Restoring the Zambezi: Can Dams Play a Role?

Lori Pottinger
Sunday, October 1, 2006

From World Rivers Review, October 2006

Kariba Dam
Kariba Dam

The Zambezi River is one of southern Africa's most important lifelines, and its delta is a Ramsar "Wetland of International Importance." However, it is also one of Africa's most heavily dammed river systems, and its health is in decline. More than 30 large dams (including two of Africa's largest, Kariba and Cahora Bassa) constrict its flow of water and sediments, and more large dams are planned. A new dam, Mphanda Nkuwa in Mozambique, is farthest along, and is expected to result in a push for industrialization in the Zambezi Valley. Dam-induced ecological changes have already had widespread consequences on wildlife and the thousands of people who call the Zambezi valley home; widespread industrialization would worsen the situation for many downstream users.

Offering a counterpoint to this situation are a number of efforts now underway that aim to restore the river's health through better water management. One important new study, "Assessing Environmental Flow Requirements for the Marromeu Complex of the Zambezi Delta”, outlines a restoration effort based on making Cahora Bassa Dam's water release patterns more closely mimic natural flows. The study states: "At present the management of [upstream dams] remains focused on maximizing hydropower production relative to other potential Zambezi River use." The researchers found that a small (less than 3%) reduction in hydropower production at Cahora Bassa would enable prescribed flood releases that could result in very significant improvements in ecosystem-based livelihoods of downstream residents. Here we talk to lead author Richard Beilfuss about the team's findings.

WRR: Your report notes that the Lower Zambezi basin supports about a quarter of Mozambique's population. Describe the natural "support systems" that sustain so many people. How would improved flows help the rural poor in this area?

RB: The Lower Zambezi is vital to the national economy in Mozambique. Under more natural flow conditions, the annual ebb and flow of Zambezi floodwaters supports extensive flood-recession agricultural systems along the length of the Zambezi. This provides more reliable food security than fickle, rain-fed agriculture on adjacent uplands. It also sustains productive freshwater fisheries, coastal prawn fisheries, and healthy grasslands with a very high carrying capacity for large herbivores in the Delta. Over the past 50 years of river regulation, these systems have all declined precipitously. There is widespread scientific opinion that the restoration of regular, annual, modest flood releases – timed to coincide with the historical period of peak flooding – would improve these and other systems, providing important socio-economic and ecological benefits.

We looked at how changing the timing, duration and magnitude of the river's flow could help restore some of the conditions that support the people and animal life in the delta. What we found was that the most significant benefit for the rural poor from environmental flows would be improved fisheries. The productivity of Zambezi Delta fisheries is directly related to the magnitude and duration of floodwaters – and even the return of small annual floods would improve matters significantly. The prawn industry would also be improved, perhaps substantially, as the lifecycle of prawns is closely linked to the seasonal pattern of high and low flows in the Zambezi. This is a large commercial industry, employing many people from the coastal region.

We're still looking at exactly what kind of flows would most improve the lives of poor rural farmers, including those involved in subsistence and cash crop agriculture, production of natural flooded rice near the coast, and irrigated commercial agriculture schemes. It's harder to put a dollar value on these benefits compared to the benefits of improving the fisheries, because part of the benefit would be reducing the risks of crop failure relative to more fickle rain-fed agriculture, but it's a very promising area as well.

WRR: What kinds of changes to dam operation are you calling for in this new report?

RB: We would like to see the Zambezi River managed as it was originally intended – for multiple objectives including “environmental reserve requirements” and integrated rural development. At present, Zambezi waters are managed almost exclusively for hydropower generation, with a secondary function of controlling large floods when excess reservoir capacity exists. The water authorities use a monthly flood rule curve that specifies the maximum reservoir water level that can be maintained without risk of overtopping during the flood season. Reservoir management according to this flood rule curve is particularly damaging to downstream users because it often results in the release of excess reservoir waters during the dry season. These releases scour standing crops from river banks and serve no ecological purpose – the antithesis of what we are trying to achieve with well-timed prescribed floods. Our proposal would include improving food security for downstream users and sustaining the floodplain ecosystems, especially the Zambezi Delta. . But keep in mind that this is a preliminary study, aimed more at helping decision-makers understand the value and potential of prescribed flooding rather than making specific prescriptions. The next phase would be for the water authorities to develop a specific flow plan for the Zambezi--for both high and low flows.

WRR: Mozambique has had very damaging large floods in recent years. As you note, "large flooding events in the Zambezi delta are now heavily influenced by reservoir operation." Would an environmental flow regime offer any improvements in flood management?

RB: With little or no reduction in total hydropower generation and firm power supply, the dam could release a prescribed flood early in the wet season that would not only have great benefits for people and wildlife downstream, but would also reduce the risk of large floods later in the flood season by increasing the available reservoir storage for incoming flows. Flow releases would be of modest size – sufficient in magnitude to spread floodwaters into the floodplains of the Zambezi Delta, and timed to enable flood-recession cropping systems, but not on the scale of the large, damaging floods such as occurred most recently in 2001. It's important to note that extreme flooding events like the 2001 flood, though infrequent, are inevitable because even large dams like Cahora Bassa and Kariba cannot fully capture Zambezi flows during years of very high regional rainfall.

WRR: How will this environmental flow plan be impacted by proposals to build future dams such as Mphanda Nkuwa?

RB: We believe there is a narrow window of opportunity to implement a new vision for managing the Zambezi waters for the health of the delta and the people of Mozambique, as new dam projects will bring new economic imperatives to maximize hydropower at the expense of other uses. Mphanda Nkuwa is always described as a "run-of-river" dam – a description that is usually meant to indicate fewer environmental impacts – but in fact it has a fairly large dam wall. There could still be environmental flow releases from Cahora Bassa if this dam is built, but it is likely to be seen as "wasted water" once there are two dams that could be using the flow to create electricity. Mphanda Nkuwa will be operated for peaking power, which will negatively affect agricultural production on the river banks immediately downstream. And Mphanda Nkuwa will capture the Liua River, one of only four significant Zambezi tributaries that occur downstream of Cahora Bassa Dam, further reducing natural variability of flows and sediment availability. We'll continue to make a strong case for environmental flow releases, but it will be a harder sell.

WRR: How might climate change impact this plan for environmental flows?

RB: Although we have not yet modeled the impact of climate change on the potential for prescribed flooding from Cahora Bassa, some of the climate change models show there will be more intensive storms in the basin – meaning more risk of large floods and less infiltration to sustain dry season baseflows. But in general, greater aridity is expected due to higher temperatures and evaporation. This would mean less water in the Zambezi, less water for all uses. It would likely result in a resurgence of plans to tap the Zambezi for water supply for South Africa and Namibia. Climate change could invalidate all of the design parameters for the dams, and result in reduced power sales from the dams. There would be a desperate need to hold back every drop of water to produce power, at the same time that the river's flows would become more precious for all downstream users and the environment. As institutional acceptance of prescribed flow releases moves forward, we will encourage the water authorities to base hydrological management on a wider range of flow scenarios that account for climate change and increased water extractions and diversions from the Zambezi basin.

WRR: What do you hope the results will be if your best-case is adopted?

RB: We would like to see the Zambezi River managed for the full range of water users and ecological systems that depend on it. It is not possible to fully restore historical hydrological conditions in the lower Zambezi River given the water needs for hydropower production. But our work has clearly demonstrated several important points. First, a range of prescribed flood releases can be implemented with little or no reduction in annual hydropower generation. Secondly, such floods can be achieved on an annual basis in all but the most extreme drought years – thus providing for example a vital water source for small-producer irrigation during years of below-average or erratic rainfall. Thirdly, prescribed flood releases would benefit a wide range of socio-economic sectors and ecological processes, and these benefits would significantly outweigh the cost of lost hydropower production. Finally, prescribed floods released early in the wet season provide increased reservoir capacity for attenuating large flooding events that might occur later in the wet season during years of very heavy rainfall.

WRR: What gives you hope that this river can be restored?

RB: There is a lot of good work on the management of the Zambezi right now, and considerable interest in the findings of our report. There is more discussion about environmental flows and water reserves to support natural areas, and more widespread understanding about the need for such flows. The two arguments used most commonly against flood releases – one, that they were strictly for “environmental purposes” and not for human benefit, and two, that they would require unrealistic reductions in hydropower generation – have been discredited. Not everyone agrees about the importance of prescribed flooding, but at least the topic is on the national agenda, and that is really significant. There is also a greater effort now to look at the Zambezi as a whole, from a basin-wide perspective, which can only help the cause.

The Zambezi River is recognized as an economic lifeline for all of southern Africa. Developments in planning include large hydropower dams, water diversions (including the pressure for inter-basin water transfers), navigation, large-scale irrigation schemes, gold and other mineral mining, commercial logging, and other forms of development. In this context, prescribed flooding is of course not a panacea for the Zambezi River, but it can be part of a new vision for sustainable management of the river basin.

Dr. Richard Beilfuss is Director of the Gorongosa Research Center for the Carr Foundation, based at Gorongosa National Park in Mozambique. He has worked on research and conservation activities throughout the Zambezi River basin since 1995.

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