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Large dams and the degradation of the Lower Zambezi Valley

Africa: > Large dams and the degradation of the Lower Zambezi Valley

Large dams and the degradation of the Zambezi Valley and Delta
The Zambezi River system is the lifeline of central Mozambique, ancient home to more than a million villagers and of immense conservation value as one of the most productive and biologically diverse tropical floodplains in Africa. Over the millennia, the valley floodplains were nourished by the annual spread of Zambezi floodwaters. The fertile floodplains provided recession agriculture, hunting, fishing, and abundant natural resources for its inhabitants. The Zambezi Delta’s vast, seasonally flooded grasslands supported diverse and abundant wildlife populations, including African elephant, Cape buffalo, and waterbuck, and numerous threatened and endangered species, including the Wattled Crane. The healthy floodplain provided spawning grounds for riverine and anadromous fishes and critical dry-season grazing lands for livestock and wildlife. Extensive coastal mangroves and estuaries supported a productive prawn fishery. Because of its rich biological diversity and national economic importance, the Zambezi Delta is a candidate for designation as a Wetland of International Importance under the Ramsar Convention.

Over the past century, however, the communities and ecosystems of the lower Zambezi have been severely affected by the management of large upstream dams and other water resource development works. By eliminating natural flooding and greatly increasing dry season flows in the lower Zambezi, Kariba Dam (completed in 1959) and especially Cahora Bassa Dam (completed in 1974) cause great hardship for hundreds of thousands of Mozambican villagers whose livelihoods depend on the ebb and flow of the Zambezi River. Although these hydropower dams generate important revenues for Zimbabwe and Zambia, and Mozambique, respectively, they are operated to maximize hydropower output at the expense of other water users. Subsistence fishing, farming, and livestock grazing activities have collapsed with the loss of the annual flood. The productivity of the prawn fishery has declined by $10 – 20 million per year — this in a country that ranks as one of the world’s poorest nations. Changes in the flooding regime have affected the availability of water supplies, fuel wood, building materials, and medicinal plants, as well as general public health and the cultural relationship between local people and the river.

The construction of large dams on the Zambezi River has also greatly diminished the diversity and productivity of the Zambezi Delta, one of the great floodplain systems of Africa. Before Cahora Bassa Dam was constructed, Bryan Davies, Ken Tinley, and others predicted that the hydrological changes imposed by the dam would result in reduced silt deposition and nutrient availability, salt water intrusion, replacement of wetland vegetation by upland species, failure of vegetation to recover from grazing, and disrupted or mistimed reproductive patterns for wildlife species in the delta. Only ten years after the construction of Cahora Bassa Dam, a team of United Nations ecologists observed,

"It is clear that in the case of Cahora Bassa there was no serious attempt to ecologically optimize the dam prior to construction…furthermore, after dam closure, proposals put forward by the ecological assessment team were not implemented and there has been no regular monitoring of the dam’s downstream effects during its lifespan. As a result, Cahora Bassa has the dubious distinction of being the least studied and possibly least environmentally acceptable major dam project in Africa."

The delta today is much drier at the end of the dry season than under natural conditions, with a reduction in wetland and open water areas, infestation of stagnant waterways with exotic vegetation, and intrusion of saltwater. Wetland vegetation communities are being replaced by upland communities, and no longer support the web of floodplain life that previously existed. There is widespread encroachment of woody savanna species onto the open floodplain. The desiccation of the floodplain has opened the area to aggressive poaching of wildlife species, with a 95% or greater reduction in grazing species such as Cape buffalo, waterbuck, reedbuck, zebra, and hippopotamus between 1979 and 1995. Grassland fires are widespread across the dry plains, degrading fire-sensitive communities. Globally endangered Wattled Cranes, an umbrella species for many of the flood-dependent waterbird species of the Zambezi system, have ceased to breed across most of the delta. The delta today is much drier at the end of the dry season than under natural conditions, with a reduction in wetland and open water areas, infestation of stagnant waterways with exotic vegetation, and intrusion of saltwater. Wetland vegetation communities are being replaced by upland communities, and no longer support the web of floodplain life that previously existed. There is widespread encroachment of woody savanna species onto the open floodplain. The desiccation of the floodplain has opened the area to aggressive poaching of wildlife species, with a 95% or greater reduction in grazing species such as Cape buffalo, waterbuck, reedbuck, zebra, and hippopotamus between 1979 and 1995. Grassland fires are widespread across the dry plains, degrading fire-sensitive communities. Globally endangered Wattled Cranes, an umbrella species for many of the flood-dependent waterbird species of the Zambezi system, have ceased to breed across most of the delta.

Although the delta is rapidly losing grond, it was recently designated as a Wetland of International Importance under the Ramsar Convention because of its immense value for wildlife and its national economic importance. This new global attention for the delta provides a vital window of opportunity for advancing sustainable management not only of the delta itself but also of the Zambezi waters that nourish and maintain it. A major aim of this program is therefore to reverse decades of hydrological degradation in the lower Zambezi Valley and Delta by promoting integrated river basin management throughout the Zambezi River system for the benefit of people and wildlife in Mozambique.

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