Over the millennia, life in Southern Africa was measured by the ebb and flow of the great Zambezi River. Every year the river’s waters spilled over into its vast floodplains, irrigating subsistence crops, rejuvenating vital grasslands for wildlife and livestock, depositing nutrient–rich sediments in coastal mangroves, and triggering the lifecycles of countless species of plants and animals. Low dry–season flows sustained the productivity of coastal prawn fisheries and enabled the people of the river’s basin to harvest riverine fishes.
For the past 40 years, however, the pulse of the Zambezi’s ancient flood cycle has been harnessed by the colossal Kariba and Cahora Bassa dams. Built with the promise that they would stimulate regional economic growth through hydropower production, these developments have come at great cost to the marginalized people and wildlife of the Zambezi basin. Nowhere has this hardship been more pronounced than in the lower Zambezi valley of Mozambique. For twenty–five years, erratic and mistimed flooding below Cahora Bassa Dam has adversely affected the living standards of hundreds of thousands of downstream households and decimated one of the most productive and diverse wetland ecosystems in Africa, the Zambezi Delta.
While Kariba was built before the impacts of dams were well–understood, Cahora Bassa has no such excuse – and in fact appears not to have incorporated many lessons from Kariba. The degree to which scientists’ concerns were ignored in the planning of the Cahora Bassa project is staggering. Before the dam was completed in 1975, South African river ecologist Dr. Bryan Davies warned of the dam’s severe consequences in a pre–project assessment: "Reduced artisanal fisheries and shrimp industry productivity, reduced silt deposition and nutrient availability, severe coastal erosion, soil salinization, salt water intrusion, replacement of wetland vegetation by invasive upland species, reduction in coastal mangroves, failure of vegetation to recover from grazing, and disrupted or mistimed reproductive patterns for wildlife species." Just ten years later, deleterious changes to the Zambezi’s riverine, wetland, deltaic and coastal ecosystems were already apparent. Fisheries ecologists G. Bernacsek and S. Lopez lamented in a UN document, "It is clear that in the case of Cahora Bassa there was no serious attempt to ecologically optimize the dam prior to construction ... Cahora Bassa has the dubious distinction of being the least studied and possibly least environmentally acceptable major dam project in Africa."
Now, a quarter century after the dam’s completion, the impacts predicted years ago have sadly come true. Except in years of exceptional local rainfall, the lower Zambezi River no longer overflows its banks at the time of natural peak flooding. Most of the delta’s distributary channels are choked with invasive vegetation and no longer convey floodwaters to the desiccated floodplains. Anthropologist Dr. Ted Scudder notes that "villagers correctly attribute the loss of crucial land for grazing and flood recession agriculture, and a drop in the productivity to their fishery, to dam construction."
Due to the lack of the natural seasonal variations in flow, the once lucrative delta prawn fishery has declined precipitously, and only one of the main channels of the Zambezi Delta mouth supports relatively healthy mangrove. According to Davies, "there are now large gaps in the mangrove forest along the entire northern and southern sectors that didn’t exist prior to 1975, and areas of coastal erosion with dead mangroves in evidence." Populations of Cape buffalo, waterbuck, reedbuck, zebra and hippo have declined by 95 percent or more as the now–dry floodplain has opened the area to commercial poaching. Grassland burning has intensified throughout the dry season, and more than 90 percent of the lower Zambezi floodplain now burns every year. Research by Mozambican ornithologist Carlos Bento suggests that the breeding cycles of many delta species, including the endangered wattled crane, have been disrupted by the irregular flooding patterns below Cahora Bassa Dam.
The devastation of the lower Zambezi seems all the more tragic in view of the fact that power lines were sabotaged shortly after the dam was completed. For more than 17 years, all but one of Cahora Bassa’s turbines remained idle while Mozambique’s civil war raged. Only in the past year has power production been restored to its original capacity.
Restoring the Flood
In 1995, a broad group of interested social and environmental scientists from Mozambique, South Africa, and North America joined together to promote the recovery of the lower Zambezi system through improved management of Cahora Bassa Dam. Through a series of comprehensive studies, this group began to assess the social, economic, and environmental advantages and disadvantages, for a broad range of stakeholders, of different strategies for managing Zambezi waters. These experts hoped to demonstrate that the "best" use of Zambezi waters requires the restoration of natural flooding patterns of the river through prescribed releases from Cahora Bassa Dam.
The benefits of prescribed flooding for downstream communities and ecosystems has gained worldwide attention in recent years. In the western United States, artificial flood releases from large dams are being tested to maintain instream flow requirements for riverine habitats, salmon fisheries, and recreational demands. In Africa, artificial flood releases below large dams are gaining acceptance as a means of promoting integrated rural development. In Nigeria, for example, artificial flood releases from the Tiga and Challawa Gorge Dams have been promising and could be used further to help restore the Komadugu–Yobe basin’s ecosystems. In the Senegal River basin, studies by the Institute for Development Anthropology have demonstrated that floodplain conditions below Manantali Dam would be improved with only a small reduction in hydropower, and that the cost of the lost hydropower is substantially outweighed by the economic benefits to agriculture and fisheries. From Cameroon to South Africa, experimental flood releases offer opportunities for reducing the damage in river basins that have been degraded by large dams. Indeed, African nations are leading the way in this emerging science.
Ideally, a prescribed flooding program for the lower Zambezi system would include the coordinated management of both Cahora Bassa and Kariba Dams (the latter controls more than 50 percent of the Zambezi catchment). Unfortunately, Kariba was designed with no consideration of prescribed water releases. Flood releases from Cahora Bassa Dam, however, are possible. Cahora Bassa’s eight sluice gates are located significantly lower on the dam wall than Kariba’s, and are below the average operating water level of the reservoir.
Flood pulses have been released from Cahora Bassa Dam since its completion to prevent the dam from overtopping. These events were not planned, and thus took a heavy human toll because they occurred as flash floods. The 1978 flood wave alone killed 45 people, displaced more than 200,000, and destroyed nearly 60,000 hectares of crops.
These floods did have unplanned benefits to the ecosystem, which has led scientists to believe that prescribed floods can play an important role in restoring ecological conditions in the lower Zambezi. Following emergency flood releases from the dam in 1978, floodplain conditions improved dramatically for local flora and fauna. South African ecologist Paul Dutton observed that Cape buffalo and waterbuck populations increased markedly, and encroaching upland vegetation receded from the floodplain. In 1997, emergency flood releases again led to overbank flooding, and again the hydrological conditions in the lower Zambezi showed striking improvement. Waterbird populations increased and dispersed more widely, and the flushing of stagnant waterways in the floodplain led to an estimated 10–20 percent reduction in the cover of invasive plants.
Incorporating planned flood releases into the operation of Cahora Bassa Dam needs to be evaluated in light of a complex economic picture muddied by past debts and the effects of the long civil war. Mozambique is deeply in debt on the project to Portugal, its former colonial overseer (Portugal still owns 82 percent of the dam, and operates the dam through the firm Hidroelectrica De Cabora Bassa). Because the Cahora Bassa debt is just one of the many burdens on Mozambique’s post–war economy, the government is pressed to generate maximum power output to eliminate the dam debt as quickly as possible. Although negotiations are underway to try to relieve some of the debt, Mozambique also remains bound by power contracts that date back to the 1970s and force the export of nearly all power from the dam.
The encouraging news is that the Mozambican government is very enthusiastic about seeing improvements in the river’s ecosystem, and has been very interested in studies that will help it evaluate options to improve the dam’s management. One step it has taken is to look at the costs and benefits of an artificial flood on the dam’s income–generating capabilities. According to the Zambezi Valley Development Authority, the total value of contracts with South Africa and Zimbabwe for hydroelectric power from Cahora Bassa is about US$200 million per annum (based on about 2,100 megawatts of power). Preliminary hydrological studies suggest that management of outflows to mimic the historic mean annual Zambezi flood might entail, on average, a 10–15 percent reduction in hydropower output. This would result in about $20–30 million in lost revenues per annum.
The economic benefits of restoring natural flood cycles to the lower Zambezi derive from the anticipated improvements in downstream conditions. Although such gains can be difficult to quantify, the benefits to the coastal prawn industry alone are eye–opening. According to Mozambican fisheries ecologist Dr. Antonio Hoguane, "the lowering of dry–season flows and a rise in wet–season flows in the Zambezi River would stimulate recruitment of juvenile prawns to the population and improve production of the industry, leading to an increase in revenue of $10–$30 million per annum within two years of improved water management."
Re–establishment of natural flood–release patterns would also have beneficial effects on the riverine fishery. Many fish species depend on overbank flooding to provide access to spawning grounds on the grassy floodplain, while lower dry season flows tend to improve the catch rate. Since Cahora Bassa Dam was built, local markets have experienced a significant decline in the quantity of large fish species taken from the lower Zambezi River. The annual value of improved flooding to the thousands of lower Zambezi fisherman would undoubtedly measure in the millions of dollars.
A more natural flood would also enhance the value of floodplain agriculture and grazing practices. Past studies by Swedish consultants argue that released floods would improve soil fertility, reduce salinity, increase the extent of flood recession cropping, and improve the species composition and carrying capacity of floodplains for native and domestic herbivores. These benefits are corroborated by findings from recent research in Senegal and Nigeria, which places the economic value of artificial flood releases for subsistence agriculture practices in the millions of dollars.
Raising the carrying capacity of the Zambezi floodplain may also allow the once legendary concentrations of Cape buffalo, elephant, waterbuck, zebra, lion, and hippo to return. According to Dutton, restoration of healthy populations of Cape buffalo and other game species would result in substantial economic returns from tourism and regulated trophy– and food–hunting.
Of course, economic analyses alone cannot begin to capture the full value of restoring the Zambezi ecosystem. The basin is of profound cultural and ecological importance to Mozambique and to the world. It is home to between 400,000–700,000 rural people, all of whom draw their livelihood from the river and its valley. The lower Zambezi is of international importance for its diversity of mammal, fish, and waterbird species, many of which are endangered or vulnerable. It supports a rich mosaic of more than 12 different natural communities, including some of the most extensive coastal mangroves in Africa. It is the Zambezi river floodwaters that have created and maintained this richness.
Further studies will provide a more complete accounting of these benefits, but clearly there are strong economic, social, and ecological arguments for restoring flooding to the lower Zambezi. The fate of the lower Zambezi ultimately depends on using these arguments to garner strong institutional support at the local, regional, national, and even international levels for more–natural water releases from the dams.
Living with Dams
Of course, prescribed flooding is not a panacea for the problems of the lower Zambezi basin. Efforts to re–establish natural flood cycles will not necessarily result in the re–establishment of historic floodplain conditions. The health of river systems depends on many other factors as well, including sediment loading, nutrient flows, and the activities of the system’s plants and animals. Riverine sediments, for example, "play a pivotal role in the ecological functioning and productivity of the river, its wetlands, the delta, and the coastal zone, and hence, greatly influence economic activities in the valley and the adjacent coastal zone," notes Davies. But Kariba and Cahora Bassa reservoirs trap most of the sediment load of the upper and middle Zambezi basins, releasing silt–free waters downstream. Artificial flood releases may increase sediment transport, but they cannot recreate the basin–wide process of sedimentation that once fertilized and aggraded the lower Zambezi. With widespread coastal erosion and mangrove die–back already in evidence, the long–term social and ecological consequences of reduced sedimentation are sobering.
Equally problematic are the changes that have taken place in land use and settlement patterns along the lower Zambezi system. Widespread settlement is occurring in historically flood–prone areas of the riverway, as villagers abandon their upland homes and adjust their livelihoods to the regulated Zambezi. To realize the anticipated benefits of restored flood cycles, temporary movement away from flood–prone areas will be required during peak flood releases. This in turn will require an appropriate flood warning system and a community–based rural development program to make best use of the flood releases. However, after 25 years of mismanagement of Cahora Bassa Dam, attitudes toward floods in the delta are very negative, and these will be difficult to overcome.
Although some village elders recall the importance of the historic floods for their livelihood, most young people think that floods are "created by the government" and do nothing but harm. They point to the unanticipated flash flood releases from Cahora Bassa, which crested much more quickly than natural floods and created hardships that slowly rising and receding floodwaters did not. Overcoming such perceptions will require extensive community outreach. According to IUCN Zambezi Basin project manager Baldeu Chande, "one of the great challenges for conservation work in the Zambezi Delta is to work with people and educate them about the values of floods ... but first, we must learn from them."
Prescribed releases are expected to produce only moderate floods. These moderate floods are likely to prove beneficial for many floodplain processes. However, large floods – characteristic of the historic Zambezi system – may be required to remove upland trees that have become established in the floodplain in the absence of flooding, and to flush accumulated organic matter and nutrients from peripheral swamps to the floodplain. As yet, little research has been conducted on species–, community–, or ecosystem–level responses to varied flood releases in the lower Zambezi or elsewhere in sub–Saharan Africa.
The process to re–operate the Cahora Bassa Dam in a more beneficial way is gaining momentum and political will, despite the many obstacles and competing considerations. Most notably, a workshop on the sustainable use of the Cahora Bassa Dam was held in October 1997 under the auspices of the Mozambique’s Zambezi Valley Development Authority (GPZ) and Arquivo do Patrimonio Cultural (ARPAC), the main government body doing social research. The workshop drew more than 50 scientists, managers, and decision–makers from Mozambique, southern Africa and beyond. Participants discussed how the Cahora Bassa Dam might be managed to optimize use of Zambezi water for local development and conservation in addition to other national interests, and discussed immediate actions that could be taken to improve water management and build consensus among Zambezi users. Participants, including three cabinet–level Mozambican officials, concluded that the Cahora Bassa Dam’s outflow should be managed to simulate the Zambezi River’s natural variability in water flow.
Future meetings will engage a widening circle of stakeholders and decision–makers in reaching consensus on an integrated management plan for Cahora Bassa Dam and the Zambezi Valley. This circle will include key international stakeholders concerned with hydropower sales from the dam, especially neighboring Zimbabwe and South Africa. Calls to allocate the Zambezi’s waters to benefit river basin communities and ecosystems, in addition to other national and international development interests, are now being received favorably in the decentralized political system of Mozambique.
The restoration of the lower Zambezi system is ultimately an exercise in adaptive management. Many conclusions and judgments will be made by observing short–term responses to flood releases and adjusting our prescriptions accordingly. Decision–makers and river basin managers alike will need to understand the uncertainties that we inevitably face in trying to ameliorate the impacts of large dams.
Implementation of a prescribed flooding program at the scale of the lower Zambezi basin is a long–term process. It will demand strong political will, constant community outreach, and continuing international scientific cooperation. The alternative – the continued degradation and decline of the lower Zambezi – would ultimately prove infinitely more costly to the people and wildlife of Mozambique.
The author is a wetland hydrologist and the Africa Program Coordinator for the International Crane Foundation in Baraboo, Wisconsin.