term fluctuations in water level, unlike the normally gradual (Fig. 103C). The rivers feeding the Hadejia-Nguru system are short daily increase and decrease of water level associated with unhampered with a steep gradient, and peak flooding is therefore already reached flooding and deflooding. WWeetlaannds in the Saahel: a summary 181.