RESTORING LIVING RIVERS

If governments really want to restore damaged rivers, they have to do more than clean up pollution or reforest damaged watersheds. They have to acknowledge the central role of dams in perpetuating and accelerating environmental degradation.

By Philip Williams

April 2000

At the end of the 20^th century, society came to the belated realisation that human activities have greatly harmed the global environment. The water world has been particularly hard hit, from the worldwide deterioration of freshwater resources, to the destruction of rivers, wetlands and estuarine ecosystems and the loss of the biodiversity these ecosystems support, to the degradation and impoverishment of millions of river-dependent people. Almost everywhere on the planet, rivers have been affected by pollution, channelisation, destruction of their watersheds, and, in profound ways, by large dams.

As we begin the 21^st century, there is increasing interest in restoring our damaged rivers. Some governments now accept the need to clean up pollution or reforest damaged watersheds. Numerous water managers have begun to recognise the benefits of reconnecting rivers to their floodplains.

But in contrast to this flood of activity to restore riverine health, most governments and river-planning agencies around the globe still do not fully acknowledge the central role of dams in perpetuating and accelerating environmental degradation. Such an understanding is essential if we are to stop repeating the mistakes of the past and start healing the problems of the present.

Living Rivers

Every river basin has a unique geologic and climatologic history that has created its particular landform. This landform, from mountains to mudflats, has been created by geomorphic processes - the action of flowing water eroding and depositing sediments.

Certain dynamic features of the landscape, such as alluvial floodplains or meandering river channels, can persist over tens of thousands of years in an evolving equilibrium, responding to periodic floods, tectonic movements, and sea-level rise. Thus a watershed is a product of its own evolution and contains a sort of virtual DNA which determines the unique character of the particular river that drains it.

Fish, plants and other species evolve to take advantage of the river’s physical processes and the forms they create. For example, fish migrate at certain times of the year when flows and water temperature are advantageous; riparian trees have adapted to seasonal flooding, and estuarine shellfish rely on nutrients released from decayed leaves from floodplain forests.

This evolutional history means that the integrity of the whole ecosystem is dependent on the integrity of the physical processes that sustain it. This dynamic interaction between river flow, river form, people, plants, fish and wildlife is what is meant by the term ‘living river’ to describe the river ecosystem.

Like any other species, humans have interacted with and taken advantage of river ecosystems for tens of thousands of years. The abundance of fish and wildlife and the fertility of floodplain soils enabled river communities to prosper even as they transformed the landscape with fields and villages.

Only in the industrial age, with the onset of widespread pollution and the construction of massive river engineering projects, has the relationship between river ecosystem and human settlement become severely out of balance. The main reason for this unsustainable interaction is that now human intervention has drastically changed river processes as well as the landscape. A dam’s impacts on ecosystems are massive and continuous, and prevent the river from healing itself.

What Dams Do

All rivers are both a river of water and a river of sediment. River systems are resilient and persistent because the erosion and deposition of sands and mud, together with regrowth of riparian vegetation, can quickly re-create disturbed landscapes - sometimes within a few decades. A straightened river will always tend to re-create its meander; a mined gravel bar rebuilds in the same place.

In other words, the ‘living river’ is inherently self-healing, always evolving to re-create its healthy form, provided the movement of sediment is not impaired. To return a river to a living state requires understanding, maintaining and restoring key physical processes that sustain the landscape, processes like regular inundation of floodplains, movement of gravels, or summer flow levels.

In the past 60 years human technology has become capable of constructing dams large enough to eliminate and transform key physical processes in the world’s largest rivers. Major dam projects have been planned and executed with little understanding of the consequences of these alterations on the health of the river.

Using a medical analogy, the initial construction of the dam is an acute shock to the system, but it is the continued operation of the reservoir that creates chronic illness in a healthy river by changing its hydrology and geomorphology. There are many ways dams affect physical processes essential for sustaining a living-river ecosystem, but in problems of river management and restoration the following are the most important pathologies:

‘Persistent bleeding’ (Diversions): The impacts of reductions in flow are easiest to see. The greatest impacts occur in the low-flow season of drought years, but in some semi-arid regions the downstream river channel has been almost permanently completely dried up by dams.

For many years, restoring more natural flows has been a main focal point of habitat restoration efforts, and more recently this has extended to efforts to restore freshwater flows to sustain major estuaries like California’s San Francisco Bay. This has led to the formulation of minimum or ‘in-stream flow requirements’ for fish.

Some dam operators now release a fraction of their inflow to provide such flow releases; proponents of this approach claim that this action alone will mitigate downstream ecological impacts. These claims are at best unproved. Restoring minimum flows does nothing to sustain and replenish critical habitats created by floods and flow variability.

‘Irregular heartbeat’ (Elimination of the flood flow pulse): Seasonal and flood flow variations are the heartbeat of a river ecosystem. Typically, large reservoirs store seasonal high flows to release them for power generation or irrigation later in the year. This has the effect of greatly reducing flow variability downstream by capturing smaller frequent floods. Larger, infrequent hazardous floods are much less affected as they tend to fill the reservoir and then spill uncontrolled at the flood peak.

These changes in flow variation and flood frequency have  major adverse impacts on key river processes. For example, they typically eliminate regular floodplain ecological interactions without eliminating flood hazards. They eliminate natural periodic disturbance essential for rejuvenating wetland habitats. They prevent pulses of freshwater which sustain estuarine ecological processes. Finally, they reduce the capability of the river to move sediment downstream and allow choking of the channel with vegetation.

Recently, attempts have been made to release small flood pulses from some reservoirs to mitigate these adverse effects. Such measures alone are unlikely to be more than marginally useful if they do not also restore the flow of sediment to the river.

‘Thinning of the blood’ (Trapping sediment): Large reservoirs can capture almost all of the sediment carried by a river. Until the reservoir completely silts up, releases downstream are ‘clear water’ flows that erode the bed and banks of the river channel until the natural sediment load has been regained. The impacts on river system habitats are profound.

Lowering the channel bed isolates the floodplain from ecological interaction with the river channel as effectively as if it had levees, and channel lowering lowers the water table, thus drying up floodplain wetlands and woodlands. Silts and muds no longer replenish floodplain soils. The complexity of habitats in the river channel is simplified into a single uniform thread. Further downstream, mud no longer replenishes estuaries and sands are no longer delivered to beaches, degrading estuarine and coastal habitats.

Bringing Rivers Back to Life

The history of dam construction has been a history of environmental devastation. However, in the past two decades some attempts have been made to mitigate adverse impacts on fisheries, endangered species and wetlands; and recently in the US there has been a growing acceptance of the need to re-operate existing large dams for this purpose. A continued failing of these attempts is that they address symptoms, not causes.

In the US we know that technical fixes like fish hatcheries are not working very well, and we also see attempts to restore landscapes without restoring physical processes. Examples are attempts to restore floodplain forests that are no longer inundated by floods, or efforts to create artificial fish-spawning channels without necessary scouring flows, which clean out spawning gravel or sediment.

It has taken us a long time to recognise that the essential processes which need to be restored are those that sustain the life of the river; the vigour of its flow, its flood-flow pulse, and its lifeblood, sediment. These are the processes most affected by large dams.

Our conceptual model of a living river leads us to a meaningful approach to restoration. We can progressively remove or limit human interventions such as dams so that the river can restore itself. Practically, this means re-establishing the seasonal and annual variation in flow, in particular the smaller floods that do most of the work in scouring and depositing sediments; inundating and replenishing floodplain woodlands and farmland, and re-watering aquifers and wetlands. It also means re-establishing a way for the river sediments to move downstream.

An Obsolete Legacy

An equally important question of the 21^st century is, How do we retrofit obsolete water management infrastructure to reflect modern societal goals? These goals now include protecting the rights and values of river-dependent people, protecting the river ecosystem, and ensuring economic accountability for river management decisions. In order to answer this question we need to change our view of the dam as an everlasting concrete structure, to the dam as a continued human decision to perpetuate a certain type of intervention on a healthy living river.

One way to begin to restore rivers to life would be to conduct a periodic audit of existing dams, to re-evaluate the rationale for their continued operation. This audit would be similar to a relicensing process used on private dams in the US. It would include an environmental and social assessment of projected future operations and determine whether the dam will meet contemporary societal goals. From such an audit, decisions can be made to continue a dam in operation, modify its operation or decommission it.

This process would lay the groundwork for a new direction in river management. Instead of being trapped into more and more extensive and expensive attempts to mitigate the ever-expanding negative impacts of obsolete poorly planned dams, we would have the freedom and opportunity to realise the cumulative social and ecological benefits of restoring and managing our river valleys.

For those dams we decide to continue operating, we have to place their rivers on permanent life support. This means developing new reservoir operations and river management regimes that mimic natural hydrologic processes. We have a pretty good idea of what they are. To achieve them may require making significant changes in flood-control operation and even retrofitting dam structures with new spillways, but these changes could significantly improve flood protection while restoring river corridor and wetland ecosystems.

New reservoir-operating criteria will require fair evaluation of the trade-offs between  irrigation  deliveries  and   ecosystem  restoration.   To   do   this  requires   first establishing a transparent and rigorous accounting system for how we manage our water resources.

But even this does not treat the ‘whole body’ of the river. Once we free ourselves from the legacy of engineering decisions made 60 years ago, we have the opportunity to realise the cumulative societal, economic and ecologic benefits of large-scale restoration. For example, we know that restoring lowland river floodplains will not only reduce flood damages, but help recharge groundwater, preserve greenbelts, and provide recreational benefits.

It is possible to envisage a future where rivers and water resources are managed in harmony to sustain a viable ecosystem and a modern economy. To get there, we have to first break free from the legacy of greed and bad planning that imposed big dams on our rivers. We need to articulate a clear, comprehensive and scientifically defensible vision for river management. And we need to begin taking bad dams down. - Third World Network Features

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About the writer: Philip Williams is president of International Rivers Network and a consulting hydrologist. The above article first appeared in World Rivers Review (February 2000, ‘The Flood Next Time: Restoring Living Rivers’).

2031/2000