6.Inner Niger Delta

Floodplains everywhere are disappearing like snow in the sun. Dams, irrigation schemes and changes in land use wreak havoc. Even as large a fl oodplain as the Inner Niger Delta, at fi rst sight pristine, is threatened by ever more dams in the Niger River upstream and downstream of the Inner Niger Delta. Dams interfere with the fl ood pulse of a delicate ecosystem. The evocative term “fl ood pulse” was coined by Junket al. (1989), to describe the seasonal fl ooding of the aquatic-terrestrial transition zone along rivers. The fl ood pulse is like a wave rolling over a saucer, magically transforming arid land into a huge but temporary wetland, a wonder to behold and a source of leg- ends and myths over the centuries. In the Inner Niger Delta, fl ood height may reach six metres, slowly engulfi ng an area of 400 by 100 km. Fortuitously, the seasonal rise and retreat of the fl ood in this area has been measured daily at several hydrological stations over many decades, producing a time series of great value, as we will show in this book. Usually, lower-level fl oods cover fl oodplains for four months only (October-February), but high fl oods inundate them for twice as long (September-April). Floodplains are highly dynamic wetlands that attract large numbers of waterbirds, and people as well. The Inner Niger Delta is no exception.

60 Living on the edge Introduction (as described in a separate Section) largely depend on its natural resources. They have reshaped the floodplains into a semi-natural The Inner Niger Delta is one of the largest floodplains in Africa. The landscape. In the remaining Sections we analyse bird counts to out- topographical maps of the Institut Géographique National (ign) re- line the significance of the area for a number of bird species. We veal that the inundation area measures 36 470 km2, including 5340 used aerial counts of the entire Inner Niger Delta, ground surveys of km2 of levees, dunes and other islands within that area. They also the central lakes in the Inner Niger Delta and counts of birds flying show that water coverage declines from 31 130 km2 in wet periods to nocturnal roosts. We plotted density counts in different habitat to 3840 km2 in dry periods (Fig. 36). The entire floodplain area is in- types on a vegetation map and correlated the result with the digital cluded in the 41 195 km2 designated as a Ramsar Wetland Site of flooding model to estimate the total number of birds present in the International Importance in January 2004. This chapter deals with Inner Niger Delta. the ornithological significance of the area, but first explores hydro- logical and ecological information to provide a background for un- derstanding the ecosystem. River flow and flooding of the Inner Niger Delta We will first describe the relationship between river flow and flooding, using satellite images to quantify the surface of the inun- The Inner Niger Delta is awe-inspiring in size and dynamics. Start- dation zone at different water levels. A simple equation accurately ing in July, the water rises about 4 m in 100 days. In years of high riv- predicts the size of the floodplains as a function of river flow. Be- er discharge, peak level may be 6 m higher than a few months previ- cause water extraction through dams and irrigation is known, it is ously (Fig. 37). Large between-year differences in flooding make the also possible to indicate the impact these infrastructures have on system even more dynamic. the flooding cycle. Next we deal with the ecological consequences of Topographical maps show the floodplain as if it were flooded at a variations in flooding. Vegetation types are clearly associated with maximum level (Fig. 36). However, the area actually inundated var- flood duration and height. We used a digital flooding model to cal- ies considerably between years. Zwarts & Grigoras (2005) used satel- culate the surface area of sets of optimal water depths for the vari- lite images to produce a continuum of 24 water maps of the Inner ous vegetation types. At a lower flood level, plants have to colonise Niger Delta, covering the range of water levels between -2 and +511 lower-lying zones which will provide optimal growing conditions. cm, as measured on the gauge at Akka in the central lakes. The maps Thus, reduced flooding affects the coverage of different vegeta- allow the determination of the relationship between water level and tion types. At least, that is to be expected in a fully natural system. the area inundated. Water maps were combined to construct a dig- However, the Inner Delta is inhabited by one million people, who ital flooding model. This was done separately for rising and reced-

er Fig. 36 The floodplains (light blue) and Nig Timbuktu permanent water bodies (dark blue) of the Inner Niger Delta, as indicated on the topographical Lake Télé maps of the Institut Géographique National. The maps are from 1956, and based on aerial Lake Horo photographs and field work in the early 1950s, a period with very high floods.

Akka Lake Debo Lake Walado Lake Korientzé

a k ia D Mopti

r Ke-Macina Nige

Djenné

Bani

50 km

Inner Niger Delta 61