Dams have provided society with substantial benefits by helping to secure reliable water supplies for cities and farms, manage floods, and generate electricity. However, their construction and operation have also brought some undesirable social and economic impacts. Some of these are well understood, such as the inundation of river valleys by reservoirs and displacement of human populations and economic activities in those places. But some of the most damaging and long-lasting effects of dam construction and operations remain poorly understood in the dam industry and regulatory community. As a result these impacts are seldom addressed adequately when siting, designing, and operating dams.

Dams can have serious consequences for the ecological health of rivers and the economic and social well-being of people dependent upon the goods and services provided by healthy rivers. Historically, assessments of the environmental and social impacts of dams have focused primarily on areas in the immediate vicinity of a dam and its reservoir. However, dams can substantially modify river ecosystems for hundreds of kilometres downstream by changing the water flow (volume and timing), water chemistry, physical structure of river channels and floodplains, and hydrologic connections between upstream and downstream and between a river and its floodplain. These physical and chemical changes in a river environment lead to changes in biological conditions, including loss of plants and animals valued by local communities for food, building materials, recreation, tourism, and other cultural purposes. While dams have necessitated the resettlement of 40-80M people worldwide, the toll of people adversely affected by ecosystem degradation downstream of dams numbers in the hundreds of millions.

A primary challenge in water resource development is siting, designing and operating infrastructure projects to provide social benefits while preventing the loss of natural ecosystem services valued by communities proximate to dammed rivers. The best and most cost-effective way of minimising the adverse consequences of dams is to avoid such impacts in the first place, through careful siting of new projects. Once a site has been chosen for a new dam, its impacts can be further minimised by designing it in a manner that minimally disrupts the flow and quality of water, sediments, nutrients, and the movements of fish and other aquatic species. Unfortunately, the vast majority of dams built in the past have been developed without adequate consideration of the environmental conditions that must be maintained downstream of dams to protect ecosystem health and support the lifestyles and well-being of local communities.

This article focuses on one critically important element of this challenge: the need to maintain proper environmental flows downstream of dams. This term refers to a variable water flow regime that has been designed and implemented – such as through intentional releases of water from a dam into a downstream reach of a river – in an effort to support desired ecological conditions and ecosystem services. The process of integrating environmental flow releases into the regular operations of an existing dam is commonly referred to as re-operation.

Any proposed changes in a dam’s operations can have implications for existing water uses and economic benefits. The specific objectives to be met by re-operating a dam for environmental flow releases must be carefully defined by qualified environmental and social scientists in collaboration with local communities, and integration of these flow releases into dam operations must consider the consequences for all other operating objectives.

The master variable

During recent decades, scientists have amassed considerable evidence that a river’s flow regime – its variable pattern of high and low flows throughout the year as well as variation across many years – exerts great influence on river ecosystems. Each component of a flow regime – ranging from low flows to floods – plays an important role in shaping a river ecosystem. Due to its strong influence on other key environmental factors (water quality, physical habitat, biological composition), river scientists refer to the flow regime as a master variable.

Over long periods of time, the plants and animals – and even the human communities situated along rivers – have become adapted to natural variations in river flow. In fact, many aquatic species will reproduce only during certain flow levels, such as fish that wait for high water levels to migrate upstream, or move out onto floodplains to feed and spawn. In essence, the variability of water flow in a river can be thought of as a musical score to which river plants, animals, and human communities are choreographed. When that musical score is altered too greatly, such as by building a dam on the river, the symphony of river life becomes discordant. Some instruments fall silent while intruders playing foreign scores take their seats. The harmony of life is broken.

Scientists have been working closely with water planners and dam managers in river basins around the world to characterise the environmental flows needed to support ecosystem-related objectives, particularly the human benefits associated with ecosystem services. Ecosystem services are defined as a variety of culturally and socially-valued goods and services that human society derives from natural ecosystems. Freshwater ecosystems provide a wealth of food and fibre, water purification, fish and wildlife habitat, tourism and recreational opportunities, shipping routes, and opportunities for cultural and spiritual renewal. In subsistence economies where two billion humans live, the connection between river health and what a family finds on its dinner table is direct and immediate.

Because environmental flows are so essential to the health of river ecosystems, maintaining adequate flow conditions is critical in supporting the ecosystem goods and services that local communities depend upon for livelihoods and food production. When integrating environmental flow management into dam operations, it is important to understand that environmental flow releases from dams can be tailored to meet an array of social and economic objectives, and attain various levels of ecosystem protection. The integration of environmental flows should not be viewed as an all or nothing decision; instead, different aspects or building blocks of the flow regime, such as provision of occasional flood releases from dams to stimulate fish spawning, can be maintained or restored to varying degrees as necessary to accomplish ecosystem-based objectives. The type of environmental flow regime to be provided should be dictated by trade-off analyses informed by an inclusive stakeholder decision-making process that fully considers the range of ecosystem benefits that are linked to environmental flows, and the social and economic costs and benefits of various options.

A recent assessment of environmental flow needs on the Savannah river in the southeastern US provides an illustrative example of the ecosystem benefits that can be restored through dam re-operation. This environmental flow assessment was conducted as part of a river basin planning process led by the US Army Corps of Engineers (USACE), which operates a cascade of multi-purpose dams on the river. USACE’s river basin plan evaluated the need for changes in dam operations to better meet the full array of values and needs of stakeholders, including those dependent upon a healthy river ecosystem.

The environmental flow assessment took a comprehensive view of the water flows needed to sustain a broad array of plant and animal species, particularly those that are highly valued by the local community, such as commercial and recreational fisheries.

The results of this assessment, as portrayed in Figure 1, suggest that different ecological objectives can be attained with varying levels of dam release. For each level of release specific ecological outcomes are anticipated. As environmental flow releases are made, ecological responses and conditions can be monitored to see if the system responds as anticipated, and dam operations can be further adjusted as necessary.

For example, a key concern on the Savannah river has been the inability of sturgeon to access their spawning grounds, located immediately below the lower-most dam on the river. Fish biologists involved in the environmental flow assessment estimated that a dam release of at least 570m3/sec during early spring would be needed to enable migrating sturgeon to move upstream through a navigation dam structure and shallow areas in the river.

However, the sturgeon didn’t respond as expected to the first high flow pulse released from the dam in March 2004. Careful monitoring of sturgeon movements using radio-tagging technology revealed that the coldness of the water – released from deep in the upstream reservoir – actually repelled the sturgeon, suggesting that in future years the pulse release should be delayed until water temperatures in the reservoir and river were more suitable. With four years of dam releases and monitoring, scientists have learned a great deal about the Savannah ecosystem, enabling them to provide improved recommendations for re-operating USACE’s dams.

Key to the successful integration of environmental flow releases into dam re-operations on the Savannah river has been river scientists working hand-in-hand with the dam operators to identify aspects of the environmental flow recommendations that could be implemented immediately, and which aspects would need to be delayed. For example, while USACE was able to begin releasing springtime high-flow pulses almost immediately, accommodating flood releases or very low releases remain problematic. The flood releases specified in the environmental flow assessment would inundate homes built recently in the historical floodplain. USACE is now studying the feasibility of constructing a floodway that could channel small dam-released floods around these homes. The very low flows specified for April through October in dry years would cause water quality problems in areas of the river that are affected by industrial wastewater discharges and require higher water flows for dilution purposes. The feasibility of reducing or cleaning up these discharges to enable lower water releases from the dam remains to be investigated.

The constraints or challenges in integrating environmental flow releases into dam operations are numerous and varied, and to some degree unique to each and every dam and its associated river basin. Constraints can be of a physical nature, such as limits on dam releases imposed by the size and arrangement of outlet works, but they can also be of an economic, social, or political nature. While it is difficult to offer generalised solutions to overcome these constraints, we highlight here four strategies that we commonly apply to create the necessary flexibility in dam operations to enable environmental flow releases.

Flood management

Flood control dams, or multi-purpose dams that include flood-control functions, are usually designed and operated to control or modify all floods. These include those that would cause considerable threat to human life and damage to downstream structures, as well as smaller floods that could be released safely and provide considerable ecological benefit in downstream river ecosystems. Considerable flexibility for accommodating small flood releases can be realised by restoring floodplain areas to enable them to again store floodwaters. By storing some portion of natural flood storage on the floodplain instead of in a reservoir, the total volume of necessary flood storage space in the reservoir can be reduced. In many instances, a triple win solution may be available (particularly at multi-purpose dams) when downstream floodplain areas can be protected or reactivated for flood storage, such as by moving settlements out of harm’s way or obtaining flood easements allowing seasonal inundation of agricultural lands.

By moving structures and human populations out of the floodplain, or by allowing agricultural areas to be flooded occasionally, the flood storage requirement in the upstream reservoir(s) can often be reduced substantially and re-allocated for hydro power use, additional water supply, or improved environmental flows downstream of the dam.

Re-flooding of natural floodplains can bring substantial ecological benefits as well, such as by providing additional spawning and feeding opportunities for fish and enabling the floodwaters to fertilise and moisten floodplain areas used for agriculture or grazing. Therefore, in addition to gaining hydro power or water supply capacity (first win), overall flood risk can be reduced (second win), and environmental flows can be implemented (third win).

The Nature Conservancy and the Natural Heritage Institute are both currently involved in a feasibility study exploring the possibility of implementing such a strategy on the Yangtze river in China. Eight large hydro power dams are planned or under construction in the river basin upstream from Three Gorges dam, including four dams to be built immediately upstream of Three Gorges reservoir by the China Three Gorges Project Corporation. Considerable flood control capacity has been planned for each of these four new dams, which greatly compromises potential hydro power generation at these facilities because reservoir levels must be lowered considerably during flood season. However, large areas of the Yangtze floodplain could be safely re-activated to store the same volume of floodwater planned for the four new dams; much of this floodplain area is presently isolated from the river by levees. In fact, initial estimates suggest that as much as US$1B per year in additional hydro power revenue could potentially be generated by restoring flood storage in the floodplain and enabling the four dams to be operated for increased hydro power production. This same strategy could be feasibly implemented in other river basins around the world.

Rhythm of the river

The aim of hydro power reoperations for environmental benefits is to emulate a more natural flow pattern downstream of the dam and its powerhouse. To do this, the water must be released from the reservoir at essentially the same time and rate as the inflow. That means that the dam will generate power at its maximum capacity during the precipitation season(s), and at a much lower level during the dry season(s). This pattern is often quite different than the customary role that hydro dams play in the mix of power facilities feeding into a power distribution system.

Typically, the hydro power dam is operated to follow the load curve. Re-operation therefore entails rescheduling the power production, not necessarily reducing it. To make this feasible, the other power generators in the system – such as oil and gas turbines – must also be re-operated to synchronise with the power dam. Thus, during the rainy season, the hydro dam will substitute for thermal generation, and during the dry season, the thermal generators will substitute for hydro. It will sometimes be necessary to integrate the hydro system into a larger regional grid with a larger number of power plants. This is the reoperation technique that is being pursued and demonstrated for the Akosombo dam on the Lower Volta river in Ghana in a collaboration involving the dam operator, the Volta River Authority, the Natural Heritage Institute, The Nature Conservancy and several governmental and non-governmental partners in Ghana.

Variable turbine-generator capacities

The ability of a dam operator to provide a range of flows for downstream environmental purposes is ultimately dependent upon a dam’s outlet and turbine-generator capacities. Many hydro power dams lack adequate turbine-generator capacity to make large releases, such as controlled floods that may be highly desirable for maintaining the ecological health of downstream floodplain ecosystems and estuaries, without sacrificing power generation. Because of these constraints, some fraction of controlled flood discharges must be released through the dam’s flood spillway. This sacrifice of power generation causes dam operators to resist such controlled flood releases.

This is the situation at the Manantali dam in the Senegal river basin. At that dam, some 2000m3/sec would need to be released to inundate the floodplain to support 50,000ha of flood-dependent agricultural production, yet the outlet and turbine-generator capacity is only 48m3/sec. The rest of the required flow would need to be released through the spillway, thereby compromising hydro power generation. Necessary structural modifications to expand the powerhouse capacity from 480-2000m3/sec would be very expensive at this point, but had the powerhouse capacity and reservoir storage tradeoff been optimised in the first place, the economics of providing floodplain inundation would likely have been more favorable. However, modifications of outlet structures to better accommodate environmental flow needs may be quite feasible and cost-effective, such as at dams that can be retrofitted with new or additional turbine-generator units that improve hydro generation potential.

Re-regulation reservoirs

The unnatural fluctuations in river levels caused by hydro power generation or capturing and releasing water for urban or irrigation uses can be mitigated to some degree by constructing a re-regulating dam.

A re-regulating dam can be operated to smooth out the unnatural fluctuations caused by dam operations even while it is generating electricity, releasing water in a pattern much closer to reservoir inflows. The ability of a re-regulating dam to restore natural flow patterns downstream will depend upon the extent to which the hydro power dam has altered them; essentially the same volume of storage capacity is needed to both alter flows at the hydro dam and to restore flows at the re-regulating dam. If hourly downstream fluctuations are undesirable, a relatively small re-regulating dam below the powerhouse can be a positive asset to hydro power and to the environment by providing a more steady downstream discharge during the day. The same benefit can be achieved by dedicating the lower most hydro power dam in a cascade to reregulate flows, which can be of considerable benefit to the downstream environment.

Key ingredients for success

After more than two decades of working with dam managers around the world to re-operate their facilities for environmental and social benefits, we have learned that – similar to the Savannah story – considerable improvement in ecosystem conditions and benefits can be attained with relatively modest accommodation of environmental flow releases and minimal disruption of existing water uses. But some essential ingredients must be in place to attain successful and mutually satisfying outcomes in a dam re-operation effort. We consider these to be essential:

• Build strong stakeholder support – The indispensable partner in dam reoperation projects is the dam operator itself, together with the governmental agencies that plan, regulate and manage water resources in the basin. Interest groups that depend on the river ecosystem for their livelihoods or value ecosystem health for other reasons also need to be actively engaged in developing specific objectives for dam re-operation. Economic assessments that place monetary value on ecosystem-related benefits can be useful in evaluating trade-offs with other water uses, but ecosystem values that cannot be quantified monetarily should not be ignored.

• Engage qualified, inter-disciplinary scientists in assessing environmental flow needs – Given the broad array of ecosystem-related values likely to be held by stakeholders, and the complex life-cycle needs of river plants and animals, any environmental flow assessment must be conducted by an inter-disciplinary team of experts familiar with the process of defining linkages between differing river flow levels and ecological responses. This team must consider the full range of river flow variation.

• Comprehensively examine options and flexibility for integrating environmental flows – When the operation of a single dam is examined, opportunities for integrating environmental flow releases can usually be identified. However, those possibilities can be expanded exponentially by considering the entire water or energy system of which the dam is just the storage component. Re-operation of an agricultural water supply dam may require assessment of the entire irrigation system, with an eye toward creating operational flexibility by integrating the surface and groundwater storage systems. Re-operating a hydro power dam may require assessment of the entire energy grid that includes the dam along with many other power generators. When the driver is flood management, strategies to accommodate larger controlled flood events in the downstream floodplain may be required.

• Stimulate engineering creativity – Water and dam engineers are some of the world’s best problem solvers. When they fully embrace environmental flow objectives on par with other dam-operating purposes, their innate sense of creativity can be channeled toward innovative and optimal solutions. The best outcomes result from active collaborations between engineers, scientists that understand how river ecosystems work, and representatives from local communities that can communicate what their people need.

• Manage for uncertainty and respond to surprises – No scientist is able to perfectly predict how an ecosystem will respond to environmental flow releases, and no engineer can fully anticipate every challenge that will arise in implementing them. Additionally, social values change over time, and dam operations should be adjusted accordingly. When appropriate indicators of success are well defined, and adequate monitoring systems are in place to track those indicators, there exists an opportunity to learn what is working as expected, and what is not, thereby providing a basis for continual improvement.

Brian Richter, Co-Director of the Global Freshwater Programme for The Nature Conservancy, a programme that is supporting conservation projects across the Americas, Asia, Africa, and the Pacific Region. Email: brichter@tnc.org

Gregory A. Thomas, President, Natural Heritage Institute. He develops and manages large-scale projects in California, throughout the US and internationally. His current concentration is on global dam reoperation projects to restore downstream ecosystems and human livelihoods. Email: gat@n-h-i.org