WCD Thematic Review Environmental Issues II.1
Dams, Ecosystem Functions and Environmental Restoration
Final Version: November 2000
Prepared for the World Commission on Dams (WCD) by:
Ger Bergkamp, Matthew McCartney, Pat Dugan, Jeff McNeely and Mike Acreman
Based on contributions from:
M.C. Acreman (Institute of Hydrology, UK) E. Barbier (University of York, UK) G. Bernacsek (FAO) M. Birley (University of Liverpool, UK) J.R. Bizer (MESAS Consultants, USA) C. Brown (University of Cape Town, South Africa) K Campbell (Natural Resources Institute, UK) J. Craig (Consultant, UK) N. Davidson (Wetlands International, The Netherlands); S. Delany (Wetlands International, The Netherlands) C. Di Leva (IUCN Environmental Law Center, Germany) F. Farquharson (Institute of Hydrology, UK) N Hodgson (Natural Resources Institute, UK) D.C. Jackson (Mississippi State University, USA); J. King (University of Cape Town, South Africa) M. Larinier (Institut de Mecanique des Fluides, France) J. Lazenby (Gibb Ltd, UK ) D.E. McAllister, (Ocean Voice International, Canada); G. Marmulla, (Fisheries Department, FAO) M.P. McCartney (Institute of Hydrology, UK) J. Morton (Natural Resources Institute, UK) D. Murray (OPIRG, Carleton University, UK) M.B. Seddon (National Museum of Wales, UK) L. Sklar (University of California, Berkeley, USA); D. Smith (Natural Resources Institute, UK) C. Sullivan (Institute of Hydrology, UK) R. Tharme (University of Cape Town, South Africa)
Secretariat of the World Commission on Dams P.O. Box 16002, Vlaeberg, Cape Town 8018, South Africa Phone: 27 21 426 4000 Fax: 27 21 426 0036. Website: http://www.dams.org E-mail: [email protected] Dams, Ecosystem Functions, and Environmental Restoration i
Disclaimer
This is a working paper of the World Commission on Dams - the report published herein was prepared for the Commission as part of its information gathering activity. The views, conclusions, and recommendations are not intended to represent the views of the Commission. The Commission's views, conclusions, and recommendations will be set forth in the Commission's own report. This manuscript has been compiled by current and former staff members of IUCN in their personal capacity, based on contributions from a wide range of sources, and comments received from the review panel and WCD Forum. It does not therefore represent any official IUCN policy.
Please cite this report as follows: Berkamp, G., McCartney, M., Dugan, P., McNeely, J., Acreman, M. 2000. Dams, Ecosystem Functions and Environmental Restoration Thematic Review II.1 prepared as an input to the World Commission on Dams, Cape Town, www.dams.org
The WCD Knowledge Base
This report is one component of the World Commission on Dams knowledge base from which the WCD drew to finalize its report “Dams and Development-A New Framework for Decision Making”. The knowledge base consists of seven case studies, two country studies, one briefing paper, seventeen thematic reviews of five sectors, a cross check survey of 125 dams, four regional consultations and nearly 1000 topic-related submissions. All the reports listed below, are available on CD-ROM or can be downloaded from www.dams.org
Case Studies (Focal Dams) Country Studies Briefing Paper • Grand Coulee Dam, Columbia River Basin, USA • India • Russia and NIS • Tarbela Dam, Indus River Basin, Pakistan • China countries • Aslantas Dam, Ceyhan River Basin, Turkey • Kariba Dam, Zambezi River, Zambia/Zimbabwe • Tucurui Dam, Tocantins River, Brazil • Pak Mun Dam, Mun-Mekong River Basin, Thailand • Glomma and Laagen Basin, Norway • Pilot Study of the Gariep and Van der Kloof dams- Orange River South Africa
Thematic Reviews • TR I.1: Social Impact of Large Dams: Equity • TR IV.1: Electricity Supply and Demand and Distributional Issues Management Options • TR I.2: Dams, Indigenous People and Vulnerable • TR IV.2: Irrigation Options Ethnic Minorities • TR IV.3: Water Supply Options • TR I.3: Displacement, Resettlement, • TR IV.4: Flood Control and Management Rehabilitation, Reparation and Development Options • TR IV.5: Operation, Monitoring and • TR II.1: Dams, Ecosystem Functions and Decommissioning of Dams Environmental Restoration • TRII.1: Dams, Ecosystem Functions and • TR V.1: Planning Approaches Environmental Restoration • TR V.2: Environmental and Social Assessment • TR II.2: Dams and Global Change for Large Dams • TR V.3: River Basins – Institutional Frameworks • TR III.1: Economic, Financial and and Management Options Distributional Analysis • TR V.4: Regulation, Compliance and • TR III.2: International Trends in Project Implementation Financing • TR V.5: Participation, Negotiation and Conflict Management: Large Dam Projects
• Regional Consultations – Hanoi, Colombo, Sao Paulo and Cairo
• Cross-check Survey of 125 dams
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
Dams, Ecosystem Functions, and Environmental Restoration ii
Acknowledgment
The WCD acknowledges contributions to this thematic review by the United Nations Environment Programme (UNEP) and the IUCN - The World Conservation Union - through their respective work programmes. The submissions to the WCD thematic review were supported by the partnership agreement between United Nations Foundation, UNEP and the WCD.
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
Dams, Ecosystem Functions, and Environmental Restoration iii
Financial and in-kind Contributors
Financial and in-kind support for the WCD process was received from 54 contributors including governments, international agencies, the private sector, NGOs and various foundations. According to the mandate of the Commission, all funds received were ‘untied’-i.e. these funds were provided with no conditions attached to them.
• ABB • Skanska • ADB - Asian Development Bank • SNC Lavalin • AID - Assistance for India's Development • South Africa - Ministry of Water Affairs and • Atlas Copco Forestry • Australia - AusAID • Statkraft • Berne Declaration • Sweden - Sida • British Dam Society • IADB - Inter-American Development Bank • Canada - CIDA • Ireland - Ministry of Foreign Affairs • Carnegie Foundation • IUCN - The World Conservation Union • Coyne et Bellier • Japan - Ministry of Foreign Affairs • C.S. Mott Foundation • KfW - Kredietanstalt für Wiederaufbau • Denmark - Ministry of Foreign Affairs • Lahmeyer International • EDF - Electricité de France • Lotek Engineering • Engevix • Manitoba Hydro • ENRON International • National Wildlife Federation, USA • Finland - Ministry of Foreign Affairs • Norplan • Germany - BMZ: Federal Ministry for Economic • Norway - Ministry of Foreign Affairs Co-operation • Switzerland - SDC • Goldman Environmental Foundation • The Netherlands - Ministry of Foreign Affairs • GTZ - Deutsche Geschellschaft für Technische • The World Bank Zusammenarbeit • Tractebel Engineering • Halcrow Water • United Kingdom - DFID • Harza Engineering • UNEP - United Nations Environment • Hydro Quebec Programme • Novib • United Nations Foundation • David and Lucille Packard Foundation • USA Bureau of Reclamation • Paul Rizzo and Associates • Voith Siemens • People's Republic of China • Worley International • Rockefeller Brothers Foundation • WWF International
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
Dams, Ecosystem Functions, and Environmental Restoration iv
Executive Summary
Introduction. The impact of dams upon natural ecosystems and biodiversity has been one of the principal concerns raised by large dams. Over the course of the past 10 years in particular, considerable investments have been made in the development of measures to alleviate these impacts. Yet today widespread concern remains that despite improvements in dam planning, design, construction and operation, they continue to result in significant negative impacts to a wide range of natural ecosystems and to the people that depend upon them for their livelihood. WCD Thematic Reviews I.1 Social Impacts of Large Dams Equity and Distributional Issues, I.2 Dams, Indigenous People and vulnerable ethnic minorities and I.3 Displacement, Resettlement, rehabilitation, reparation and development examine this complex set of issues. It does so by first reviewing the importance of natural river basin ecosystems and examining the impact of dams on these ecosystems. It then examines the current status of approaches being taken to addressing these impacts through the continuum of “avoidance-mitigation-compensation-restoration”. Based upon this analysis the report concludes with an assessment of the areas of convergence and divergence on these issues within the dams debate and provides a set of recommendations to the Commission.
River Basin Ecosystems and Biodiversity. Each river basin contains many natural ecosystems including not only the aquatic habitats associated with water in the river channel, but all of the elements of the river catchment that contribute water, nutrients and other inputs to the river. These ecosystems include: the headwaters and the catchment landscapes; the channel from the headwaters to the sea; riparian areas; associated groundwater in the channel/banks and floodplains; wetlands; the estuary and any near shore environment that is dependent on freshwater inputs.
These ecosystems perform functions such as flood control and storm protection, yield products such as wildlife, fisheries and forest resources, and are of aesthetic and cultural importance to many millions of people. The total global value of ecosystem goods and services is estimated at US$ 33 trillion per year of which roughly 25% relates directly to freshwater ecosystems. With widespread and still growing recognition of these ecosystem values, river basin development needs to determine how much water is required for the maintenance of ecosystems to provide environmental goods and services, and how much water should be used to support agriculture, industry and domestic services.
Ecosystem Impacts of Large Dams. The current state of knowledge indicates that the impacts of dams on ecosystems are profound, complex, varied, multiple and mostly negative. By storing or diverting water dams alter the natural distribution and timing of stream flows. This in turn changes sediment and nutrient regimes and alters water temperature and chemistry, with consequent ecological and economic impacts. Reduction in downstream annual flooding in particular affects the natural productivity of floodplains and deltas.
These ecosystem impacts result in a significant impact of dams on freshwater biodiversity, which is already under special threat. Global estimates of endangered freshwater fish reach 30% of the known species. And in North America detailed studies indicate that dam construction is one of the major causes of freshwater species extinction. Dramatic reductions in bird species are also known, especially in downstream floodplain and delta areas. Some reservoirs also provide habitats for birds and other fauna but this often does not outweigh the loss of habitat downstream.
Multiple dams on a river significantly aggravate the impact on ecosystems. Sediment entrapment can reach 99% if a cascade of dams is developed. Fish migration is affected even by a single dam, and multiple dams worsen this situation dramatically. In the Northern hemisphere 77% of the largest rivers are affected by dams and on many rivers fully natural reaches are restricted to headwaters. The global impacts of dams on the global water cycle are increasingly recognised.
The review highlights the complexity of the processes that occur when a dam impacts an ecosystem. It is therefore extremely difficult and rarely possible to predict in precise detail the magnitude and This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
Dams, Ecosystem Functions, and Environmental Restoration v
nature of impacts arising from the construction of a dam or a series of dams. The precise impact of any single dam is unique and dependent not only on the dam structure and its operation, but also upon local hydrology, fluvial processes, sediment supplies, geomorphic constraints, climate, and the key attributes of the local biota. There is therefore no normative or standard approach to address ecosystem impacts and these have to be looked at on a case-by-case basis. In addition the acceptability of ecosystem changes will vary with the nature of human societies, cultures, and expectations.
The Economic and Social Implications of Ecosystem Impacts. Because natural ecosystems fulfil functions and yield a range of services that are of substantial economic and cultural value to society, the ecosystem changes that result from the creation of dams lead in turn to substantial economic and social impacts. Entire communities depend on the functions provided by freshwater wetlands, yet it is still difficult to translate the value into monetary terms. As a result the value of ecosystem functions is not properly accounted for in conventional market economics, and the value of these functions and the cost of their loss, is excluded from the economic decision-making process.
This externalisation of costs is a major factor leading to the loss of natural ecosystems. By reducing or eliminating access to resources flooded by the reservoir, through degradation and loss of agricultural and grazing resources on downstream floodplains, and through loss of riverine and coastal fisheries dependent upon the river flood, many dams have very high external costs. Policy-makers need to identify the value of this loss of welfare and implement financial and institutional mechanisms to assimilate these costs into the accounting structure.
The review stresses however that, even when these steps are taken, the valuation of ecosystems and the consideration of development options is not a straightforward accounting exercise. It needs to be recognised that not all ecosystem values can be expressed in economic terms. Ethical and societal considerations also need to be included. The monetary value serves as an input to multi-criteria decision-making and raises awareness of costs that are currently hidden and negated in the accounting exercise.
Responding to the Ecosystem Impacts of Dams. There are four principal categories of measures that may be incorporated into dam design or operating regime in order to respond to the environmental impacts identified through an EIA. These are: i) measures that avoid anticipated adverse effects of a dam; ii) mitigation measures that are incorporated into a new or existing dam design or operating regime in order to eliminate, offset or reduce ecosystem impacts to acceptable levels; iii) measures that compensate for existing or anticipated adverse effects that cannot be avoided or mitigated; iv) de-commissioning of the dam and restoration of the riverine ecosystem.
Within this framework of avoidance, mitigation, compensation and restoration, there are a wide range of specific measures that can be taken appropriate to specific circumstances of each dam. The Thematic Review evaluates experience in each approach and reveals that the most widely used approach, mitigation, is problematic. It concludes that there are always residual impacts that cannot be mitigated, simply by the nature of the dam’s impact on ecosystems themselves. Whether these impacts are significant varies from case to case.
While there is experience of good mitigation, this success is nevertheless contingent upon stringent conditions of:
• a good information base and competent professional staff available to formulate complex choices for decision-makers; • an adequate legal framework and compliance mechanisms; • a co-operative process with the design team and stakeholders; • monitoring of feedback and evaluation of mitigation effectiveness, and • adequate financial and institutional resources; This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
Dams, Ecosystem Functions, and Environmental Restoration vi
If any one of these conditions is absent, then the ecosystem values are likely to be lost. In practice the extent to which these conditions are met varies enormously from country to country and dam to dam. The review therefore concludes that mitigation, though often possible in principle, has many uncertainties attached to it in field situations and is therefore at present not a credible option in all cases and all circumstances. In addition the weaknesses of the EIA process for many projects (cf Thematic Review V.2) reduce the possibilities for positive outcomes. This would tend to encourage a strategy of avoidance and minimisation rather than one of mitigation if the aim is to maintain biodiversity and ecosystem functions and services for the foreseeable future. Alternative tools for maintaining ecosystem health therefore need to be pursued.
The review argues that improved scientific predictive capacity and improved institutional and human capacity will take several decades. In the short term therefore focused attention needs to be given to the development and application of effective tools that can allow environmentally sound development of river water resources and the management of dams within this context. Three such tools are described: i) Indicators for Hydro-project selection; ii) Indicators of Ecological Integrity; iii) Environmental Flow Requirements.
Trends in the International Debate/Approach to Dams. The Thematic Review examines current trends in the international debate over dams and their environmental impacts. It concludes that considerable steps have been taken to address the environmental concerns and that there are today many areas of broad agreement between those who are generally supportive of building dams and those who are generally philosophically opposed to large dams. However differences remain. At the most general level these differences concentrate on the value systems adhered to by the different groups involved and especially the value to be attached to the intrinsic value of nature. This highlights the importance of ensuring that project approval be based on multi-criteria decision- making, not just economic cost-benefits analyses or on a purely eco-centric view of the world. Techniques also need to be improved to offer better methods of economic valuation that are acceptable to both proponents and opponents of dams. Clearer guidelines on how costs and benefits can be distributed among those people affected by a dam may necessitate the establishment of appropriate institutions to promote equitable water use, especially between upstream and downstream ecosystems and livelihoods.
The Review argues that most success in bridging the differences outlined is likely to be made by strengthening options assessment and the evaluation of the true cost and benefits of projects for the short and medium term. Discrepancies are likely to remain on value systems and development paradigms for decades to come. Therefore efforts to deal with environmental impacts of dams should concentrate on developing legitimate and accepted processes for dam planning, design and management within the river basin context. Secondly, much effort could be invested in improving the economic tools for analysis and improving incentives for better dam design and operation.
Policy Recommendations. The review concludes by providing ten policy recommendations to the WCD.
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
Dams, Ecosystem Functions, and Environmental Restoration vii
Table of Contents
List of Figures...... x
List of Tables ...... xi
List of Boxes...... xii
1. Introduction ...... 1
2. River Basin Ecosystems and Biodiversity ...... 3 2.1 Introduction ...... 3 2.2 Why Ecosystems are Valuable ...... 4 2.2.1 Ecosystems as Regulators...... 5 2.2.2 Ecosystems as Habitats...... 6 2.2.3 Ecosystems as Providers of Resources...... 7 2.2.4 Ecosystems as Providers of Information ...... 8 2.3 The value of ecosystem goods and services ...... 8 2.3.1 Economic valuation techniques ...... 8 2.3.2 The Monetary Value of Freshwater Ecosystems ...... 9 2.4 Ecosystems and River Basin Development ...... 10 2.5 International and national recognition of ecosystem values...... 11 2.6 Conclusions ...... 13
3. Ecosystem Impacts of Large Dams ...... 14 3.1 Introduction ...... 14 3.2 Scale and Variability of Impacts...... 14 3.3 Framework for Analysis ...... 18 3.4 Information Constraints...... 20 3.5 Upstream Impacts ...... 21 3.5.1 First-Order Impacts on Key Parameters ...... 21 3.5.2 Second Order Impacts – Changes in Primary Production ...... 23 3.5.3 Third-Order Impacts on Fauna ...... 25 3.6 Downstream Impacts on Rivers, Floodplains and Deltas ...... 26 3.6.1 First-Order Impacts on Ecosystem Driving Variables...... 27 3.6.2 Second Order Impacts on Primary Production ...... 33 3.7 Third-Order Impacts on Fauna ...... 36 3.7.1 Freshwater Species Diversity Changes...... 36 3.7.2 Bivalve and Gastropod Molluscs...... 40 3.7.3 Impact of Dams on Fish Diversity...... 42 3.7.4 Dams and Waterbirds ...... 44 3.8 Cumulative Impacts of Dams ...... 46 3.8.1 Conceptual Framework for cumulative impact assessment...... 47 3.8.2 Case studies on cumulative impacts ...... 48 3.9 Estimating the Costs of the Impacts of Dams on Ecosystems...... 51 3.9.1 Externalities and Livelihoods ...... 52 3.9.2 Trade-offs between Economic and Ethical Considerations...... 52 3.10 Conclusions ...... 53
4. Responding to the Ecosystem Impacts of Dams ...... 55
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
Dams, Ecosystem Functions, and Environmental Restoration viii
4.1 Introduction ...... 55 4.2 Types of Response...... 55 4.2.1 Avoidance...... 56 4.2.2 Mitigation ...... 58 4.2.3 Compensation ...... 61 4.2.4 Dam Decommissioning and River Restoration ...... 62 4.3 How Effective is Mitigation?...... 64 4.3.1 The Example of Fish Ladders...... 66 4.3.2 Why Avoidance, Mitigation, and Compensation are Difficult...... 68 4.4 How to Make Mitigation More Effective? ...... 69 4.4.1 Indicators for Hydro-Project Site Selection...... 70 4.4.2 Indicators of Ecological Integrity ...... 71 4.4.3 Environmental Flow Requirements (EFRs)...... 72 4.5 Conclusions ...... 73
5. Trends in the International Debate/Approach to Dams ...... 75 5.1 Introduction ...... 75 5.2 Summary of the debate ...... 75 5.3 Summary of Trends ...... 76 5.3.1 IEA 76 5.3.2 International Commission on Large Dams (ICOLD) ...... 78 5.3.3 The World Bank ...... 79 5.3.4 New approaches of the Organisation for Economic Co-operation and Development (OECD) ..80 5.3.5 The International Movement Against Large Dams ...... 80 5.3.6 Requirements of International Conventions ...... 81 5.4 Areas of Convergence/Divergence ...... 82
6. Conclusions and Policy Recommendations for WCD ...... 84 6.1 Conclusions ...... 84 6.2 Recommendations...... 85 6.3 Options for Operationalising the Recommendations...... 86
7. References ...... 89
Annex 1: Potential Environmental Impacts of Dams, Reservoirs and Hydroelectric Projects ...... 100
Annex 2: Reservoir Fisheries ...... 104
Annex 3: Comparison of Pre vs. Post Impoundment Conditions...... 107
Annex 4: Sediment Discharges ...... 108
Annex 5: Large Dam Projects: Adverse Environmental Impacts and Mitigation Options...... 111
Annex 6: Environmental Flow Requirements (EFR) ...... 114
Annex 7: Example of Mitigation Measures ...... 116
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
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Appendix I – List of Contributing Papers to Thematic Review II.1...... 117
Appendix II - Submissions for Thematic Review II.1 ...... 119
Appendix III – Comments Received for Thematic Review II.1 Dams, Ecosystem Functions & Environmental Restoration ...... 129
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
Dams, Ecosystem Functions, and Environmental Restoration x
List of Figures
Figure 2.1: General description of the inter-relationships between a river basin and the water cycle (after Shiklomanov 1999)...... 4 Figure 2.2: Cumulative number of countries ratifying the main environmental Conventions(World Heritage (1), Ramsar Convention on wetlands (2), trade in endangered species (3), biological diversity (4), and climate change (5))...... 12 Figure 3.1: Distribution of reservoir area for dams over 15m high in Turkey(DSI, 1999)...... 15 Figure 3.2: Total area of large-dam reservoirs (1000's km2) by region (ICOLD 1999)...... 16 Figure 3.3: Average area of large dam reservoir (km2) per region(ICOLD 1999)...... 16 Figure 3.4: Comparison of pre and post impoundment flows in the Murray River, Australia:variation in the average monthly flow at a) Albury (2225 km from the mouth) and b) at Barrages (1 km from the mouth). Source: Murray-Darling Basin Ministerial Council, 1995...... 28 Figure 3.5: Daily Streamflow Variations in the Colorado River at Lee's Ferry in September. Peak flows are associated with the power generation between 14.00 and 19.00 daily, with minima at 04.00 am, and the fluctuation in demand also varies from day to day...... 29 Figure 3.6: Fish species richness decreases at higher latitudes indicating that dam construction in tropical regions could potentially have more impacts than at higher latitudes (WCMC 1998)...36 Figure 3.7: Fragmentation of rivers in 225 basins in the world (Source: Nilsson et al. 2000)...... 46 Figure 3.8: Dams in the river systems of Sweden. Only four major rivers remain undammed...... 47 Figure 3.9: The impact of dams on the hypothesised downstream pattern exhibited by a given river characteristic in situationa) with no cumulative impacts and b) with cumulative impacts (modified from Ward and Stanford, 1995)...... 48 Figure 3.10: a) Cumulative useable storage in reservoirs in the Platte River basin and b) associated cumulative change in island and channel area ...... 50 Figure 3.11: a) Longitudinal and altitudinal profile of the Gunnison River and b) changes in speciosity and biomass with distance downstream...... 51 Figure 4.1: Number of dams removed in the USA, as a function of a) dam height and b) year of removal (after, Doyle et al., 2000)...... 63 Figure 4.2: Figure 4.1: Distribution of EMP preparation & environmental problem evaluations for dam projects in Latin America co-financed by the IDB from 1960-1999(IDB 1999)...... 65
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
Dams, Ecosystem Functions, and Environmental Restoration xi
List of Tables
Table 2.1: Natural ecosystems provide many goods and services (functions) to humankind that are often neglected in (economic) planning and decision making...... 6 Table 2.2: Global monetary values of freshwater and wetland functions (in US$ billion, 1994)...... 9 Table 3.1: The potential scale of the impacts of dams...... 16 Table 3.2: Some databases of dams ...... 17 Table 3.3: Upstream and downstream impacts according to first, second, and third order as described...... 18 Table 3.4: A framework for assessing the impact of dams on river ecosystems(modified from Petts, 1984)...... 19 Table 3.5: Fragmentation of rivers in 225 basins in the world (Source: Nilsson et al. 2000)...... 46 Table 5.1: Distillation of arguments used by proponents and opponents of large dams...... 77 Table 5.2: Trends in the Planning of Hydropower Projects (IEA, 2000)...... 78 Table 6.1: Options for establishing sub-principles under each recommendation ...... 87
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
Dams, Ecosystem Functions, and Environmental Restoration xii
List of Boxes
Box 1.1: Why large dams are built ...... 1 Box 2.1: Important ecological concepts of rivers and floodplains...... 3 Box 2.2: Why biological diversity is important...... 7 Box 2.3: Watershed Management...... 10 Box 2.4: International recognition of ecosystem values ...... 12 Box 3.1: Types of dams, in descending order of impacts on ecosystems...... 15 Box 3.2: Construction impacts...... 20 Box 3.3: Invasive species and large dams...... 24 Box 3.4: Species richness of the planet’s major environments (Source: McAllister et al. 1997)...... 37 Environment % area of % of known Relative species...... 37 Box 3.5: Global hotspots for freshwater molluscs...... 40 Box 3.6: Mollusc species present within reservoir region, USA (Source: Neves, 1999) ...... 41 Box 3.7: Fish species richness in selected river basins (after: World Bank 1998, WCMC 1998)...... 43 Box 3.8: Dams as Wildlife Habitats...... 45 Box 3.9: Example of cumulative affect on first order impacts (i.e. the hydrology) of the Murray River, Australia (after Maseshwari et al., 1995)...... 49 Box 3.10: Example of cumulative affect on second order impacts (i.e. the geomorphology) of the Platte River, USA (after Hadley et al., 1987) ...... 50 Box 3.11: Example of cumulative affect on third order impacts (i.e. zoobenthos) in the Gunnisson River, USA (after Hauer et al., 1989)...... 51 Box 3.12: Ethical principles for decision-makers involved in water and energy planning (di Leva 1999) ...... 53 Box 4.1: Demand management, water recycling and rainwater harvesting: examples of ...... 57 Box 4.2: Avoidance of impacts on sensitive species during blasting ...... 58 Box 4.3: Decommissioning of the Edwards Dam, USA...... 63 Box 4.4: Possible consequences for salmon of removal of the Elwha Dam, Washington (after Doyle et al., 2000) ...... 64 Box 4.5: Improving fish passage design to make them work better ...... 67 Box 4.6: Why fish passes may fail...... 67 Box 4.7: Environmental Indicators To Guide Site Selection...... 70 Box 4.8: Indicators of Ecological Integrity...... 71 Box 4.9: Case study: The Colorado River ...... 72 Box 4.10: Case study: Kromme River ...... 73 Box 5.1: The Curitiba Declaration...... 81 Box 5.2: Ramsar Convention: Guidelines for Contracting Parties relating to reducing the impact of water development projects on wetlands ...... 82 Box A2.1: Fisheries yields of selected reservoirs...... 104
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
Dams, Ecosystem Functions, and Environmental Restoration 1
1. Introduction
Dams, large and small, are planned, constructed and operated to meet human needs in the generation of energy, irrigated agricultural production, flood control, supply of drinking water, and various other purposes. While seen originally as a relatively straightforward solution to many of these needs, the history of dams over the past 100 years has shown that their many benefits to society come together with an array of environmental and social costs. The decision to construct a dam and the design and operation of its management regime therefore need to be based upon a rigorous analysis of these costs and benefits.
There is today widespread recognition of this challenge amongst governments, industry, the development assistance community, NGOs, community groups, and many others concerned with the issues of large dams. Indeed over the course of the past 10 years there has been substantial change in the approach to dams with much greater attention to environmental and social issues. Various types of environmental and social guidelines now exist and are increasingly applied.
Differences in judgements towards dam development are based on different value systems, development paradigms, options analysis and practical actions. At the level of value systems, the dilemma focuses on whether ‘environmental conservation’ and ‘development’ are antagonistic. Within the environmental conservation movement one can distinguish ‘conservationists’ and ‘preservationists’ (Norton 1991). Conservationists see natural ecosystems and species as resources and are concerned mainly with the wise use of them. ‘Preservationists’ on the other hand are committed to protecting large areas of landscape from any human alterations. In a simplified way, the development community can be divided in those seeking ‘development per se’ and those that are looking for ‘sustainable development’. Obviously the preservationist and those seeking ‘development per se’ adhere to a different development paradigm. For conservationists and those seeking sustainable development, paradigms often lie close to each other, with the debate focussing on what constitutes “acceptable” change.
The majority of dams (75%) are developed and operated to irrigate land and generate power or are used for both (Box 1.1). In many cases, these dams have provided profits for a range of beneficiaries. At the same time, dams have negatively impacted affected people and the environment. As such the development of water resources using dams has created many conflicts of interest and it is becoming increasingly clear that environmental and social dimensions need to be addressed more substantially.
Box 1.1: Why large dams are built
Irrigation only 37% Multi-purpose 22% Electricity generation only 16% Water supply only 12% Flood control only 6% Recreation only 3% Other 4%
TOTAL: 100%
(Source: ICOLD World Register of Large Dams, 1998)
Despite this progress there remain significant and widespread concerns about the environmental impacts of dams at the more practical level linked with option analysis and practical actions. The conservationist view in short argues that dams, even when designed to minimise environmental impacts, result in significant negative impacts to a wide range of natural ecosystems and to the people
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
Dams, Ecosystem Functions, and Environmental Restoration 2
that depend upon them for their livelihood. At a time when pressures upon the diversity and productivity of the world’s natural resources continue to rise, it is argued that firm action is required to prevent loss of these resources through further dam construction (McCully, 1996). In response, those engaged in the planning, construction and operation of dams argue that with continuing improvements in knowledge and technology it is increasingly possible to avoid, mitigate or compensate for the environmental impacts of dams, so yielding win-win solutions in most cases (ICOLD, 1997).
As a contribution to improved decision-making about large dams, this report examines the nature of the effects of dams upon upstream and downstream ecosystems and the reported experience with methods to avoid, mitigate and compensate those effects. It does not attempt an exhaustive assessment of the impact of dams on ecosystems world-wide as data for this task are currently unavailable. The focus of the report is deliberately on the medium to long-term impacts of dams on ecosystems rather than the short term impacts of construction.
The paper approaches these issues by first examining the nature of river basin ecosystems, asking why they are important and why the international community has signed up for promoting their protection within the framework of sustainable development. It then reviews current understanding of the nature of the impact of dams upon these ecosystems and their associated values. This includes both their intrinsic spiritual, ethical and biodiversity values and their economic values to local people and their livelihoods, as well as wider ecosystem values for society as a whole.
The report then examines the current status of approaches to addressing the consequences of dam impacts on ecosystems through the continuum of “avoidance-mitigation-compensation-restoration” of ecosystem losses. Based upon this analysis the report concludes with an assessment of the areas of convergence and divergence on these issues within the dams debate and set of recommendations to the Commission.
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
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2. River Basin Ecosystems and Biodiversity
2.1 Introduction
Freshwater covers only 2.7% of the Earth’s surface, of which 66% by volume is in snow and ice, 29% in groundwater, 2.5% in lakes and rivers and less than 0.001% in reservoirs (Shiklomanov 1999). Rivers therefore cover a very small part of the Earth, yet they are intricately linked to the vast area of the planet that lies within river basins, as well as the coastal and near-shore marine ecosystems that are dependent on freshwater inputs. Rivers are central elements in many landscapes. Their string-like shape and dendritic drainage pattern mean they are effectively interspersed into the landscape despite their small total area (Nilsson and Jansson, 1995). They are important natural corridors for the flows of energy, matter and species (Malanson, 1993).
Viewed holistically, river- related ecosystems encompass all the components (both biotic and abiotic) of the environment linked to that river, including people. This includes not only the aquatic habitats associated with water in the river channel, but all the elements of the river catchment that contribute water, nutrients and other inputs to the river. Thus the complex of ecosystems that constitute a river basin includes: the headwaters and the catchment landscapes; the channel from the headwaters to the sea; riparian areas; associated groundwater in the channel/banks and floodplains; wetlands; the estuary and any near shore environment that is dependent on freshwater inputs. Each of these environments is dependent to a greater or lesser degree on connectivity with the active channel of the river and the ecological character of the main channel depends on the interactions with those environments (Petts and Amoros 1996) (Box 2.1).
The hydrological cycle provides an important linkage between the component parts (Figure 2.1). Another important linkage is formulated in the ‘flood-pulse’ concept which describes the periodic, two-way exchange of nutrients between the main river channel and riparian ecosystems (Junk et al. 1989, Bayley 1995, Sparks 1995) (Box 2.1). In order to understand the relationship between large dams and the rivers on which they are built, it is essential to understand the nature and values of the different ecosystems along a river’s course from its catchment to the sea.
Box 2.1: Important ecological concepts of rivers and floodplains
1. The “river continuum concept” encompasses the linkages upstream and downstream from a river’s source to the coastal zone, including any deltas or lagoon systems. This concept includes the gradual natural changes in river flows, water quality and species, that occur along the rivers length. Nutrients and sediment generated in the headwaters are recycled downstream, driving plant growth and biotic productivity. One of the most obvious characteristics of the river continuum concept is the migration of fish from the sea to spawning grounds in the headwaters. River engineering projects, such as dams, can break this continuum causing radical changes in flows, water quality and stopping the movement of species.
2. The “flood pulse” concept is based on the importance of lateral connectivity between rivers and their floodplains and sees the inundation of floodplains as the main driving force behind river life, not as a problem that needs eradicating. Rivers provide the floodplain with nutrients and sediment, whilst the floodplain provides a breeding ground for river species and improves water quality through settlement of sediment and absorption and re-cycling of nutrients and pollutants.
Reviews of river ecosystems can be found in a number of volumes, for example Petts (1984) and Davies and Day (1998).
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Figure 2.1: General description of the inter-relationships between a river basin and the water cycle (after Shiklomanov 1999).
2.2 Why Ecosystems are Valuable
Each ecosystem is composed of a number of physical, biological or chemical components such as soils, water, plant and animal species, and nutrients. Processes among and within these components allow the ecosystem to perform certain functions such as flood control and storm protection, and generate products such as wildlife, fisheries and forest resources. There are also ecosystem scale attributes such a biological diversity and cultural uniqueness/heritage, that have value either because they induce certain uses or because they are valued themselves. It is the combination of these functions, products, and attributes that make ecosystems important to society.
Whether a natural or man-made ecosystem performs a certain function, yields specific products, or possesses certain attributes, is determined by the interaction between chemical and physical characteristics of the site. Characteristics vary greatly between and within each major ecosystem group. Thus forests perform different functions from wetlands and amongst wetlands there is variation both in terms of the types of functions, and the degree to which they are performed.
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Ecosystem functions can be grouped into four categories (after De Groot, 1992): regulation functions, habitat functions, production functions, and information functions (Table 2.1). The following sections summarise examples of functions of river basin ecosystems under each of these categories.
2.2.1 Ecosystems as Regulators
Ecosystems along the course of a river serve both as regulators of water quantity and water quality. Several types of ecosystems, notably forests and wetlands, are known to act as hydrological buffers, absorbing water when it rains and releasing it gradually over several weeks and months. This not only helps to protect downstream communities from flooding, but helps to ensure that water continues to flow during the drier periods of the year. For example, the forest of La Tigra National Park (Honduras) sustain a well-regulated, high quality water flow throughout the year, yielding over 40% of the water supply of the capital city (Acreman and Lahmann, 1995).
Wetland ecosystems are able to reduce rates of water flow and store water above the surrounding water table (for example in a raised bog). The vegetation and hydrology enables the wetland ecosystem to function as a ‘sponge’ and provide the services of flood prevention and water storage. The value of these services may be considerable. Often technical alternatives to regulate the quantity of flow are much more expensive. New York City ensures the quality of its water supply through the protection of the biological and hydrological processes of the upper parts of the catchment on which the water supply depends. Building water treatment plants would cost ten times as much, US$ 7 billion (Abramovitz, 1997).
Ecosystems also regulate water quality. On sloping ground, for example, vegetation anchors soil and prevents it from being washed into the watercourse where it would cause siltation and nutrification and reduce light penetration. This would reduce water quality, the health of aquatic ecosystems and the suitability of the water for aquaculture and other uses. The physical structure of watercourses and the organisms that inhabit it also regulate water quality. For example, waterfalls, rapids and aquatic vegetation oxygenate the water, and riverbanks, river beds and vegetation trap sediment. These hydrological and biological processes enable the watercourse to function as a water purification unit providing fresh water.
Riverine wetlands play an important role in regulating water quality. They remove toxins and excessive nutrients from the water both by processes of decomposition and uptake by vegetation (Baker and Maltby, 1995). As wetlands hold water for long periods of time, decomposition processes and vegetation are given enough time to remove nutrients and toxins from the water. For example, vegetation found in the Melaleuca wetlands in SE Asia reduces the acidity of polluted water and removes toxic metal ions making the water suitable again for the irrigation of rice (Ni et al., 1997). In this way, the combination of hydrological and biological processes allows these wetlands to function as filtration and purification systems and to provide the service of water purification.
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
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Table 2.1: Natural ecosystems provide many goods and services (functions) to humankind that are often neglected in (economic) planning and decision making. (adapted from de Groot 1992).
REGULATION FUNCTIONS PRODUCTION FUNCTIONS
The capacity of natural and semi-natural Resources provided by natural and semi- ecosystems to regulate essential ecological natural ecosystems processes and life support systems
• Maintenance of biogeochemical cycling • Food (e.g. edible plants and animals) (e.g. air-quality regulation and CO2- • Raw materials (e.g. thatch, fabrics) buffering) • Fuel and energy (renewable energy • Climate regulation (e.g. buffering resources) extremes) • Fodder and fertiliser (e.g. krill, litter) • Water regulation (e.g. flood protection) • Medicinal resources (e.g. drugs, models, • Water supply (filtering & storage) test organisms) • Soil retention (e.g. erosion control) • Genetic resources (e.g. for crop resistance) • Soil formation & maintenance of fertility • Ornamental resources (e.g. aquarium fish, • Bioenergy fixation souvenirs) • Nutrient cycling (i.e. maintenance of the availability of essential nutrients) • Waste treatment (e.g. water purification) • Biological control (e.g. pest control and INFORMATION FUNCTIONS pollination) Providing opportunities for reflection, spiritual enrichment and cognitive development
• Aesthetic information (e.g. valued scenery) • Recreation and (eco-) tourism HABITAT FUNCTIONS • Religious and cultural values • Cultural & artistic inspiration (i.e. nature Providing refugia to wild plants and animals as a motive and source of inspiration for (and native people) in order to maintain human culture and art) biological and genetic diversity • Spiritual and historic information (based on ethical considerations and heritage values) • Refugium function (for resident & • Scientific educational information (i.e. migratory species) nature as a natural field laboratory and • Nursery function (reproduction habitat for reference area) harvestable species)
2.2.2 Ecosystems as Habitats
Riverine floodplains and river courses, together with their catchments are important habitats for many species of plants, fish, birds and others animals. Wetlands areas are known as prime areas for biodiversity conservation and as important nursery and feeding areas for many aquatic and terrestrial migratory species. In contrast to their fringing wetlands, the main watercourses of rivers function as habitats for animals that require fast-flowing oxygen-rich water. Together, freshwater ecosystems support over 10,000 species of fish and over 4,000 species of amphibians described so far. Freshwaters support a relatively high proportion of species, and more per unit area than other
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environments; 10% more than land and 150% more than oceans (McAllister et al., 1997; WCMC, 1998).
Coastal deltas and the estuaries of the major rivers, are also important providers of habitats. They provide food and shelter for marine animals that require freshwater conditions for part of their life cycle. Consequently, these coastal wetlands function as habitats for crabs, oysters and shrimp, and provide the service of supporting fisheries based on these goods. For example, it has been calculated that over 90% of the shrimp harvest of the Gulf of Panama is dependent upon the estuaries and mangroves of the region (D’Croz and Kwiecinski, 1980) that are, in turn, dependent on fresh water inflows.
Box 2.2: Why biological diversity is important
Biological diversity, or biodiversity, is a measure of the variability of genes, species, and ecosystems in a region. This region can vary from a small forest patch to a sub-contintent. Biodiversity is important because plants and animals have made our planet fit for the forms of life we know today. They help maintain the chemical balance of the Earth, stabilise climate, protect watersheds and renew soil. All societies continue to draw on a wide array of ecosystems, species and genetic variants to meet their ever-changing needs. The diversity of nature is a source of beauty, enjoyment, understanding, and knowledge. It is the source of all biological wealth, supplying all our food, much of our raw materials, and a wide range of goods and services and genetic materials for agriculture, medicine and industry worth many billions of dollars per year. Biological diversity should be conserved as a matter of principle, because all species deserve respect regardless of their use to humanity, and because they are all components of our life support system.
Prudence dictates that we keep as much biodiversity as possible, but the trend is steadily downward, as more habitats are converted to exclusively human uses. While we are still uncertain about how many species now exist, leading experts calculate that if present trends continue, up to 25% of the world's species could become extinct, or be reduced to tiny remnants, by the middle of the next century. Many more species are losing a considerable part of their genetic variation, making them increasingly vulnerable to pests, disease, and climatic change.
2.2.3 Ecosystems as Providers of Resources
Many riverine ecosystems provide large quantities of water, food and energy for direct human consumption, agriculture, fisheries, watering livestock, industry and energy production. Harvesting these goods while respecting the production rate and the regenerative capacity of each species can generate great benefits to human society. One of the most important products of riverine ecosystems is fish. In many areas, river-dependent fisheries form a fundamental pillar of the local and national economy.
Direct harvest of forest resources of many floodplains also yields important products, ranging from fuelwood, timber and bark to resins and medicines, which are common non-wood ‘minor’ forest products (Dugan, 1990). Wildlife provides important commercial products such as meat, skins, eggs and honey. Extensive riverine floodplain ecosystems also support substantial seasonal grasslands that are grazed by livestock. For example the Brazilian Pantanal supports over 5 million cattle (Adamoli, 1988). Wetlands also contain a large genetic reservoir for certain plant species, fish and other animals. For example, wild rice continues to be an important resource of new genetic material used in developing disease resistance and other desirable traits.
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
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2.2.4 Ecosystems as Providers of Information
Water-based ecosystems provide many opportunities for recreation, aesthetic experience and reflection. Recreational uses include fishing, sport hunting, birdwatching, photography, and water sports. The economic value of these can be considerable. For example in Canada the value of wetland recreation was estimated in 1981 to exceed US$ 3.9 billion (Dugan, 1990). Maintaining the wetlands and capitalising on these uses can be a valuable alternative to more disruptive uses and degradation of these ecosystems. They are important repositories and stores of palaeontological information. Under anaerobic conditions biological material such as pollen, and diatoms and even human bodies can be preserved in peats and lake sediments.
In addition, many people gain spiritual or aesthetic benefits from visiting, appreciating and experiencing free-flowing rivers. The symbolism of nature untamed, the bubbling of mountain streams and the majesty of lowland rivers can be an uplifting personal experience, and also provides inspiration for literature and music.
2.3 The value of ecosystem goods and services
We all depend on functioning ecosystems for our survival. For many of the World’s poorest people the biological resources of river ecosystems often provide the single most important contribution to their livelihoods and welfare in the form of food supplies, medicines, income, employment and cultural integrity. Such communities often have limited alternative livelihood options and this makes them particularly vulnerable to changes in the condition of the natural resources on which they depend.
2.3.1 Economic valuation techniques
Attempts to quantify economic values for ecosystems have been made, both at the micro (Echeverria et al., 1995; Sullivan, 1999) and at the macro levels (Costanza et al., 1997; Alexander et al., 1998). These have demonstrated that replacement costs for ecosystems and their functions are likely to be far higher than the opportunity cost of maintaining the natural system intact. However they have highlighted the difficulties faced by those trying to assess environmental values in monetary terms. The complexity of such systems in particular makes accurate assessments very difficult, since feed- back effects and interactions are not yet fully understood.
While immature, the science of valuation of ecosystems has, however, allowed a clear distinction to be made between: i) ecosystem products that can be sold on the market (and for which prices exist, revenue is generated and jobs maintained); ii) non-marketable services (such as water quality maintenance or groundwater recharge) that are more difficult to price in evolving circumstances; and iii) intrinsic values such as the beauty of natural landscapes. Less clear guidance is available on how to price "free services", particularly in developing economies that need to generate real, not virtual, incomes and where society may value these services more highly 20 years hence as the economy develops. The assessment of intrinsic, cultural and aesthetic values cannot usually be addressed in monetary terms in the same way, as there is no replacement when these values are lost. These values are therefore usually addressed through a political or ethical process rather than a process of economic valuation.
Some current valuation techniques are based on preferences, and money provides a measure of value. Preferences are both subjective and dynamic, and measurement of such ‘fuzzy’ variables is a difficult task. The fact that preferences are subjective means that different groups within society are likely to have different values, and this demonstrates the importance of consulting a wide variety of stakeholders when considering the question of environmental values.
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It should be stressed that this review does not see the valuation of ecosystem services as a straightforward accounting exercise where the calculated value is simply added to the cost:benefit balance sheet. As the ecosystem dimensions of dam projects also include ethical and societal values, the monetary value serves as an input to multi-criteria decision-making and raises awareness of costs that are currently hidden and negated in the accounting exercise.
2.3.2 The Monetary Value of Freshwater Ecosystems
Many functions of freshwater ecosystems and wetlands have direct and indirect economic importance. Entire communities depend on the functions provided by freshwater ecosystems and, as such, ecosystems have enormous value. It is still difficult to translate this value into monetary terms, leading to the continuing loss and degradation of water systems due to undervaluation and neglect in economic accounting procedures. In Nigeria, for example, it has been shown that the net economic benefits of the Hadejia-Nguru floodplain ecosystem are much larger than those from irrigated land: US$ 32 versus US$ 0.15 per 1,000m3 of water, not including benefits of floodplain inundation for groundwater recharge and water supply to the productive ecosystem of Lake Chad (Adams 1992).
A first attempt to synthesise existing knowledge on the monetary benefits of the services of ecosystems on a global scale was published in 1997 (Costanza et al. 1997). Table 2.2 gives a summary of the main functions, and monetary values, of freshwater and wetland ecosystems.
Table 2.2: Global monetary values of freshwater and wetland functions (in US$ billion, 1994). (functions based on de Groot 1997; values based on Costanza et al. 1997).
Ecosystem functions (goods & services) Active or direct Passive or Per cent of use values indirect use Global Total (mainly values (for a market prices) (mainly shadow particular price) function) 1. REGULATION FUNCTIONS 1.1 Climate regulation & biogeochemical ? 44 3 % cycling (e.g. CO2) 1.2 Water buffering (e.g. flood prevention) ? 350(a) 40 % 1.3 Waste treatment ? 5,300 31 % 1.4 Biological control ? 14 3 % 2. HABITAT FUNCTIONS 2.1 Refugium function ? (c) (c) 2.2 Nursery function 62 62(a) 100 % 3. PRODUCTION FUNCTIONS 3.1 Water 840 840(a) 99 % 3.2 Food (mainly fish) 186 (b) 13 % 3.3 Raw materials & energy 40 (b) 6 % 3.4 Genetic material & medicines (d) (d) (d) 4. INFORMATION FUNCTIONS 4.1 Aesthetic information (e.g. views) ? 5 2 % 4.2 Recreation and tourism 304 (b) 37 % 4.3 Cultural values (e.g. art, science) (d) (d) (d) Total (in US$ billion/year) 1,782 + 6,905 Average 26%
Notes: (a) The total value of the flood prevention, nursery function and water supply given in Costanza et al. (1997) was based on a combination of market and shadow prices. For simplicity, it has been estimated that 50% of the calculated value is included in market prices. This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
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(b) The values given for food, raw materials and tourism are based only on market prices. However, these resources also have an unknown (direct) consumptive use value (many people depend on freshwater systems for these resources directly, without market intervention). (c) In addition to active and passive use values, many ecosystem functions have so-called non-use or intrinsic value. In this study it is not attempted to place a monetary value on the intrinsic importance of nature but it could, in part, be derived from the money people are willing to spend to maintain the refugium function of natural ecosystems. (d) Freshwater and wetland systems are important sources of genetic material, medicines and cultural values but little or no information is available on the monetary value of these ecosystem functions.
This analysis shows that, world-wide, freshwater and wetland systems account for approximately 26% of the total economic value of all ecosystem services (which vary substantially by function, as the last column shows). It can be concluded that still only about 20% (US$ 1,782 billion) of the economic value of coastal and freshwater systems is accounted for in market pricing mechanisms. All other values, which mainly relate to regulation and habitat functions, are not yet (properly) accounted for.
2.4 Ecosystems and River Basin Development
The central issue of river basin development is to decide how to allocate water to maximise the benefits it provides to society as a whole. In the past little consideration was given to the importance of ecosystems and their multiple values to society. Very regularly these were overlooked when single- use developments were predominantly considered. Today, in some societies high value is placed on sustaining healthy “pristine” river ecosystems because they are believed by many to have an intrinsic value in themselves. Human – nature interactions within river basins are so strong that the system as a whole is the logical level for environmental and water management measures.
The sustainable development of river basins requires the development and implementation of management plans at the level of the entire basin (Newson 1997, Mostert 1999). The conservation of valuable ecosystem goods and services forms an essential element in these (Box 2.3). To manage a river basin implies to optimally allocate scarce resources among competing users now and in the future. This requires political will, accurate information and knowledge of the basin, sustainable technologies, appropriate institutional and legal arrangements, stakeholder participation and economic viability (Burton 1999). There is no single best approach for river basin development as each basin is unique in its configuration and state of development.
Box 2.3: Watershed Management
Watershed management programs generally include a variety of subprograms designed to reduce erosion through establishment or expansion of protected areas, improved management of protected areas, restoration and rehabilitation of forest or other biotypes, and the introduction of improved agricultural technologies or alternative types of production. One of the more promising approaches is to introduce agroforestry practices that have the dual benefit of increasing forest cover in the basin and replacing existing agricultural practices with cultivation and promotion of non-destructive, but economically beneficial use of those resources.
As the importance of the inter-relationship between dam development and the surrounding watershed has become more fully realised, measures to protect and manage the watershed are being promoted in association with the construction of new dams. These include management of agricultural, urban and natural areas throughout the basin. While initially viewed as compensation to ameliorate the negative impacts of dams (section 4.2.3), these measures for protecting and enhancing environmental resources in the basin are increasingly seen as also sustaining the operational life of the project.
In 1996 the Compania Nacional de Fuerza y Luz, a private utility in Costa Rica (but owned by the state utility), started a management program for the upper watershed (142 km2) of its Brasil hydropower plant on the Virilla River. The project consists of reforestation, forest conservation, and
This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
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environmental education components, and will cost US$9 million over a ten-year period. It is expected that such a change in land use management upstream of the dam will improve the water regime by 2%, therefore resulting in an increased power production of 9%. The reforestation component will change the land use from pastures to tree plantations in 1,250 ha, while an additional 1,250 ha will be managed as agroforestry systems. 3,400 ha of forest will be conserved, which together with the plantations will yield the additional benefit of the fixing of 584,000 tonnes of carbon (Mora 2000/ENV223).
Dams have been an important technology in river basin development. Planning for new dams and upgrading of existing dams should be carried out within the context of river basin development and management plans. National sector reviews, plans and efforts for the implementing of an integrated water resources management approach need to be taken into account in dam development and management.
The WCD’s Thematic Review V.3, “River Basins- Institutional Frameworks and Management Options” deals with this subject in more detail. Also, Thematic Review V.1 “Planning Approaches” also looks at the planning level of the dam building decision process.
2.5 International and national recognition of ecosystem values
During the past two decades, legal experts have attempted to understand and clarify the basic concept underlying the governing principles regarding respect for all forms of life. The three resulting ‘over- arching’ principles are to be read in conjunction with principles regarding human needs – development and poverty eradication. Di Leva (1999) defines the three principles as:
1. Recognise that the enjoyment of basic human rights is dependent on the continued existence of a ecologically sustainable natural environment; 2. Recognise that decisions impact on future generations which have inherent rights (inter – generational equity); 3. Respect all life forms independent of their value to humanity (UN 1982).
To date there is a growing recognition of these values (Box 2.4). At the international level, this has lead to the development of a UN Charter for Nature and a range of environmental conventions to protect species and specific ecosystems. The UN Charter for Nature (UN 1982) was adopted by consensus by the UN General Assembly to provide a high-level guiding principle to govern humankind’s responsibility for nature conservation and management. The Charter has several principles. One of these includes ‘Ecosystems and organisms as well …. resources utilised by men shall be managed to achieve and maintain optimum sustainable productivity, but not in such a way as to endanger the integrity of those ecosystems or species with which they coexist.’
As Figure 2.2 illustrates the attention given to ecosystem conservation by national governments from all parts of the developing and developed world, as reflected in the signatures of international treaties, has increased markedly in recent years. For example, The Convention on Wetlands (Ramsar 1971) has been ratified by 110 contracting parties and 177 countries have ratified the Convention on Biological Diversity to date, that is 96% of all UN-recognised countries in the world (185) (UN-CBD 2000). This argues strongly that every effort should be made to avoid irreversible loss of resources that are likely to become more valuable to all societies in future.
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Box 2.4: International recognition of ecosystem values
Contracting Parties give the following motivations for respecting ecosystem values: “…the intrinsic value of biological diversity and of the ecological, genetic, social, economic, scientific, educational, cultural, recreational and aesthetic values of biological diversity and its components…”
“…the importance of biological diversity for evolution and for maintaining life sustaining systems of the biosphere…”
“…that conservation and sustainable use of biological diversity is of critical importance for meeting the food, health and other needs of the growing world population, for which purpose access to and sharing of both genetic resources and technologies are essential…”
Convention on Biological Diversity of June 5, 1992 ______“…that wetlands constitute a resource of great economic, cultural, scientific, and recreational value, the loss of which would be irreparable…” Convention on Wetlands of International Importance Especially as Waterfowl Habitat-The Ramsar Convention of 1971
Figure 2.2: Cumulative number of countries ratifying the main environmental Conventions(World Heritage (1), Ramsar Convention on wetlands (2), trade in endangered species (3), biological diversity (4), and climate change (5)).
Currently, national governments and civil society are faced with the challenge of deciding how the continuous process of the implementation of these conventions can be strengthened. Often this relates to strengthening national legislation and its implementation / enforcement. An important practical issue to resolve is defining how much water should be used for the maintenance of ecosystems to provide environmental goods and maintain elemental services. Recent changes in the South African
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law defined specific allocations for maintaining river flows for ecological reasons. The basic human needs and environmental requirements are now identified as ‘The Reserve’ and have priority of use by right (Asmal 1998). Increasingly, a range of techniques are available to determine the requirements of downstream ecosystems (King et al. 1999).
This has to be determined along with how much water should be used to support agriculture, industry and domestic services to provide basic goods. Obviously, the value that society places on these alternative goods and services will determine the pattern of allocation. It is important therefore that the costs and benefits to society of allocating water to maintain ecosystems or to support agriculture, industry and domestic uses are well understood.
2.6 Conclusions
Ecosystems provide goods and services to human society. These have high values and provide the basis for sustainable livelihoods. The goods and services these systems provide, such as food, timber, fisheries and drinking water, form an important natural resource base for many societies throughout the world. The total global value of ecosystem goods and services is estimated at US$ 33 trillion per year, of which roughly 25% relates directly to freshwater ecosystems. Freshwater ecosystems are also known to regulate water quality and quantity and to provide habitats for tens of thousands of species.
To maintain natural ecosystem goods and services it is essential to conserve and sustainably manage species and ecosystem processes. Together they form the integrity of healthy ecosystems. For the maintenance of healthy river ecosystems, the integrated management of land and water resources is required within an entire river basin. Dams therefore cannot be an objective in themselves, but should be seen as a tool that should be used with great care and prudence.
A large majority of sovereign states have committed themselves to conservation of nature through a range of international conventions and national legislation and policies, as exemplified by the extensive ratification of the five major international conventions on nature conservation. The Convention on Biological Diversity alone is ratified by over 96% of all countries. The effective implementation of these, however, often remains weak in relation to dam development.
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3. Ecosystem Impacts of Large Dams
3.1 Introduction
Dams are structures designed to store or divert water. They are intended to alter the natural distribution and timing of streamflows in order to meet human needs. As such, they also alter essential processes for natural ecosystems. Dams constitute obstacles for longitudinal exchanges along rivers. By altering the pattern of downstream flow (i.e. intensity, timing and frequency), they change sediment and nutrient regimes and alter water temperature and chemistry. Storage reservoirs flood terrestrial ecosystems, killing terrestrial plants and displacing animals. As many species prefer valley bottoms, large scale impoundment may eliminate unique wildlife habitats and extinguish entire populations of endangered species (Nilsson and Dynesius, 1994).
Terrestrial ecosystems in reservoir areas are replaced by lacustrine, littoral and sublittoral habitats and pelagic mass-water circulations replace riverine flow patterns. As such, dams and their reservoirs also provide new opportunities as they create new habitats and over time could be considered to become part of the new environment. The degree to which these ‘new’ habitats can compensate the loss of original habitats, species and ecosystem goods and services is however often contested.
Within these broad patterns of change, there is a wide diversity of specific impacts that vary from dam to dam, catchment to catchment, ecosystem to ecosystem, and species to species. For example loss of some ecosystems may benefit some species (e.g. waterfowl and fish that favour deep water), but others may suffer significant loss of population, or even extinction.
The purpose of this chapter is to review current understanding of these impacts. An initial assessment of the scale and variability of impacts is followed by an analysis of specific ecosystem impacts divided according to Petts (1984) – Fig 3.4. He suggests the following breakdown that is used for the basic structure of this chapter.
1. Assessment of first-order impacts that influence the key abiotic driving variables of the riverine ecosystem (e.g. temperature and hydrological flows). 2. Definition of second-order impacts that include primary productivity as the basis for the food chain. 3. Third-order impacts on the food web – implications for fauna.
This approach is adopted for both the upstream and downstream areas, followed by an assessment of specific impacts on biodiversity and cumulative impacts within the catchment.
3.2 Scale and Variability of Impacts
There are different types of dams each with their own operating characteristics. Similarly, dams have been built in a wide array of conditions, from highlands to lowlands, temperate to tropical regions, fast-flowing to slow-flowing rivers, urban and rural areas, etc. The combination of dam types, operating systems, and the contexts where they are built, yields a multitude of conditions that are site- specific and very variable. This complexity makes it difficult to generalise about the impacts of dams on ecosystems as each specific context is likely to have different types of impacts and to different degrees of intensity. However, at a certain level of generality, some indications can be given of the most likely impacts and their relative order (Box 3.1).
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Box 3.1: Types of dams, in descending order of impacts on ecosystems
Storage dams Large reservoirs with or without river diversions.
Diversion (run-of-river) Uses flow with limited or no storage; diverts all or part of in-stream or across catchments.
Run-of-river Uses flow with limited or no storage and no river diversion.
In addition to dam type, the height of dams and their reservoir areas are extremely variable. ICOLD recognises a large dam as one that is higher than 15 m and/or, between 5-15 m high and impounding more than 3 million cubic meters of water. Within any individual country there is a wide range of different dam types, dam heights and reservoir sizes. For example, Figure 3.1 shows the distribution of reservoir size for dams in Turkey where reservoir size varies from a few hectares to over 80,000 ha.
Figure 3.1: Distribution of reservoir area for dams over 15m high in Turkey(DSI, 1999)
Although dam and reservoir size are highly variable, it is possible to examine the broad scope of the impacts. Figure 3.2. shows the total area of reservoirs by continent, while Figure 3.3. shows the average area of reservoirs in each continent. These highlight both the large total areas involved and the great variation between regions in terms of the average size of the reservoirs.
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Figure 3.2: Total area of large-dam reservoirs (1000's km2) by region (ICOLD 1999)
Figure 3.3: Average area of large dam reservoir (km2) per region(ICOLD 1999)
Table 3.1 gives some examples of the scale of the impacts of water diversion on water flows from countries where data is available, which indicate the significant effects of man’s activities on water flows in major catchments. These impacts are likely to differ between northern countries, where temperate climates and little irrigation mean that there is little water diversion and semi-arid countries, which may have extensive out-of-river uses and high evaporation rates to contend with.
Table 3.1: The potential scale of the impacts of dams
River Example of Scale of Impact Source Indus, India Only 28% of the Indus’ total Anonymous 1997, WCD annual streamflow reaches its Tarbela report 1999 delta. For dry season flows it is only 10%. The dams along the river retain ~75% of the silt carried by the river. Various, South Africa There are 520 major Davies & Day 1998 regulating structures in South Africa that capture nearly 50% of the mean annual runoff This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
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Murray-Darling, Australia Mean annual outflow from the Murray Darling Basin Murray to the sea has been Commission 1999, quoted in reduced from some 13,700 WWF Australia GL/yr under natural conditions 1999/ENV220, to 4,900 GL/yr, or as low as www.mdbc.gov.au 35% of natural flows. Japan Of 35,000 rivers; only two McAllister et al. 1997, Dams have not been either dammed Yearbook 2000 or modified in any way. North America north of 77% of the total water Dynesius and Nilsson 1994 Mexico, Europe, and former discharge of the 139 largest Soviet Union river systems is strongly or modestly affected by fragmentation of the river channels by dams and by water regulation resulting from reservoir operation, inter-basin diversion, and irrigation. United States Only 42 free-flowing rivers Abramovitz 1996 longer than 125 miles remain – less than 2% of the country’s 3.1 million miles of rivers and streams Europe There are more than 10,000 Kristensen & Hansen 1994, major reservoirs in Europe, ICOLD 1999 covering a total surface area of ca 140,000km2, which is equal to app. 4 times the national territory of the Netherlands. Columbia, USA 5% to 14% of adult salmon are Collier et al 1996 killed at each of the eight dams through which they pass on their way up the river
Studying the importance of dam impacts on ecosystems at a global scale, and incorporating the complexity of temporal and spatial variability is made difficult by the lack of available information. Although there are several databases containing some information on dams (Table 3.2.) there is, at present, no comprehensive database of dams with geo-referenced locations in the public domain. This means that there is no global data set that can easily be used to locate and map large dams. Hence, it is not possible, except in very general terms, to relate dam distribution to major biotopes and to deduce the different effects dams have in different regions of the world.
Table 3.2: Some databases of dams
Name Source World Register of dams ICOLD 1999 World atlas of large dams International Journal on Hydropower and Dams, Hydropower & Dams 1999 European Lakes, Dams and Reservoir European Environment Agency 1999 database (ELDRED) National registers of dams e.g. USA ICOLD national committees, USCOLD 1999 register of dams
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Including the temporal and spatial variability of ecosystems and dams increases the complexity of the analysis. Temporal scales are important because some ecosystem changes caused by dams may not become evident for many years, or may evolve through time (see for example section 3.7). Similarly, the time factor is essential when considering some impacts which might become irreversible with time, such as the accumulation of toxic sediments in a reservoir that can make dam decommissioning more difficult or even economically impossible. Spatial scales are also important for ecosystems as some impacts of dams are felt far away from the dam site. For example, if the dam reduces the amount of detritus in the streamflow, this might affect the fisheries production in the river’s estuary many kilometres downstream. The cumulative effects of many dams in a catchment might also be different from the sum of the impacts of each individual structure, reinforcing the need for a spatial assessment (see section 3.8).
3.3 Framework for Analysis
In considering the impact of dams on riverine ecosystems it is important to recognise the interconnected nature of the ecosystems concerned and the often far-reaching consequences of change in individual ecosystem components. Analysis of this complexity can be approached from various different perspectives. Many groups and individuals, such as McCully (1996), Davies & Day (1998), Veltrop (1999), and USGS (1996), World Bank, have attempted this in different ways. Two of these approaches are presented in Annex 1 and Annex 5 – A summary of impacts derived from ICOLD Bulletins and USCOLD, and another from the World Bank’s Environmental Assessment Source Book. Overall such an analysis is only possible by breaking down the impacts into categories and there is no agreed or definitive way of doing this. This report adopts the approach of Petts, 1984. (Figure 3.4) as he dissagregates the components of ecosystem complexity by structuring impacts according to their level.
First-order impacts are the immediate abiotic effects that occur simultaneously with dam closure and influence the transfer of energy and material into and within the downstream river and connected ecosystems (e.g. changes in flow, water quality and sediment load).
Second-order impacts are the abiotic and biotic changes in upstream and downstream ecosystem structure and primary production, which result from first-order impacts. These depend upon the characteristics of the river prior to dam closure (e.g. changes in plankton, macrophytes and periphyton), and these changes may take place over many years.
Third-order impacts are the long-term biotic changes resulting from the integrated effect of all the first- and second-order changes, including the impact on species close to the top of the food chain (e.g. changes in invertebrate communities and fish, birds and mammals). Complex interactions may take place over many years before any new “ecological equilibrium” is achieved.
Table 3.3: Upstream and downstream impacts according to first, second, and third order as described. Location in Category of Impact Impact Relation to (as in Petts 1984) the Dam Upstream First-Order Impact Modification of the Thermal Regime Accumulation of Sediment in the Reservoir Changes in Water Quality Groundwater along reservoir
Second-Order Impact Plankton and Periphyton Growth of Aquatic Macrophytes Riparian Vegetation Third-Order Impact Invertebrates, Fish, Birds and Mammals
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Downstream First-Order Impact Daily, Seasonal and Annual Flows Water Quality Reduced Sediment Flows Changes to Channel, Floodplain and Coastal Delta Morphology Groundwater in riparian zone Water temperature – thermal pollution Ice formation Second-Order Impact Plankton and Periphyton Growth of Aquatic Macrophytes Riparian Vegetation Carbon flows and cycle distortions
Third-Order Impact Invertebrates, Fish, Birds and Mammals Estuarine Impacts Marine Impacts
In general terms the complexity of interacting processes increases from first- to third- order impacts. Since ecosystem functioning is guided by abiotic steering variables related to hydrology (i.e. water quantity and flow regime), geomorphology and water quality, observations related to these ecosystem components can be used as primary indicators of river ecosystem conditions. Changes in abiotic steering variables are key to understanding the long-term ecological consequences of dams as they are the underlying mechanisms by which many habitats are maintained. As Ligon et al (1995) stated , “If [a] stream’s physical foundation is pulled out from under the biota, even the most insightful biological…program will fail to preserve ecosystem integrity.”
Birds Mammals THIRD-ORDER IMPACTS Fish
Invertebrates
Primary Morphology production SECOND-ORDER IMPACTS Plankton Channel form Terrestrial environment Aquatic Macrophytes Substrate composition Algae
Water Hydrology Sediment quality load Water Flow FIRST-ORDER quantity regime IMPACTS
Barrier effects
Table 3.4: A framework for assessing the impact of dams on river ecosystems(modified from Petts, 1984).
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Box 3.2: Construction impacts
The framework presented in Figure 3.4 and used here does not include the often significant impacts on non-aquatic ecosystems caused by the construction of physical infrastructure, transmission lines and roads. Power transmission lines are typically cleared of shrubs and trees by cutting or use of herbicides. Habitat is thus modified on the cleared swathes and runoff or winds may result in off-site effects of the herbicides. Hydro-Québec reports that the length of its transmission lines totals 32,000 km (HQ and GDG 1999). To deal with this problem, it has established, as outlined in its environmental code, specific measures to minimise construction impacts on the environment (Hydro Quebec 1991).
HQ and GDG (1999) point out that powerlines have less environmental impact than roads and railway lines, which are essentially vegetation free. Some powerline constructions support species, like the smoky shrew and southern bog lemming, that are in danger of extinction in Québec. The lines and roads, at times, fragment habitat for organisms both large and small.
Access roads to dam sites can also cause a significant direct impact on natural ecosystems, while also providing access to previously remote areas for settlers and hunters. Blasting at construction sites can also be a major source of disturbance, in particular during certain times of the life cycles of animals such as calving caribou in Canada (Kiell 2000/ENV202).
A comprehensive analysis of indirect impacts due to the development of dams, for example impacts of new irrigation schemes, development of navigation or tourism, and human health impacts, do not form part of this review. Other papers prepared for the WCD secretariat will deal with these issues, such as Thematic Review IV.2 Assessment of Irrigation Options and I.1 Social Impacts of Large Dams: Equity & Distributional Issues.
3.4 Information Constraints
Over the last 30 years, the finding of numerous scientific studies relating to the environmental impacts of dams have been reported in the scientific literature. Some of these findings have been summarised within wide-ranging compilations (e.g. ICOLD, 1981; Petts, 1984; McCully 1996, USGS 1996, ICOLD, 1988). Research continues and research findings are constantly being up-dated. Other sources of information include a significant body of “grey” literature (e.g. consultant reports), usually written during the planning of a river impoundment. Most of these case studies consist of pre- regulation investigations. Finally, there is now an increasing amount of information and related “position papers” published by various organisations. To an extent the perspective of the people and organisations involved cloud the latter and the information presented may be selective in nature.
In order to effect a thorough investigation of the impacts of dams on ecosystems, data are required on both the abiotic and the biotic components of ecosystems (eg Annex 3). Pre- and post- impoundment information is required on: the hydrology of the river (both at the site of the dam and downstream); hydraulic characteristics of the river; water quality; geomorphological characteristics (i.e. sediment transport); aquatic biota and their habitat requirements; riparian vegetation and associated fauna; vegetation and associated fauna in the upper watershed; and the direct use of the river and its associated resources by local people.
To date however, most studies have investigated the impact of one dam or a few dams on specific components of ecosystems rather than on the ecosystem as a whole. Most studies are focussed primarily on the abiotic, primarily first-order impacts. Relatively few studies have assessed second- and third-order impacts, possibly because of the longer time frame required before new equilibrium states are attained and total change becomes apparent. At higher trophic levels (e.g. impact on
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terrestrial vertebrates), very limited amounts of data relate to long-term change caused by dam construction, though possible impacts are subject to much speculation (Nilsson and Dynesius, 1994).
Further, most studies of the environmental impacts of dams have been conducted in temperate climates. Relatively little is known of the possible third-order impacts in tropical climates, where biological processes often proceed faster and so ecological changes become apparent more quickly (Bardach and Dussart, 1973). Scientists and conservationists suspected that the flooding of large areas in the tropics is especially likely to contribute to global species extinction. Although largely unproven because the data are unavailable, this suspicion exists because of the high species richness and possible endemicity of many of the affected areas. One well-documented example is the case of the Kihansi Spray Toad in Tanzania (Finlow-Bates, Gentle and Lovett, 2000).
This report therefore draws on the available literature while being aware of the dangers of generalising from “worst case” examples that have been well studied. It seeks to lay out and illustrate the generic impacts that are known to occur while recognising that the nature and scale of these impacts will vary from site to site. It emphasises the need to look holistically at the impacts on a case- by-case basis rather than addressing them piecemeal.
3.5 Upstream Impacts
The construction of a dam results in post-impoundment phenomena that are specific to reservoirs and do not occur in natural lakes. One difference is that with first reservoir filling terrestrial habitats are submerged and destroyed. Another difference is that level fluctuations may be much larger than those normally found in a natural lake. Non-earth storage dams often have a bottom outlet. This may allow both sediment flushing and water releases from deep below the surface. Both management measures cannot be carried out with most natural lakes. Nevertheless some older reservoirs can be considered as lakes and the challenges presented in managing them are often the same (Dinar et al., 1995), such as the management of the riparian wetland habitats and fisheries. In this section a summary is given of the first-, second- and third-order impacts on upstream ecosystems comprising the reservoir and upper reaches of the river.
3.5.1 First-Order Impacts on Key Parameters
3.5.1.1 Modification of the Thermal Regime
Temperature is an important regulator of many important physical, chemical and biological processes. In particular temperature, in conjunction with nutrient dynamics and seasonal availability of minerals and light conditions, controls primary productivity. Reservoirs act as thermal regulators that may fundamentally alter the seasonal and short-term fluctuations in temperature that are characteristic of many natural rivers. The relatively large mass of still water in reservoirs allows heat storage and produces a characteristic seasonal pattern of thermal behaviour. Depending on geographical location, water retained in deep reservoirs has a tendency to become thermally stratified (Hutchinson, 1957). Typically, three thermal layers are formed: i) a warm, well-mixed, upper layer (the epilimnion); ii) a cold, dense, bottom layer (the hypolimnion) and iii) an intermediate layer of maximum temperature gradient (the thermocline). Water in the hypolimnion may be up to 10oC lower than in the epilimnion and in the thermocline the temperature gradient may be up to 2oC for each metre.
A range of factors, including climatic characteristics, controls the exact nature of thermal stratification. Reservoirs closest to the equator are least likely to become stratified. At higher latitudes the overall controlling factor is the variable input of solar energy. Considerable variability may occur within a region as a consequence of different topographies and different reservoir- catchment morphometrics. Shallow reservoirs respond most rapidly to fluctuations in atmospheric conditions and are less likely to become stratified. Strong winds can affect rapid thermocline
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oscillations. The patterns of inflows into, as well as the nature of outflows from, the reservoir also influence the development of thermal stratification.
Currents generated by large water level fluctuations in reservoirs caused by the operating regime can sometimes prevent thermal stratification. Many deep reservoirs, particularly at mid and high latitudes do become thermally stratified as do natural lakes under similar climatic and morphological conditions. However the release of cold water into the receiving river from the hypolimnion of a reservoir is the greatest “non-natural” consequence of stratification. This is addressed in Section 3.6.1.
3.5.1.2 Accumulation of Sediment in the Reservoir
River currents transport particles from the fine ones in turbid water to the coarser ones such as sand, rocks, and boulders. The speed and turbulence of currents enable transport of the geologically- derived materials. As waters slow down and turbulence declines, the particles tend to drop out. Lowered currents and turbulence occur when the river bed gradient diminishes, as in the lower reaches of many rivers, upon entry into lakes or the sea. This also happens when river flow reaches man-made reservoirs.
Many reservoirs retain a large proportion of the sediment load supplied by the drainage basin. About 1,100 km3 of sediment has accumulated in the world’s reservoirs, taking up almost 20% of the global storage capacity (Mahmood, 1987). The Glen Canyon Dam on the Colorado River, USA, traps 66 million tons of sediment per year, equivalent to 95% of the sediment load. (Collier et al., 1996).
As with a natural lake, the “trap efficiency” of a reservoir depends on: i) the size of the reservoir’s catchment; ii) the characteristics of the catchment that affect the sediment yield (i.e. geology, soils, topography, vegetation and human disturbance) (see Kettab and Remini, 1999/ENV048); and iii) the ratio of the storage capacity to the river flows into the reservoir. However, unlike a natural lake, the type of outlet on the dam will also affect the trap efficiency of a reservoir. Sediment transport shows considerable temporal variation, both seasonally and annually. The amount of sediment transported into reservoirs is greatest during floods but also depends largely on the management of the upper catchment.
Sediment transport and deposition have both positive and negative impacts. Sedimentation can create new habitats in the reservoir, especially at the mouth of the river, while sedimentation reduces storage capacity. For example, Nepal’s Kulekhani hydro dam, estimated to have a useful life of 85 years when commissioned in 1981, had lost nearly half of its 12 million cubic metres of dead storage capacity by 1993, while El Salvador’s Cerron Grande reservoir was found to have a useful life of 30 years, instead of the originally expected 350 years (Dorsey et al 1997). In North Africa, severe autumn rains and a mountainous terrain mean that reservoirs receive enormous sediment loads. For example, the Mellegue reservoir in Tunisia has lost 92% of its storage capacity since filling in 1954, and the Mohamed V reservoir in Morocco has lost 58% of its storage capacity since filling in 1967 (Kettab and Reminin, 1999/ENV048).
3.5.1.3 Changes in Water Quality
Water storage in reservoirs induces physical, chemical and biological changes in the stored water and in the underlying soils and rocks, all of which affect water quality. The chemical composition of water within a reservoir can be significantly different from that of the inflows. The size of the reservoir, its location in the river system, its geographical location with respect to altitude and latitude, the storage retention time of the water and the source(s) of the water all influence the way that storage detention modifies water quality.
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Major biologically-driven changes occur within thermally stratified reservoirs. In the surface layer, phytoplankton often proliferate and release oxygen thereby maintaining concentrations at near saturation levels for most of the year. In contrast, the lack of mixing and sunlight for photosynthesis in conjunction with the oxygen used in decomposition of submerged biomass can result in anoxic conditions in the bottom layer.
Nutrients, (i.e. phosphorous and nitrogen) are released biologically and leached from flooded vegetation and soil. Although oxygen demand and nutrient levels generally decrease over time as the organic matter decreases, some reservoirs require a period of more than 20 years to develop stable water-quality regimes (Petts, 1984). After maturation, reservoirs, like natural lakes, can act as nutrient sinks particularly for nutrients associated with sediments. Eutrophication of reservoirs may occur as a consequence of large influxes of organic loading and/or nutrients. In many cases these are a consequence of anthropogenic influences in the catchment (e.g. application of fertilisers) rather than a direct consequence of the presence of the reservoir. For example, eutrophication of the heavily- regulated Waikato River system in New Zealand was enhanced by sewage and stormwater discharges (Chapman, 1996). Nutrient pulses, in conjunction with the specific environmental conditions, can result in water blooms of blue-green algae which (in addition to being aesthetically unpleasant) can cause oxygen depletion and increased concentrations of iron and manganese in the bottom layer and increased pH and oxygen in the upper layers of stratified reservoirs (Zakova et al., 1993).
Mercury and other heavy metal contamination has recently been highlighted as a major reservoir problem in some countries (Friedl, 1999/ENV079). Mercury is naturally present as a harmless inorganic form in many soils. However, bacteria breaking down decomposing matter under a new reservoir transforms this inorganic mercury into methylmercury, a toxin of the central nervous system. Plankton and other creatures at the bottom of the aquatic food chain absorb the methylmercury. As the methylmercury passes up the food chain it becomes increasingly concentrated in the bodies of the animals eating contaminated prey (Paterson et al. 1998). Through this process of bio-accumulation, levels of methylmercury in the tissues of large fish-eating fish or birds at the top of the food-chain can be several times higher than in the small organisms at the bottom of the chain. The degree to which fauna have been intoxicated with mercury has been shown to be variable (Friedl, 1999). In other reservoirs no effects are reported (Lucotte et al. 1999).
Water quality changes due to the reservoir will be reflected throughout the downstream watercourse, affecting primary productivity and the invertebrate fauna that provide the basis for the foodweb.
Annex 3 provides a comparison of a series of water quality variable parameters under pre and post impoundment conditions of two reservoirs in the Mekong river basin, together with corresponding data on second and third order variables, notably phytoplankton, zooplankton and fish.
3.5.2 Second Order Impacts – Changes in Primary Production
3.5.2.1 Plankton and Periphyton
Within natural fast-running river (lotic) systems, phytoplankton production is often negligible, only derived from lakes, low velocity backwaters and benthic algal communities. Natural rivers, particularly clean, slow-moving lowland rivers, do contain free-floating micro-organisms, but the plankton populations are inherently unstable and dependent upon the frequency of high discharges. The introduction of a reservoir into a river system, particularly in headwater areas, can markedly alter its primary productivity. The hydrological characteristics and thermal and chemical regimes of reservoirs are unique, so the character of primary production within reservoirs is highly site- and catchment-specific.
Upon dam closure, the river (lentic) system resets itself as the reservoir fills. Often a microbial population explosion releases nutrients as the newly submerged organic matter begins to decompose.
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This stimulates the rapid development of the phytoplankton.. The enrichment of reservoir water by large amounts of nitrogen and phosphorous, as a result of the decay and mineralisation of organic matter flooded by the reservoir, may lead to a multiplication of blue-green algae. This in turn drives invertebrate productivity and fisheries production. In many reservoirs, fisheries thrive for 4-5 years after closure and then decline as primary productivity drops.
The occurrence of lacustrine plankton assemblages varies seasonally and with individual reservoirs, depending upon their geographical location and catchment inputs. In temperate and high latitude climates plankton populations are lowest in the cold winters and greatest during the warm summers. Tropical reservoirs have no seasonal check to plankton growth comparable to winter in temperate regions. Given the favourable thermal regime, the productivity of tropical reservoirs is mainly limited by the introduction of highly turbid waters and wind-induced turbulence during the wet season.
Periphyton are layers of algae attached to any submerged object, including larger plants. Diatoms normally dominate the attached algae of lotic systems. Conversion from a lotic to a lentic environment will provide opportunity for some species of periphyton, while destroying the habitat for others. Periphyton are most likely to proliferate where light penetrates, in the shallow water close to the reservoir edge. The exact species composition will be determined by the nature of the substrate, the presence or absence of aquatic macrophytes, the temperature and chemistry of the reservoir water and the operation of the dam.
3.5.2.2 Growth of Aquatic Macrophytes
There may be increased opportunity for aquatic macrophytes in the littoral and sub-littoral zone of reservoirs. The rapid build up of delta deposits near river inlets to the reservoir reduces water depths and can encourage macrophyte growth. However, their ability to colonise these areas may be limited if there are large fluctuations in reservoir level. Further out in the reservoir opportunity for aquatic macrophytes may be limited by lack of light penetration to depth, yet in windless conditions with high nutrient levels, colonisation by floating invasive species is possible (Box 3.3).
The growth of macrophytes can be an advantage as they create wetland-like conditions with biodiversity values, support fisheries and assist in structuring habitats. However, they may also provide habitat for disease vectors such as bilharzia-carrying snails, mosquitoes and intermediate hosts for flukes.
Box 3.3: Invasive species and large dams
The modified habitats resulting from large dams often create environments that are more conducive to non-native and exotic plant, fish, snail, insect and animal species. These resulting non-native species often out-compete the native species and end up developing ecosystems that are unstable, nurture disease vectors, and are no longer able to support the historical environmental and social components. The short-term gain in having a reservoir or hydroelectric plant may not compensate for the loss of critical ecosystem functions.
Species of floating and submerged weeds that are particularly virulent when introduced into new habitats (so-called "alien invasive species") such as water hyacinth Eichornia crassipes, water lettuce Pistia stratiotes, and water fern Salvinia molesta, pose a major threat to the efficiency of dams and irrigation systems. These floating plants can form thick mats that cover the surface of the reservoir completely. By shading out phytoplankton and through increased input of organic matter (when they die and sink), they add to oxygen depletion, which in turn has impacts on fish and may have other ecologically detrimental impacts and serious economic implications (Joffe and Cooke, 1999/ENV057). Managing invasive species that threaten dam and water systems in a proactive manner is far more cost efficient than the usual reactive, crisis-driven manner that is expensive and typically has had only limited success.
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3.5.2.3 Riparian Vegetation
The riparian ecosystem will inevitably change when its adjoining aquatic environment changes. The largest upstream impact of dam construction on riparian vegetation is biomass submergence. In arid locations the shallow groundwater in the vicinity of a reservoir provides opportunity for vegetation that require access to water throughout the year. A study of radial stem growth of coniferous trees near Swedish reservoirs found a significant increase in the variation in growth following dam construction in trees located close to a reservoir where regulation produced short-term (daily and weekly) variation in water levels.
Variation in the water levels of reservoirs can have a negative impact on plants in the immediate vicinity of the reservoir. For example, in Sweden, regulated water level fluctuations may exceed 30 m in height. This has resulted in riparian corridors that are several hundred meters wide. However, because the pattern of water level fluctuations is not synchronised with the natural regime, the riparian vegetation cover is extremely sparse and the riparian ecosystem gives the impression of a barren strip across the landscape (Nilsson and Jansson, 1995). The impact of reservoir level fluctuations are directly related to the gradient of the drawndown zone. Where fluctuations are significant, steep gradient drawndown zones are often characterised by baren strips along the reservoirs. With flat gradients much wider areas can be affected, causing both a disappearance of species and the creation of new habitats for amphibians, birds and drawdown-area plants.
3.5.3 Third-Order Impacts on Fauna
3.5.3.1 Invertebrates, Fish, Birds and Mammals
Filling of the dam reservoir results in permanent flooding of riverine and terrestrial habitat, and depending upon the topography and habitats of the river valley upstream from the site of the dam, these impacts can vary greatly in extent and severity. For example, the 500-megawatt Chile Pehuenche Hydroelectric Project floods only 400 hectares of land (with minimal damage to forest or wildlife resources) and has no water quality problems. By contrast, the Suriname Brokopondo Dam Project inundated about 160,000 hectares of biologically valuable tropical rainforest and suffers from severe water quality and aquatic weed problems (Ledec et al., 1997).
The effects of inundation are especially severe when the reservoirs are situated close to mountains, in dry areas, or at higher latitudes where the river valleys are usually the most productive landscape elements. Due to impoundment, all terrestrial animals disappear from the submerged areas and populations decrease within a few years in proportion to the habitat area that is lost (Nilsson and Dynesius, 1994). Flooding can result in both local and global extinctions of animal and plant species. Particularly hard hit are the species dependent upon riverine forests, and other riparian ecosystems, and those adapted to the fast-flowing conditions of the main river course (McAllister et al. 1999).
Dams also serve as a physical barrier to movement of migratory species, notably fish. This prevents broodstock from reaching their spawning grounds during the breeding season, resulting in massive failure of recruitment and eventual extinction of the stock above the dam. Dams in coastal locations prevent fingerlings and juveniles migrating from brackish water in breeding and nursery areas from reaching freshwater habitats upstream, leading to similar impacts (Bernacsek, 1999). This issue is dealt with further in Section 3.7.3.
Flooding of the dam impoundment creates a new ecosystem, which can vary enormously in ecological value and productivity according to the physical and biological characteristics of the site and the management regime of the dam. Reservoirs have been described as an ‘ecological hodgepodge’ (Helfman in preparation). When a dam is built, some riverine species trapped behind the structure
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survive although most of the lotic species cannot tolerate the lentic conditions. Most of the riverine fish stay close to the shores of the reservoir, the mouth of tributaries and in shallows. The pelagic and deep water is poorly used unless fishes adapted to these conditions were present before the reservoir was formed. Exotic species are often introduced to fill these vacant niches and these increase the number of species. To determine their effectiveness in providing environmental services and enlarging biodiversity requires a rigorous examination of initial biophysical conditions and comparison with altered conditions especially in terms of species diversity and presence of indigenous species.
During the first years of submergence, reservoirs may experience an initial increase in aquatic productivity as a result of nutrients released from decomposing plant biomass. The species of colonising fish capable of rapidly using this new and abundant food will tend to dominate until biomass decomposition has stabilised. Submerged vegetation provides habitats favourable to some invertebrates, which attract in their wake fish capable of deriving benefit from them, followed by predatory species that feed off these fish (De Silva, 1988; CIGB, 1985). Fish catch undergoes an ‘evolution’ in terms of quantity during the first 10 years after the dam closure. Typically (and assuming there is sufficient fishing effort), catch rises very quickly to a peak level 3-5 years after dam closure and then declines to a more-stable level thereafter. This is a normal feature of almost all reservoirs throughout the world and should not be misconstrued as fishing effort-induced stock depletion or that the reservoir is losing its productivity (Bernacsek, 1997). However, it should not be assumed that reservoir fish biomass will in all cases exceed pre-dam river system biomass.
In addition to their importance for fisheries many reservoirs are also important for waterbirds (see Section 3.7.4.) and other wildlife, in particular in drier regions.
3.6 Downstream Impacts on Rivers, Floodplains and Deltas
Rivers are part of the hydrological cycle and it is the variable nature of runoff processes that give rivers their dynamic characteristics. The ecological integrity of river ecosystems is dependent on the variation in flow regime to which they are adapted. Floods cause hydraulic disturbance that determines the composition of biotic communities within the channel, the riparian zone and the floodplain (Junk et al., 1989; Webb et al., 1999). The spatio-temporal heterogeneity of river systems is responsible for a diverse array of dynamic aquatic habitats and hence ecological diversity, all of which is maintained by the natural flow regime.
It is flooding and the consequent transfer of material that makes rivers and floodplains among the most fertile, productive and diverse ecosystems in the world. Floodplain communities are characterised by resilience and the ability to respond quickly to changing hydrological conditions. The rich productivity of floodplains allows them to sustain large populations of organisms that are interdependent on one another. Regular floods keep the vegetational successions in young, productive stages, creating excellent conditions for an abundant wildlife. The diverse vegetation favours animal diversity. Consequently, floodplains are also rich in species endemic to small geographical areas.
Coastal marine wetlands are often highly dependent on inputs of freshwater and associated nutrients and sediments from rivers. Coastal wetlands are ecologically and environmentally diverse because of the gradual and often fluctuating dynamic boundaries between salt, brackish and freshwaters. Salt water may penetrate considerable distances upstream, but boundary patterns vary with flow regimes and landscape forms. These patterns influence not only vegetation, but also animal behaviour, such as the extent to which marine species can range into the food-rich wetlands. Dams constitute obstacles for longitudinal exchanges along fluvial systems. Dams not only alter the pattern of downstream flow (i.e. intensity, timing and frequency) they also change sediment and nutrient regimes and alter water temperature and chemistry. These changes and others directly and indirectly influence a myriad of dynamic factors that affect habitat heterogeneity and successional
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trajectories and, ultimately the ecological integrity of river ecosystems. The changes induced by large dams may affect ecosystems and the people who depend on them for tens to thousands of kilometres downstream. In this section the first-, second-, and third-order impacts on downstream ecosystems are summarised.
3.6.1 First-Order Impacts on Ecosystem Driving Variables
3.6.1.1 Daily, Seasonal and Annual Flows
In general, discharge control resulting from the development and operation of storage dams changes flow variability downstream from the dam. For major floodplain rivers, dams may increase flood peaks by altering the timing of the floodpeak to coincide with floodpeaks from tributaries downstream. Peak discharges can also increase when reservoirs are used for generating peak power. In most cases, however, the magnitude and timing of flood peaks is reduced by storage dam development and operation.
The effect of a reservoir on individual flood flows depends on both the storage capacity of the dam relative to the volume of flow and the way the dam is operated. Reservoirs having a large flood- storage capacity in relation to total annual runoff can exert almost complete control upon the annual hydrograph of the river downstream. However, even small-capacity detention basins can achieve a high degree of flow regulation through a combination of flood forecasting and management regime. An example of the changes in average annual flow regime following dam construction on the Murray river (Australia) is shown in Figure 3.5. Discharge close to the Yarrawonga weir has no resemblance to natural flow pattern. At the mouth of the river, the timing of the annual peak discharge under natural conditions is similar to altered conditions (Figure 3.5b). However, river discharge is reduced to 21 % and especially medium size flow peaks are affected. At Albury, a seasonal inversion of river flows is observed due to releases for rice, dairy and orchard irrigation (Figure 3.5b). Reduction in flow velocity in weir pools is considered a key cause of the decline in silver perch in Murray river basin. A decline in river mouth wetlands due to reduced flows is also observed (MDBMC 1995). a)
1000
800
600
400 Flow (GL) 200
0 Jul Apr Oct Jan Jun Feb Mar Aug Sep Nov Dec May
Natural Conditions Current Conditions
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b)
1800 1500 1200 900
Flow (GL) 600 300 0 Jul Apr Oct Jan Jun Feb Mar Aug Sep Nov Dec May
Natural Conditions Current Conditions
Figure 3.4: Comparison of pre and post impoundment flows in the Murray River, Australia:variation in the average monthly flow at a) Albury (2225 km from the mouth) and b) at Barrages (1 km from the mouth). Source: Murray-Darling Basin Ministerial Council, 1995.
A consequence of reduced flood peaks is reduction in the frequency of overbank flooding and reduced extent of flooding when it does occur. For example, in the Hadejia-Nguru wetlands (Nigeria) annual flooding prior to construction of dams for irrigation was typically about 3,000 km2 and this was reduced to less than 1000 km2 after construction (Hollis et al., 1993). Reduced floodplain inundation and altered hydrology downstream of dams may reduce groundwater recharge in the riparian zone, resulting in lowering of the groundwater table, with consequent impacts on riparian vegetation. Equally there is a direct and significant relationship between flood extent and the number of wintering ducks in these wetlands (WWF 2000/ENV224).
A range of operational procedures can result in fluctuations in discharge that occur at non-natural rates. Hydroelectric power and irrigation demands are the most usual causes, but peak-discharge waves have been utilised for navigational purposes and to meet recreational needs (e.g. white water kayaking and rafting). For many purposes, so called “pulse releases” are made regularly (e.g. daily releases through power turbines which reflect diurnal variation in power demand). Downstream from the West Point Dam (USA), discharge ranges from 14 m3s-1 during low flow generation to 445 m3s-1 during peak generation, resulting in changes in stage height of more than 2 m. The pattern of daily fluctuation in the Colorado River is shown in Figure 3.5. As hydropower represents one of the most easily activated form of peaking power available to most national electricity grids, these kind of fluctuations are frequently associated with hydropower dams.
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25,000
20,000
15,000
10,000
5,000 Stream Flow (cubic feet per second) - 1-Oct 1-Sep 3-Sep 5-Sep 7-Sep 9-Sep 11-Sep 13-Sep 15-Sep 17-Sep 19-Sep 21-Sep 23-Sep 25-Sep 27-Sep 29-Sep Day
Figure 3.5: Daily Streamflow Variations in the Colorado River at Lee's Ferry in September. Peak flows are associated with the power generation between 14.00 and 19.00 daily, with minima at 04.00 am, and the fluctuation in demand also varies from day to day. (U.S. Bureau of Reclamation, Upper Colorado Region, 2000)
In addition to altering the flow regime of rivers, dams also affect the total volume of runoff. These changes may be either temporary and permanent. Temporary changes arise primarily from filling the reservoir, which may take several years where reservoir storage greatly exceeds the mean annual runoff. Permanent changes occur because: i) water is removed for direct human consumption and not returned to the river (e.g. for irrigation or interbasin transfers); ii) water is lost from the reservoir through evaporation – worldwide it is estimated that evaporation from reservoirs is of the order of 188 km3 y-1, which equates to more than 8% of the total human consumption of freshwater (Shiklomanov 1999).
The hydrological effects of a dam become less significant the greater the distance down stream (i.e. as the proportion of discharge from the uncontrolled catchment increases). The frequency of tributary confluences below the dam and the relative magnitude of the tributary streams, largely determine the length of river affected by an impoundment. Catchments in semi arid and countries with significant storage may never recover their natural hydrological characteristics even at the river mouth, especially when dams divert water for agriculture or municipal water supply.
Flow regimes, including volume, duration, timing, frequency and lapse time since last flooding, are the key driving variables for downstream aquatic ecosystems and are critical for the survival of communities of plants and animals living downstream. Small flood events may act as biological triggers for fish and invertebrate migration, major events create and maintain habitats, and the natural variability of most river systems sustains complex biological communities that may be very different from those adapted to the stable flows and conditions of a regulated river. It should also be noted however, that natural flood events can also be detrimental to ecosystems. After the Saguanay flood (Canada) in 1996, for example, salmon habitats had to be restored over a large area (Gaétan pers. com.).
Changes in river discharge can have significant effects on downstream groundwater resources. A reduction in flooding can considerably reduce the amount of recharge to downstream aquifers.
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3.6.1.2 Water Quality
Water storage in reservoirs induces physical, chemical and biological changes in the stored water. As a result the water discharged from reservoirs can be of a different composition to that flowing into the reservoir. Reservoirs act as thermal and chemical regulators so that seasonal and short-term fluctuations in water quality are altered. The salinization of water below dams in arid climates (arising from increased evaporation) is particularly problematic and is exacerbated in areas of marine sediments and where saline drainage water from irrigation schemes is returned to rivers downstream of dams. Salinization has also proved to be a problem on floodplain wetlands in the absence of periodic flushing and dilution by flood water. If sufficiently high and prolonged, elevated salinity will affect aquatic organisms (Hart et al., 1991).
Water temperature is an important quality parameter for the assessment of reservoir impacts on downstream aquatic habitats because it influences many important physical, chemical and biological processes. In particular, temperature drives primary productivity. It has been proposed that thermal changes caused by water storage have the most significant effect on in-stream biota (Petts, 1984). Temperatures downstream of the dam may be affected by the reservoir level from which the discharge is drawn, e.g. cool deep temperatures or warm surface temperatures. In New South Wales (NSW), cold water pollution impacts on average a river stretch of 300 km below each dam with water temperatures 5 degrees or more below normal. The total amount of river stretch affected in NSW amounts to 2650 km (Lugg 2000). Changed temperatures may affect spawning, growth rates and length of the growing season for many species. For example juvenile silver perch grown in the cold water released from Burrendong Dam, Australia, increased only 16% in weight over one month compared to a 112% increase in water warmed to natural levels (Blanch 1999/ENV204). In the case of the Gariep Dam in South Africa, for example, the temperature changes due to impoundment extend for 130 to 180 km downstream (Davies 1999).
Even without stratification of the storage, water released from dams may be thermally out of phase with the natural temperature regime of the river. The Hume dam on the Murray River, Australia alters the thermal regime of the river and its effect is still discernible 200 km downstream (Walker, 1979). Water temperature changes have often been identified as a cause of the reduction in native species of fish, particularly impacting spawning success negatively (Petts, 1984). Cold-water release from high dams of the Colorado River is still measurable 400km downstream and this has resulted in a decline in native fish abundance (Holden and Stalnaker, 1975). The fact that various introduced trout species replaced some twenty native species of fish has been attributed to the change from warm water to cold water.
The quality of water released from a stratified reservoir is determined by the elevation of the outflow structure relative to the different layers within the reservoir. Water released from near the surface of a stratified reservoir is often well-oxygenated, warm, nutrient depleted water. In contrast water released from near the bottom of a stratified reservoir is often cold, oxygen-depleted, nutrient-rich water which may be high in hydrogen sulphide, iron and/or manganese. Water depleted of dissolved oxygen is not only a pollution problem in itself, affecting many aquatic organisms (e.g. salmonid, fish that require high levels of oxygen for their survival), but one that may be exacerbated because such water has a reduced assimilation capacity and so a reduced flushing capacity for domestic and industrial effluents (ICOLD, 1994). The problem of low dissolved oxygen levels is sometimes mitigated by the turbulence generated when water passes through turbines. Water passing over steep spillways may become supersaturated in nitrogen and oxygen and this may also be fatal to fish immediately below a dam (ICOLD, 1994; Fidler and Miller 1997: Bouck, G.R. 1980). This is known as the gas bubble disease. It is for example a problem on the Columbia river (USA), where very high dams in the upper catchment generate high total dissolved gases that are not dissipated downstream (Bell and DeLacy 1967 in Bizer 2000).
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Davies (1999) argues that some of these key water quality parameters can recover to their natural levels. If the river flows freely for long enough, they can be “reset”, although each has its own “recovery distance”. Some, such as depleted oxygen levels, may recover within several hundred metres. Others, such as temperature, may take hundreds of kilometres.
3.6.1.3 Changes in Sediment Loads
Under natural conditions sediment feeds floodplains, creates dynamic successions, and maintains ecosystem variability and instability (Petts and Amoros 1996). Changes in sediment transport have been identified as one of the most important environmental impacts of dams. The reduction in sediment transport in rivers downstream of dams not only has impacts on channel, floodplain and coastal delta morphology (section 3.6.1.4), and so alters habitat for fish and other groups of plants and animals, but through changes in river water turbidity may effect biota directly. For example, plankton production is influenced by many variables, including turbidity. Turbidity interferes with photosynthesis and algal development may be attenuated by the presence of suspended inorganic particles. If turbidity is reduced, as a consequence of impoundment, plankton development may be enhanced and may even be stimulated to appear in new sections of rivers.
The selective release of highly turbid waters from a reservoir is a technique often used to reduce sedimentation. Sediment sluicing involves drawing down a reservoir at the start of the flood season and then allowing as much sediment-laden water as possible to pass through the dam before it has a chance to settle. The sudden release of tonnes of sediment can be disastrous for some biota. For example, the introduction of large quantities of fine silts and clays into permeable gravel substrates can have a catastrophic effect on fish eggs and fry. Thus, even though reservoirs generally trap sediments, reservoir operations can result in extreme and unnaturally high concentrations of sediment, which may produce a major stress effect on downstream aquatic ecosystems. Contaminated sediments in particular form a potential threat to downstream ecosystems if sediment flushing is carried out.
Reservoirs tend to serve as sediment traps because river velocities and therefore carrying capacities for particles decrease in reservoirs. However, sometimes, fluctuating water levels in reservoirs erode the shores and add to the turbidity of the reservoir discharge. Furthermore, the selective release of highly turbid waters from a reservoir is a technique often used to reduce sedimentation.
3.6.1.4 Changes to Channel, Floodplain and Coastal Delta Morphology
Complex relationships exist between channel form and processes. In general the frequency of flood discharges and the magnitude and particle-size distribution of the sediment load are the dominant controls of channel and floodplain morphology. Reservoirs alter the processes operating in the downstream river system by isolating upstream sediment sources, reducing the frequency of floods and regulating the flow regime (section 3.6.1.1). A unique combination of climate, geology, vegetation, size of impoundment and operational procedures produce the effect of any individual dam upon the fluvial processes downstream. Hence, a wide range of geomorphological responses can be generated by river regulation.
Some physical changes caused by dams are immediate and obvious while others are so gradual that they may go unrecognised by humans using the river for many years. Three examples of these slow and not always intuitive impacts are:
• Reduced sediment transport can result in lowering of the riverbed downstream and deepening of the channel as a result of sediment starvation. This channel incision impacts the frequency of floodplain inundation, as the deeper channel requires a higher discharge to overtop its banks and spill out over the floodplain.;
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• A reduction in lateral migration of the river channel can reduce the recruitment of spawning gravel from the floodplain. Lack of channel avulsion and bank cutting eliminates coarse sediment recruitment (Dietrich 1999/ENV082); • Where non-transportable materials are present in the bed sediments, the selective transport of the smaller sediment sizes results in the formation of a coarse sediment layer at the surface that protects the underlying material from erosion, a phenomenon known as channel armouring. A single grain thickness of coarse material may effectively prevent degradation although rare high magnitude floods may disturb this surface layer. Degradation occurs most rapidly in the upstream reaches closest to the dam so the armouring and degradation shifts progressively downstream.
Channel Erosion and Sedimentation The geomorphological effects of changes in flow and sediment regime have been analysed by many, for example Galay (1983), Williams and Wolman (1984) and Carling (1996). If the post-regulation flows remain competent to move bed material, the initial effect is degradation downstream from the dam, because the entrained sediment is no longer replaced by material arriving from upstream. According to the relative erodibility of the streambed and banks, the degradation may be accompanied by either narrowing or widening of the channel. A result of degradation is a coarsening in the texture of material left in the streambed; in many instances, a change from sand to gravel is observed and, in some, scour proceeds to bedrock.
Channel degradation below a dam persists until the reduction of channel slope reduces the flow velocity below the threshold for sediment transport. However, degradation is rarely able to progress freely. It is complicated by interrelated hydraulic, sedimentological and biotic factors. For example, degradation may be limited by the local hydraulic conditions within the channel: the interaction of a low channel slope, large cross-section and rough boundary can reduce flow velocity below the threshold for sediment transport. Consequently on many rivers these effects are constrained to the first few kilometres or tens of kilometres below the dam. Degradation of up to 7.5 m has been observed on large rivers immediately below the dam and decreasing downstream (e.g. the Colorado below the Hoover Dam). Typically, 1-3 m of degradation occurs within a decade or two of regulation (Church, 1995).
Further downstream, increased sedimentation (aggradation) may occur because material mobilised below a dam and material entrained from tributaries cannot be moved so quickly through the channel system by the regulated flows. Channel widening is a frequent concomitant of aggradation. Most degradation is observed during the first 10-15% of the period of adjustment as a certain armouring and stabilisation starts to occur (Brookes 1996). Thus both channel erosion and sedimentation take place in response to dam construction and operations.
Floodplains Damming a river can alter the character of floodplains as the reduction in high-magnitude flows reduces the number of occasions and extension of floodplain inundation. In this sense the river becomes divorced from it floodplain. Effects on floodplain ecosystems are specifically critical as they often are matured systems with a large biological diversity and complicated foodweb structures that are difficult to restore once lost (if at all). In some circumstances the depletion of fine suspended solids reduces the rate of overbank accretion so that new floodplains take longer to form and soils remain infertile. In other circumstances channel bank erosion results in loss of floodplains. For example, between 1966 and 1973, some 230 ha of land were lost from 10% of the total bank length of the Zambezi below the Kariba dam. Erosion was particularly pronounced at alluvial sites with non- cohesive sandy bank materials and was attributed to: the release of silt free water; the maintenance of unnatural flow-levels, sudden flow fluctuations, and out-of-season flooding (Guy, 1981). However, in some places the reduction in the frequency of flood flows and the provision of stable low flows may encourage vegetation encroachment which will tend to stabilise new deposits, trap further sediments and reduce floodplain erosion. Hence, depending on specific conditions, dams can either increase or decrease floodplain deposition and erosion. This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
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Coastal deltas In contrast to the impact on river and floodplain morphology, where aggradation may occur, impounding rivers invariably results in increased degradation of at least part of coastal deltas, as a consequence of the reduction in sediment input. For example, the slow accretion of the Nile Delta was reversed with the construction of the Delta Barrage in 1868. Today, other dams on the Nile including the Aswan High Dam have further reduced the amount of sediment reaching the delta. As a result much of the delta coastline is eroding at rates of up to 5-8 metres per year, but in places this exceeds 240 metres per year (Khafagy and Fanos 1993; AbdelMegeed and Aly Makky 1993; Stanley and Warne 1993).
Similarly, erosion of parts of the Rufiji Delta, by up to 40 metres per year, is attributed to the construction of dams (Horrill, 1993). The consequence of reduced sediment may also extend to long stretches of coastline eroded by waves which are no longer sustained by sediment inputs from rivers. It is estimated that the entire coastlines of Togo and Benin are being eroded at a rate of 10-15 metres a year because the Akosombo Dam on the Volta River in Ghana has halted the sediment supply to the sea (Bourke 1988).
Another example is the Rhone River, where a series of dams retain much of the sediment that was historically transported into the Mediterranean and fed the dynamic processes of coastal accretion there. It is estimated that these dams and associated management of the Rhone and its tributaries have reduced the quantity of sediment transported by the river to 12 million tons in the 1960s and only 4-5 million tons today. This has contributed to erosion rates of up to 5 meters per year for the beaches in the regions of the Camargue and the Languedoc (Balland 1991), requiring a coastal defence budget running into millions of dollars.
Further consideration of these issues is given in Annex 4.
3.6.2 Second Order Impacts on Primary Production
3.6.2.1 Plankton and Periphyton
The introduction of a reservoir into a river system as a result of impoundment can markedly alter the plankton component of the river system below the dam. Dams affect the plankton component of the river system in two ways:
1. by changing the conditions affecting the development of riverine plankton (e.g. through modification of the flow regime and alteration of chemical, thermal and turbidity regimes), and 2. by usually, but not always, augmenting the supply of plankton into the downstream system.
These changes will affect not only the total plankton present, but also plankton assemblages. Three factors govern the contribution of lentic plankton to the river downstream: the rate of water replacement within the reservoir (i.e. retention time); the seasonal pattern of lentic plankton development, and the character of outflows from the reservoir. Pulses of plankton output from reservoirs are often linked to season, hydrological conditions, nutrient supply and reservoir operation.
The flood mitigating characteristics of dams tend to promote the maintenance of higher than natural plankton populations within regulated rivers, by both sustaining populations released from the reservoir and promoting conditions for plankton development. For example, flow regulation imposed by the Eildon Reservoir, Murray River, Australia has allowed increased development of phytoplankton within backwaters, billabongs and fringing reed beds (Shiel, 1978). Furthermore, dams tend to enhance plankton development through temperature moderation, reduction of turbidity and reduction of effluent dilution (from incoming downstream tributaries etc.). This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
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Within impounded rivers, in temperate climates, the maintenance of higher summer discharges, the reduction of flood magnitude and frequency, reduced turbidities and the regulation of the thermal regime (i.e. higher winter temperatures) often promotes algal growth (Petts, 1984). Moderately swift currents and stable flow favour the growth of periphyton, but the effect of flow regulation on substrate stability may be the most important control. The periodic disruption of periphytic communities under, natural, variable flow conditions may be eliminated, or decreased in frequency, as a result of flow regulation. This allows the full development of a periphyton assemblage, at least in channels of relatively steep slope where moderate current speeds can be maintained.
Downstream from deep release reservoirs the composition of the attached algae and the proportion of the substrate covered changes as temperature, turbidity and substrate stability vary in response to tributary and anthropogenic inputs. Typically, algal growth occurs in the channel immediately downstream from dams, because of the nutrient loading of the reservoir releases, and diminishes downstream due to processes of self-purification. Increased algal density has been observed immediately below the Veyriers dam, on the Fontaulière River (France). However, although algal biomass was up to 30 times greater than at an upstream reference site, species composition was considerably altered. The differences have been attributed to nutrient pollution, lowered water temperature, flow constancy and substrate stability (Valentin et al., 1995).
3.6.2.2 Growth of Aquatic Macrophytes
Water depth and light penetration are important controls upon the composition and spatial patterns of higher plants. Together with current velocity and the susceptibility of the substrate to scouring, they are the dominant controls upon plant distribution. Thus it is the influence of dams on these factors that tends to dominate their impact on aquatic plants.
Of particular significance is the often general increase in bed stability downstream from dams. Compared with the situation in the natural river, the root systems of plants experience reduced effects of scour, the plants themselves suffer less stress from high discharges and the rate of channel migration is reduced, so that an area of the channelbed available for the development of aquatic plants can be stabilised. For example, in the years since the creation of Lake Kariba, flow regulation has allowed the rapid development of rooted plants (Panicum repens and Phragmites mauritanus) within the Zambezi (Jackson and Davies, 1976) where previously there were unstable sandbanks.
Flow regulation not only decreases the frequency of high flows and inhibits bed-material movement, but also induces the deposition of finer sediments where supplies are available from tributary or effluent sources. Channel sedimentation, particularly involving nutrient-rich silt, can markedly alter plant distributions. For example, sedimentation is often associated with the invasion and spread of Zannichellia palustris, which traps further sediments as it develops.
The elimination of high discharges to flush systems has allowed the extensive development of the aquatic weeds Water Hyacinth Eichornia. crassipes and Water Fern Salivinia molesta in both Africa and Australia. E. crassipes infested the lower reaches of the Fitzroy River, Australia after upstream dam construction stabilised flows thereby reducing floods and preventing salt water incursions to the upper tidal reaches (Mitchell, 1978). These growths may be supplemented by the discharge of floating weeds from infested reservoirs. Thus it is estimated that 150 000 S. molesta mats per hour, supplied by Lake Kariba, passed the Luanga confluence on the middle Zambezi in January 1974 (Davies, 1979). Colonisation by reeds of 41,000 ha of riverbed has occurred as a result of stablised flows on the Orange River, South Africa (Davies 1999).
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3.6.2.3 Riparian Vegetation
The characteristics of riparian communities are controlled by the dynamic interaction of flooding and sedimentation. Many riparian species are dependent on shallow floodplain aquifers that are recharged during regular flood events. Dams can have significant and complex impacts on downstream riparian plant communities. An important downstream manifestation of river impoundment is the loss of pulse-stimulated responses at the water-land interface of the riverine system. High discharges can retard the encroachment of true terrestrial species, but many riparian plants have evolved with, and become adapted to the natural flood regime. Species adapted to pulse-stimulated habitats are often adversely affected by flow-regulation and invasion of terrestrial weeds in these habitats is frequently observed (Malanson 1993).
Typically riparian forest tree species are dependent on river flows and a shallow aquifers. Therefore the community and population structure of riparian forests is related to the spatial and temporal patterns of flooding at a site. For example, the Eucalyptus forests of the Murray floodplain, Australia, depend on periodic flooding for seed germination and regeneration has been curtailed by headwater impoundment (Walker, 1979). Conversely, artificial pulses generated by dam releases at the wrong time – in ecological terms – have been recognised as a cause of forest destruction. For example, Acacia xanthophloea is disappearing from the Pongolo system below Pongolapoort Dam, South Africa as a result of mis-timed floods (Furness, 1978). The direct loss of annual silt and nutrient replenishment as a consequence of upstream impoundment is thought to have contributed to the gradual loss of fertility of formerly productive floodplain soils. It has been shown that given sufficient time after dam construction, riparian forest vegetation may be replaced by forest types more characteristic of unflooded upland areas (Thomas, 1996). Similar effects caused by the decoupling of basin hydrology from riparian vegetation (i.e. caused by changes in both high and low flow regimes) have been documented in the USA (e.g. Crawford, et al., 1994; Rood et al., 1995; Miller et al., 1995; Johnson, 1992).
A study in Sweden indicated that both storage reservoirs and run-of-river impoundments permanently altered and reduced the diversity of riparian vegetation. In comparison to natural river reaches there were one-third fewer species around storage reservoirs and 15% fewer species near run of river sites (Nilsson et al., 1997).
The Kariba dam has reduced downstream flood magnitudes within the Zambezi valley by about 24% (Masundire, in press). Within Zimbabwe’s Mana Pools National Park, flood extent has declined since the construction of Kariba Dam, reducing regeneration of the floodplain woodlands (Anonymous 1997).
3.6.2.4 Delta and Coastal Vegetation
Reduction in streamflow can also have considerable impacts on vegetation in downstream delta and coastal areas. Dam construction and operation in the Indus basin, for example, has reduced flow by more than 80% (McCully 1986). With the increased abstraction of water upstream, the quantity of silt reaching the delta has been reduced. Especially impacted are estuarine mangroves that once covered over 1 millon ha. The sediment brought down to the Delta is now estimated at about 60 Mt per year, about one fifth of original quantities. The active delta is only 10% of its original area and the reduction in the sediment discharge has meant that the balance between erosion due to high energy waves and sediment deposition has changed towards erosion. Mangrove forest needs sediment as part of its habitat renewal mechanism that provides direct benefits to people such as fuel, fodder and fibre and forms rich nursery grounds for fish. The reduced freshwater and sediment flow plus human encroachment contributes to further mangrove degradation. These changes have even affected the total site biodiversity as mangrove species in the Delta have decreased from the eight recorded species to a virtually mono-specific mangrove stand (WCD Tarbela Case Study 2000).
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3.7 Third-Order Impacts on Fauna
3.7.1 Freshwater Species Diversity Changes
Only a modest fraction – perhaps 10% – of the planet’s species have been discovered by science, named and classified: the known species. Of the 1.87 million recorded species of plants, animals, and micro-organisms, 44 000 or 2.4% occur in freshwater, 14.7% in the sea, and 77.5% on land (Box 3.4). However, the diversity of freshwater species is 10% higher than that on land when the fact that freshwaters comprise only 0.8% of the surface area of the planet is taken into account. The disproportion is even greater for the fishes; about 42% of known fish species occur in the tiny fresh water area, compared to 58% in the far greater marine area. Freshwater fish species diversity generally increases at lower latitudes. This has specific consequences for dam construction impacts at these latitudes as their impact on species loss can be potentially much higher than at higher latitudes (Figure 3.6).
Figure 3.6: Fish species richness decreases at higher latitudes indicating that dam construction in tropical regions could potentially have more impacts than at higher latitudes (WCMC 1998).
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Dams, Ecosystem Functions, and Environmental Restoration 37
Species populations may be located along a spectrum from common to rare. Declining species status can be measured in simple terms by lowered populations, extirpations (loss of populations from a part of the species range), or extinctions (loss of all individuals of a species). IUCN (1994) has developed a scientifically objective means for assessing such populations, and many governments have enshrined species status considerations in their national legislation.
Box 3.4: Species richness of the planet’s major environments (Source: McAllister et al. 1997)
Environment % area of % of known Relative species
Planet living richness surface species (%species / %area)
Fresh water 0.8% 2.4% 3.0 Terrestrial 28.4% 77.5% 2.7 Marine 70.8% 14.7% 0.2 Symbiotic N.A. 5.3% N.A.
According to IUCN's 1996 Red List, 1 107 bird species (11% of the total) are threatened and 104 (1%) are extinct. Among the more threatened of bird groups are the aquatic rails and cranes with 54 species threatened, and the partially aquatic kingfishers and bee-eaters with 11.5% threatened, while 18% of the grebes are threatened. Extinct aquatic birds include the Colombian Grebe (Podiceps andinus) and the Atitlan Grebe (Podilymbus gigas). The IUCN Red List of Threatened Plants concluded that 33 375 species or 13.8% of the world’s 242 000 vascular plant species are threatened, and 376 are extinct.
At regional scale few detailed data are available. The exception is North America where freshwater animals have been shown to be the most endangered species group on the continent, dying out five times faster than those that live on land, with a rate similar to the loss of rainforest species. Since 1900, at least 123 species have been lost from North America’s waters. A further 190 fish, 27 amphibian, 35 reptile, 84 bird and 94 mammal species are currently threatened with extinction, as 51% of species decline in numbers (Riccardi and Rasmussen, 1999). In the United States alone data on the conservation status of freshwater species groups give an alarming picture:
• 67% of freshwater mussels are vulnerable to extinction or are already extinct • 303 fish species – 37% of the US freshwater fish fauna – are at risk of extinction • 51% of US crayfishes are imperilled or vulnerable • 40% of amphibians are imperilled or vulnerable • at least 106 major populations of salmon and steelhead trout on the west coast have been extirpated, and an additional 214 salmon, steelhead trout, and sea-run cut-throat trout stocks are at risk of extinction (Nehlsen et al. 1991).
While these figures give an indication of the scale of the threats to freshwater biodiversity, the information constraints highlighted in section 3.4. mean that there are limited data available on the specific impacts of dams on species diversity. However, useful studies have been carried out on some groups and these can serve as indicators. The following sections on molluscs, fish and waterbirds therefore serve to illustrate how dams impact upon the biology of individual freshwater species and thus lead to changes in species diversity.
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3.7.1.1 Invertebrates, Fish, Birds and Mammals
The preceding sections have shown that when dams are constructed the variability in water discharge over the year is reduced; high flows are decreased and low flows may be increased. Reduction of flood peaks reduces the frequency, extent and duration of floodplain inundation. Reduction of channel-forming flows reduces channel migration. Truncated sediment transport (i.e. sedimentation within the reservoir) results in complex changes in degradation and aggregation below the dam. These changes and others directly and indirectly influence a myriad of dynamic factors that affect the diversity and abundance of invertebrates, fish, birds and mammals downstream of dams. In light of the information constraints outlined in section 3.4., comprehensive data are not available. It is only therefore possible to provide here a first indication of the third-order impacts on ecosystem functioning and productivity. Section 3.7 then examines the specific issues of the impact of dams on species diversity using three more thoroughly-studied groups as indicators.
Most aquatic species cannot live for long without water, e.g. those breathing with gills. When a dam closes off river flow, some species may avoid dehydration for short periods, e.g. snails by closing their operculum. Some downstream populations will be reduced but may manage to hang on in pools or tributaries. Survival in such pools may be reduced by predation as individuals are more accessible because they are concentrated in the shallows. These effects can lead to declines in downstream fisheries.
Larger aquatic species such as sturgeons, crocodiles and dolphins require minimal flows in which to navigate, feed, etc. Such species may be seriously affected by reduced flows which mean reduction of area of habitat. Habitat reduction may mean simply smaller populations or reduced growth rates, or where populations are already at risk, it may lead to extirpation (loss of a population) or extinction (loss of an entire species).
Large woody debris plays an important and until recently unrecognised role in providing fish and food base habitat. Wood contributes to complexity of channel form and habitat in many rivers. In some cases, woody debris is removed from the downstream environment by the storage dam operations. If the original amount of debris input into the river is large compared to the downstream input the impacts of dam construction and operation can be considerable (Kondolf 1999/ENV083/ENV085/ENV088).
River-dwelling species have several migratory patterns. These include the well-known anadromous fishes like salmon and catadromous fishes like eels. Adults of the first migrate up rivers to spawn and the young descend, while the reverse occurs with the latter. But many other freshwater fishes move up rivers or their tributaries to spawn, while the glochidia larvae of freshwater mussels hitch rides on host fishes. Migration between marine and freshwater ecosystems and within freshwater ecosystems are known. Dams block these migrations to varying degrees.
Biological linkages also extend laterally away from the river, extending the effect of river changes to a band of varying width, parallel to the river. As long as the river flow is sufficient, other wildlife such as deer, antelope and elephants will come to the water, especially in the dry/hot season, for drinking water. These lateral movements can extend to several kilometres from the river. Many wildlife species in a fairly wide strip of land on either side of the river depend upon it, and they may all be affected when the flow of the river is disrupted by the construction of a large dam.
The blockage of fish movements upstream is probably the most significant and negative impact of dams on fish survival and biodiversity. Many stocks of Salmonidae and Clupeidae have been lost as a consequence. In the Columbia River alone, more than 200 stocks of anadromous salmonids have been
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extirpated. Sturgeon populations in the Caspian Sea now rely on hatcheries, mainly in Iran, since Russian dams block natural spawning migrations.
The control of floodwaters by large dams, which usually reduces flow during natural flood periods and increases flow during dry periods, leads to a discontinuity in the river system. This together with the associated loss of floodplain habitats normally has a marked negative impact on fish diversity and productivity. The connection between the river and floodplain or backwater habitats is essential in the life history of many riverine fishes that have evolved to take advantage of the seasonal floods and use the inundated areas for spawning and feeding. Loss of this connection can lead to a rapid decline in productivity of the local fishery and to extinction of some species.
An assessment of 66 case studies of the impacts of dams on fish biodiversity concluded that 27% of cases had positive impacts (i.e. increase in species richness) compared to 73% having negative impacts (i.e. decrease in species richness) . Of the latter, 53% were downstream of the dam, affecting upward fish migrations and connections to floodplains. Within regions, negative impacts of this kind are more common in temperate than in tropical zones. In tropical regions, the extent of positive impacts is much greater than in temperate ones, particularly in reservoirs upstream of the dams (McAllister et al., 2000). Where fish biodiversity increases it occurs because the reservoir provides “new” habitat for fish species preferring lentic habitats. In many cases people introduce exotic (i.e. non-native) species to improve fisheries.
Fish migrations in the tropics are probably best known in the Neotropical region. Hydroelectric dams in the Amazon basin as a whole have halted the long distance migrations of several species of catfish although the available data are not quantified (Ribeiro et al.,1995). The dams have also interrupted the downstream dispersal of catfish larvae. On the Araguaia-Tocantins River Basin, several species of fish which undergo long distance migrations have been drastically reduced in abundance as a result of dams blocking their routes. Downstream fisheries have been reduced by 70%, probably as a result of recruitment failure.
In Africa the recent droughts have made it difficult to differentiate between the effects of reduced flow resulting from dams and from lack of rainfall, for example in the central delta of the Niger River (Läe 1995). However, substantial losses to overall fishery production in river basins have been reported in Africa as a result of dam construction. For example, 11 250 tonnes of fish per year from the Senegal River system were lost following dam construction (Reizer, 1971). A major concern throughout Asia is that movements of migratory fishes along river courses are being blocked by dams.
Dams can enhance some riverine fisheries, particularly tailwater fisheries immediately below dams that result from discharge of nutrients (seston) (primarily plankton) from the upstream reservoir. However, discharge of seston is typically attenuated quickly downstream from the dam, with corresponding attenuation of the associated fisheries. If discharge is from the hypolimnion of the reservoir, lowered temperatures in the receiving tailwater can curtail or eliminate warmwater river fisheries and require stocking of exotic coldwater species such as salmonids (assuming that the water is sufficiently oxygenated). Productive tailwater fisheries targeting these coldwater fishes can result but generally require supplemental hatchery programs and introduction of coldwater invertebrates to serve as food for these fish. In North America, yields from cold tailwater fisheries have been recorded for up to 753 kg/ha/year with fishing effort 7-16 times higher than the respective upstream reservoir.
Estuarine Impacts. Reduction in freshwater flow can result in an increase in salinity in estuarine areas and upset the complex nature of water currents which in turn can alter fish biodiversity. Increased salinity has occurred in the Nile Delta although the effects on fishes have not been well documented (Aleem, 1972; Stanley and Warne, 1993). Marine fishes were found in higher reaches of the Eastman Estuary after 90% of the river water was diverted to the La Grande River (Canada) (Ochman and Dodson, 1982).
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Marine Impacts. Freshwater flows support marine fish production. The effect of reduced freshwater flow is probably greatest in the first year of life of a fish population. Fish abundance is normally determined during the egg and larval stages (Drinkwater and Frank 1994). Thus, although the annual discharge through a hydropower dam may not differ much from unregulated flow, unless water is diverted, the seasonal timing of discharge may be significantly different and have negative impacts on marine fishes. Many marine fishes spawn in estuaries or floodplains generally at times of peak run- off. A decrease in freshwater flow and in nutrients may affect the nursery areas in a number of ways including increasing salinity, allowing predatory marine fishes to invade and reducing the available food supply. These impacts are well illustrated by the effect of the Aswan High Dam on the coastal waters of the Mediterranean (Aleem 1972; Drinkwater and Frank 1994). Here reduction in nutrients transported to the sea has reduced production at all trophic levels, resulting in a decline in catches of sardines and other fish. In the Zambezi delta the impact of modified seasonal flows on shrimp fisheries has been estimated at 10 million dollars per year (Gammelsrod 1992a, 1992b).
3.7.2 Bivalve and Gastropod Molluscs
Bivalve molluscs are especially important elements of riverine ecosystems because of their ecosystem functions and economic value.
Box 3.5: Global hotspots for freshwater molluscs
River System Species % Endemic
Mobile Bay, USA 192 78 Balkans region 190 95 Lake Baikal, Russia ±180 67 Lower Mekong 160 72 Lower Zaire 96 25 Lower Uruguay/Rio de la Plata 93 37 Lake Tanganyika 83 64 Western Ghats, India 71 18
They are also highly endemic, and therefore subject to extinction (Box 3.5). The ecology and life history traits of one group of molluscs, the freshwater mussels (Unionoidea), makes them an important indicator of ecosystem health and of the impact of physical and biological changes in the ecosystem on species diversity. Freshwater mussels are filter feeders requiring a rich and plentiful supply of diatoms, desmids, filamentous algae and other algal species. They are therefore especially vulnerable to the second and third order ecosystem impacts described above.
In addition their reproductive cycle may be seriously disrupted by dams. This involves a larval stage (called the glochidia), which is retained in the female brood pouch or gills and released for their intermediate stage as a parasite of a host fish before being transformed to bottom-dwelling juveniles. Dam building activity which blocks migratory fish or changes fish communities can also reduce the reproductive success of the freshwater mussel communities which depend on the fish as glochidial hosts. For example, dam construction at Lake Pepin on the Mississippi River led to the demise of mussels upstream of the dam, as the runs of skipjack herring, their host species, were blocked (Eddy and Underhill, 1974). Unfortunately few of the host fish for mussels have been identified.
Stresses associated with dam construction resulting in physical disturbance of the river bed may also cause mussels to prematurely empty their brood pouches of glochidia, resulting in reproductive decline (Howells et al., 1996). The changes in water chemistry that occur in reservoirs may in turn This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
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affect spawning of mussels (Isom, 1971). Bivalve species are very sensitive to water chemistry. Translocation experiments on endangered mussel species in Europe have shown that changes in water chemistry can lead to stress and trigger release of glochoidea during unsuitable flow/water conditions. Changes in temperature may affect spawning of mussels, growth and the duration of the growing season (Isom, 1971). Low dissolved oxygen levels may cause stress to freshwater mussels although some species can withstand brief periods of low oxygen levels.
Increased siltation can be a major problem in some areas. The greatest diversity in the prosobranch gastropod fauna in the USA is found in the Mobile Bay river basin, and the Tennessee river basin, although 7% of total taxa are now extinct (Bogan, 1998). Most of the extinctions in this group (38 out of 42 taxa) are in the Mobile Bay fauna, and occurred when the river shoal fauna was impounded and covered by deep standing water and subsequent siltation.
Many mussel species also have extended life cycles, some of which span over 100 years, where maturity is delayed until the individual reaches 6–15 years of age (Bauer, 1993; Chesney and Oliver 1998). This can lead to the impression that populations are secure, when in fact no active recruitment is taking place and the populations may well be functionally extinct.
Box 3.6 demonstrates the impacts of dams and reservoirs, using data from the USA. Impacts are evident from construction, after construction, and downstream. Species richness of molluscs has declined between 40% and 80% from the original diversity levels in certain USA rivers, over a period of 50 years. The figures in post-dam richness also indicate the stretches downstream of the dam where the bed may be devoid of mussels.
Box 3.6: Mollusc species present within reservoir region, USA (Source: Neves, 1999)
Reservoir Preimpoundment Postimpoundment Date Richness Richness Norris, Clinch R. (1937) 40 species (1935-37) 12 species (1990’s) Center Hill, Caney Fork (1948) 39 species (pre-1940) 2 species (1993) Cumberland, (1952) 59 species (1947-49) 16 species (1961) Wheeler, Tennessee R. (1936) >60 species (pre-1935) 18 species (1991) Demopolis, Tombigbee R. (1936) 50 species (1933-35) 8 species (1954) Demopolis/Warrior (1954/57) 48 species (pre-1950) 13 species (1972-75)
Stein and Flack (1996) conclude that the current decline of freshwater mussels in the Mississippi Basin will have a detrimental impact upon the entire ecosystem. They point out that the freshwater mussels play an important role in sediment mixing and nutrient recycling, and given their dominance in terms of biomass, their removal could have long-term repercussions that are as yet unknown. They are also a major food source for aquatic vertebrates.
Water level fluctuation also affects gastropod species. Brown (1994) described the gastropod diversity of several African reservoirs which are comparable in size to large natural lakes. The outflow from these reservoirs differs from the natural lakes, with most suffering large seasonal draw- down as outflows from the lakes are regulated to ensure that the rainy season floods can be contained. This gives a very unstable littoral zone, which stresses aquatic life at the margins, restricting the number of mollusc species which can survive in the lake. Conversely, stabilisation of flows in the Senegal river following construction of the Manantali and Diama dams allowed colonisation by bilharzia-carrying snails, that were previously absent from the dynamic flood river ecosystem.
Riparian habitats also hold unique species of molluscs. Disturbance during the construction phase, especially the destruction of habitats for temporary roads, can lead to loss of these species. These losses may be permanent or temporary, depending on the degree of degradation and the amount of habitat fragmentation. In South America, possible species extinctions have been related to loss of This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
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gallery forest adjacent to rivers which are now submerged following construction of the Salto Grande Dam. The land-snail Anthinus albolabiatus (Jaeckel, 1927) was formerly endemic to gallery forests of the Uruguay River and has been submitted for inclusion in the IUCN Red List of Threatened Animals.
3.7.3 Impact of Dams on Fish Diversity
Fish are the most species-rich of all vertebrates. Valid scientific descriptions exist for about 24 600 living species of fishes in 482 families (Nelson 1994). One third of the fish families have at least one member spending at least part of their life in freshwater. Freshwater fish diversity is therefore large compared to other systems since freshwater lakes and rivers account for only 0.8% of the earth’s surface and less than 0.01% of its water. Approximately 10 100 species are entirely freshwater and 2 500 move between the sea and freshwater during their life cycles (Helfman et al.,1997). An indication of fish species diversity in some river systems is presented in Box 3.7.
The largest number of species occurs in the tropics and the diversity of fishes, in general, increases from the poles to the tropics. Southeast Asia, South America and Africa have the most freshwater fishes. However, many have not yet been described, so taxonomists are needed to describe unknown species especially in these species-rich areas. It is also important to protect genetically distinct stocks within a species. For example, Ryman et al.,(1995) suggested that it is just as important to protect the intraspecific diversity of the Atlantic salmon as to protect the cichlid flock in Lake Malawi.
The 1996 IUCN Red List of Threatened Animals lists 617 freshwater fishes (including euryhaline – salinity-level tolerant – species), about 7% of the known number of freshwater fish species. Studies that take into account the fact that the Red List has evaluated only a fraction of freshwater fishes estimate conservatively that 20% of freshwater fishes are either extinct, endangered or vulnerable; a more realistic estimate might reach 30-35% (Stiassny, 1996).
Fish populations are highly dependent upon the characteristics of their aquatic habitat that support their biological functions. Migratory fish require different environments for the main phases of their life cycle: reproduction; production of juveniles; growth; and sexual maturation. The life cycle of diadromous species takes place partly in fresh water and partly in seawater; the reproduction of anadromous species takes place in freshwater; and catadromous species migrate to the sea for breeding purposes and back to freshwater for trophic purposes. There are also migrations of potadromous species, whose entire life cycle is completed within the inland waters of a river system.
The disruption of movement of species upstream has probably been the most significant and negative impact on fish biodiversity and many examples illustrate the point from all regions. Large dams halt long distance migrations and the fish fail to reach their spawning grounds. Many anadromous fish populations such as Salmonidae and Clupeidae (e.g. shads) have died out as a result. The sturgeon populations in the Caspian Sea now rely on stocking from hatcheries (mainly in Iran) as natural spawning migrations were halted by dams built by the former USSR on rivers entering the sea.
The best-documented examples of disrupted migrations are from the west coast rivers of the USA, in particular the Colorado and Columbia Rivers. In the Columbia River more than 200 stocks of anadromous salmonids have become extinct. Catadromous species such as Anguillidae have been less affected although adults are often killed in hydroelectric turbines. Eels are not restricted to specific rivers, like salmonids, and can move into new rivers if their path is blocked by a dam (Drinkwater and Frank 1994). Even when fish passes have been installed successfully, migrations can be delayed by the absence of navigational cues such as strong currents. This causes stress on the energy reserves of the fish as anadromous fish such as salmonids do not feed during migration.
Mortality resulting from fish passage through hydraulic turbines or over spillways during their downstream migration can be significant. The risk of injury varies according to fish size and species, but typically ranges from <1% for young fry to between 5-20% for 15cm Atlantic Salmon smolt for This is a working paper prepared for the World Commission on Dams as part of its information gathering activities. The views, conclusions, and recommendations contained in the working paper are not to be taken to represent the views of the Commission
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example. Mortality in adult fish may reach 100% without special protection measures. Problems associated with downstream migration can also be a major factor affecting anadromous or catadromous fish stocks. Habitat loss or alteration, discharge modifications, changes in water quality and temperature, increased predation pressure, and delays in migration caused by dams are significant issues.
Box 3.7: Fish species richness in selected river basins (after: World Bank 1998, WCMC 1998)
Watershed/Continent Number of fish species Number of species 100/000 km2 of watershed
Kapuas, Indonesia 320 360 Mekong – Asia 1,200 147 Chao Phrya, Thailand 222 124 Xi Jiang (Pearl), China 290 71 Amazon - South America 3,000 49 Orinoco - South America 318 33 Yangtze – China 322 19 Paraná - South America 355 14 Congo - Africa 900 13 Mississippi – USA 375 12
In Australia, dams have generally resulted in negative impacts upon native riverine fishes while encouraging exotic species. This has been attributed, in part, to disruption of seasonal flood cycles, and to dams acting as barriers to fish movements. The Murray-Darling, which has 84 main reservoirs with capacities of 10 000 ML capacity and over, now has the lowest commercial fish yield per sq km of floodplain of any of the world’s major rivers, although historical catches were comparable (Jackson, 1999).
Fish diversity in reservoirs is usually not as extensive as in natural lakes, because natural lakes have more stable conditions under which the fishes evolve. Riverine species have to live under harsher and more variable conditions. During reservoir formation the river and possibly associated wetland areas become inundated. As the reservoir fills, riffles, runs and pools of the river are lost beneath the rising waters leading to the extinction of habitat-sensitive riverine species with tightly defined niche requirements (e.g. species of darter (Percidae) found in streams above dams in the Tennessee River system (Neves and Angermeier, 1990). During construction, downstream flow may be severely restricted, as at Cahora Bassa, Mozambique (Jackson 1999), eliminating the fishes present below the dam. However many fishes can quickly recolonise once a flow is re-established. The filling of reservoirs may take a few months (e.g. Kainji) or years (e.g. Volta, Kariba and Nasser/Nubia), and fishes adapt better to prolonged filling.
Reduced number of species in reservoirs may also be an artefact created by inappropriate timing of dam closure and poor control of environmental impacts during dam construction. The initial natural stocking with native species is of high importance in determining the species composition of the stabilised reservoir. If dam closure occurs during the dry season, the number of naturally stocked species will likely be minimised and not be representative of the full complement of fish species which occur in the river all year round. This is because many larger fish species migrate downstream to refuge habitats during the dry season and only migrate upstream into low order tributaries during the rainy season for spawning purposes. The disruption in normal hydrological flows which can occur during the dam construction phase, compounded by excessive erosion and siltation of the river in the vicinity of the dam site, may result in disturbance of fish stocks and migrations, and reduce the magnitude of fish biodiversity and quantity available for initial natural stocking (Bernacsek, 1997).
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3.7.4 Dams and Waterbirds
Waterbirds, both migratory and non-migratory, are important components of the biodiversity of wetlands throughout the world. This is recognised in international conventions and agreements, which place requirements on Parties to safeguard waterbirds throughout their range and distribution. This is achieved in several ways, notably through the designation of wetlands of international importance for waterbirds through the Ramsar Convention on Wetlands of International Importance, and the development and implementation of flyway-scale migratory waterbird conservation strategies, notably the Bonn Convention African-Eurasian Migratory Waterbird Agreement (AEWA) and the Asia-Pacific Migratory Waterbird Conservation Strategy. Such flyway-scale initiatives recognise the vital need to safeguard the international networks of key sites upon which these birds depend throughout the year for their survival, and to put in place a range of management measures to maintain these populations.
Waterbirds may use natural and dammed open water wetlands for breeding, and during their non- breeding seasons for feeding and for roosting. Wildfowl (divers, grebes, cormorants, swans, geese, ducks, coots and rails) are particularly characteristic of open water systems. Large numbers of waterfowl migrate south and south-west from arctic, sub-arctic and boreal breeding areas in Europe, North America and Russia to overwinter in the relatively mild climate of western Europe, Africa, and tropical America. Other major waterbird guilds such as waders (shorebirds) chiefly use the shallow emergent shorelines of such wetlands for feeding during migration staging or wintering.
Of 957 Ramsar sites designated by December 1998, 10% included artificial wetland types, compared to 25% including natural lake types (Frazier, 1999). Many of the designated artificial wetlands are dammed sites: of the almost 100 artificial wetlands designated as internationally important, 78 are listed as having water storage areas either as a primary or occurring wetland type. Of these 78 sites 57 were designated either wholly or partly for their internationally important waterbird populations. Nineteen regularly support over 20 000 waterbirds (Ramsar Criterion 5), 13 sites regularly support more than 1% of the biogeographic population of one or more waterbird species (Ramsar Criterion 6), and a further 22 sites meet both of these waterbird criteria (Frazier, 1999)).
In inland South Africa for example, almost all permanent waterbodies are dammed sites, constructed for water storage purposes. The total capacity of these impoundments amounts to some 52% of annual run-off. These range from large scale impoundments several kilometres long to many small farm dams. At least 517 major reservoirs were constructed by 1986, along with many tens of thousands of farm dams of a few hectares each in area (Taylor et al., 1999). The overall impact of these many artificial open water bodies has been to greatly increase the year-round availability of permanent lakes in inland South Africa (Cowan and van Riet, 1998) and this has undoubtedly had very major effects on the distribution and numbers of waterfowl in the region.
Artificial wetlands are included in many Important Bird Areas (IBAs) identified in South Africa (BirdLife International, in prep.), and at least 12 impoundments support major and important concentrations of waterbirds. Overall large dams have provided increased areas of suitable habitat for several species that favour deep open-water conditions. The suitability of such dammed lakes for other species depends, as elsewhere, on the extent to which they provide areas of fringing emergent vegetation and shallow shorelines, features which generally are found in the upper parts of impoundments. Large dams in South Africa have provided generally beneficial conditions for Pelecaniformes (pelicans, darters and cormorants). They provide suitable habitats for moulting sites for waterfowl: for example at least 70% of the global population of the South African Shelduck Tadorna cana moults at only 23 localities in South Africa, 21 of which are large dams. Dams also provide dry season or drought refuges for many waterfowl species, and breeding sites for many South African waterfowl, including some species of national conservation concern, notably the Pink-backed Pelican Pelecanus rufescens and Caspian Tern Hydroprogne caspia.
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While dams in South Africa have increased the amount of suitable year-round habitat for species of waterbird that prefer open-water habitats, and in some cases species that feed along the shallow margins of the dams, the overall waterbird assemblage that naturally occurs in southern Africa has suffered from major negative impacts. These include: