Freshwater is an essential natural resource on which all humans depend. Dams have been used for thousands of years to regulate river flows and ensure adequate supply of water during dry periods. In the future, as populations increase and water consumption rises many people believe there will be a need for more dams. However, in recent years proposals for new dams have, in many places, aroused intense opposition. There are many social and economic arguments used against dams, but underpinning many of these arguments is the fact that dams, particularly large dams, produce major ecological changes in river ecosystems.
Dams have impacts on both upstream and downstream ecosystems. They constitute obstacles for longitudinal exchange along rivers and disrupt many natural environmental processes. Flooding upstream of dams results in the permanent destruction of terrestrial ecosystems through inundation. All terrestrial plants and animals disappear from the submerged area. Reservoirs trap waterborne materials including sediment and obstruct migration pathways for some aquatic species. Downstream there are changes in flow regime, sediment transport, and water temperature and quality. Many of these changes are immediate and obvious. However, others are gradual subtle and more difficult to predict. For example, changes in thermal regime, water quality and land-water interactions result in changes in primary production, which in turn has long-term implications for fish and other fauna higher up the food chain. Dams may cause changes in ecosystems at great distances from the dam. For example, changes in sediment transport result in changes in river, floodplain and even coastal delta morphology sometimes many hundreds of kilometres from the site of the dam.
The response of river ecosystems to dams are multiple, varied and complex. Consequently, in attempting to determine the effect of dams on ecosystems there is a requirement for large amounts of data relating to: hydraulic characteristics of the river; water quality; geomorphological characteristics; aquatic biota and their habitat; riparian vegetation and associated fauna and the direct use of the resources of the river and its floodplain by local people. There is a need for fundamental research linking changes in abiotic processes caused by dam construction to changes in ecology, particularly in tropical climates where much of the remaining potential for new dams resides, but where very little research has been conducted to date.
The complex inter-relationship between dams and their environment make it extremely difficult to predict all the consequences that dam construction will have for any particular river ecosystem. The impact of each dam is unique and dependent not only on the dam structure, but also local sediment supplies, geomorphic constraints, climate and the key attributes of the local biota. However, climate exerts a general and pronounced influence over the basic pattern of catchment processes and in this report a preliminary appraisal of possible variation in impacts under different climatic regime is attempted. However, while the approach provides a very broad indication of likely response, current understanding is too imperfect to provide a basis for precise predictions.
The problem faced by those responsible for power and water management concern the ways that dams impact on the environment, and how these impacts can be measured and taken into account by decision makers. Ecosystem impacts are effectively costs to society, and need to be included into accounting structures and applied at the project appraisal stage of dam planning. If this does not happen, the dam development may have spatial and temporal impacts which create a situation of both intragenerational and intergenerational inequity. This document contains a summary of those aspects of ecosystems that have relevance for the environmental economist, and how these may be valued. Through a literature review, a range of valuation methodologies are identified, along with examples of how these have been applied in practice. Some discussion is provided of how environmental values can be used to contribute to decision-making.
Over the last 20 years there has been an increasing awareness of human dependence on functioning ecosystems which has recently culminated in what is presented by many as a new paradigm in environmental management; the ecosystem approach. This recognises the need to manage land and water resources in an integrated manner in such a way that essential ecosystem functions are maintained. This current state of environmental awareness is to a large extent reflected in the guidelines for dam construction, proposed by both organisations that fund large dams (e.g. the World Bank) and organisations that support large dam building (e.g. ICOLD). However, mainly because of the fact that conditions are unique for each dam constructed, the guidelines, which are necessarily generic, are primarily limited to the recommendation that environmental assessments should be conducted. The guidelines do not specify exact environmental criteria (i.e. thresholds of acceptability) on which to judge whether or not a particular dam is environmentally acceptable.
Since the mid 1980s a well organised international movement against current dam building practices has evolved. The principal argument of opponents of large dams is that existing dams have largely failed to meet their economic and social objectives and at the same time resulted in significant environmental and social damage. Conversely, dam proponents argue that large dams have played a key role in human development throughout the world and significantly improved the well-being of humankind. They argue that many of the adverse environmental impacts can be mitigated through appropriate dam siting and operation. A brief review of the arguments used by both sides indicates that although usually presented as a strict dichotomy, the reality is more complex. Indeed, on many issues the proponents and opponents of large dams appear to be broadly in agreement. Both sides agree that large dams have significant environmental impacts, but they differ in their assessment of the true costs and benefits of large dams and the distribution of these costs and benefits between different sectors of society.
Dams modify, in both obvious and subtle ways and across a wide range of spatio-temporal scales, the conditions to which local ecosystems have adapted. In the past, flow regulation has frequently caused serious degradation of natural ecosystems and in places, this has resulted in the loss of natural processes that add to the well-being of sectors of human society. Land and water resources are linked so it is essential that they are managed in an integrated manner. To be successful management practises must be based on knowledge of ecosystem processes and an understanding of the manner in which human intervention alters those processes. To this end an understanding of the impact of large dams on ecosystems is a pre-requisite to better management of water and other natural resources.
Citation: MP McCartney, C Sullivan and MC Acreman; Ecosystem Impacts of Large Dams; Background Paper No 2; International Union for Conservation of Nature and Natural Resources and the United Nations Environmental Programme; 2001.