The legacy of large dams

The Davis Dam is located on the Colorado River near Laughlin in Nevada

One less explored aspect of large dams is that the development of dams and reservoirs can enable agricultural expansion and urban growth, which in turn increase water consumption. As such, dam development influences, while being influenced by, the spatial and temporal distribution of both supply and demand of water resources. 

A recent paper by Giuliano Di Baldassarre, Maurizio Mazzoleni and Maria Rusca, looks at this in more depth and explores the interplay between large dams, patterns of population growth and agricultural expansion in the US over the past two centuries. 

Offsetting original benefits

By supplying more water, food and energy, reservoirs enable agricultural, urban and industrial expansion that, in turn, lead to growing demands. This feedback, Baldassarre et al say, can quickly offset the original benefits of reservoirs. 

“For instance,” they state in their paper, “new reservoirs that secure water supply for irrigation increase the profitability of crop production, thereby triggering agricultural expansion and increasing water consumption. These growing demands can then set in motion further reservoir expansion or construction of additional dams.”

This phenomenon has been described as supply-demand cycles which, the authors claim, can generate accelerating spirals towards unsustainable water consumption, environmental degradation and peak water limits.

In their paper, the authors examine the legacy of dam development in the US by exploring how changes (in time and space) in human population, water supply and agricultural expansion have shaped each other over the past two centuries. They draw from methods and concepts of the interplay of human and water systems that have been recently developed in environmental history and political ecology, as well as social-ecological systems and socio-hydrology.

The authors explain that their large-scale analysis of spatial and temporal trends in the US showed that the coevolution in space and time of people and dams over the past two centuries was characterised by three distinct phases, in which different processes dominated the interplay. These are:

• Go West Young Men – 19th century. Human population grew faster than reservoir capacity. Geographical mobility shifted towards the western US with greater access to land and water resources.
• Hydraulic Mission – 1900-1980. The era of greater dam expansion and the pursuit of iconic hydropower or largescale public irrigation projects in the US. 
• Plateau phase – 1980s to ongoing. Reservoir capacity has remained essentially stable and dam development is no longer a dominant process. Water demand levels remain high.

Four workers riding on a suspended section of pipe as it is moved into position during construction of the Grand Coulee Dam in Washington (1933-42).

Lock-in condition

The authors claim that the US has got into “a lock-in condition” with unsustainable levels of water consumption. As a result, the agricultural sector is increasingly relying on groundwater while drought conditions lead to severe water crises, especially in the southwest region. The focus on agricultural water use in the southwest has shown that there is neither cause nor effect in the interplay, but rather a chicken-and-egg dynamic as water supply partly meets and partly enables water use.

It is critical, the authors urge, that government and the agricultural sector embrace a sustainable trajectory in view of water availability and climatic conditions in the region. Moreover, while the US reached a plateau and recognised the importance of reducing heavy reliance on large water infrastructure, climate change poses new challenges and policy-makers in the US are currently advocating again for supply-side solutions, including dam augmentation and aquifer storage and recovery.

To provide policy insights, Baldassarre et al refer to the concept of legacy risks, which was originally proposed in the fields of mining and nuclear waste but later applied to a range of processes, including environmental management. Reducing legacy risks means identifying development trajectories that may result in being unsustainable for the next generations. 

“In our study,” the authors state, “we proposed the identification of legacy risks as one way to examine the sustainability of dam development over a long-time horizon. We showed how the legacy of large dams can generate lock-in conditions that are difficult (if not impossible) to reverse. Our results in the US, and in particular the southwest, are similar to previous studies in other parts of the world in which large water infrastructure has secured water in otherwise dry areas, including Athens, Cape Town and Melbourne. As past decisions were a legacy to the present, current decisions will be a legacy for the future. 

The authors add that the results of this study are also relevant to the current debate about the sustainability of new dams and reservoirs which are being planned or built in many places around the world, such as Brazil , that are “mobilising resources to quench the thirst of growing cities or increase agricultural production”.

“Our analysis of the legacy of dams in the US provides insights into what type of socio-environmental trajectories might follow these developments,” the authors conclude.