A complex balance: assessing perspectives on decommissioning large dams to restore river ecosystems

In recent years, critics of large dams have raised fundamental questions around the necessity and benefits of large dams. A large dam is defined as one with a height of 15 m or more from the lowest foundation to the crest, or a dam between 5 and 15 m impounding more than 3 million cubic metres of water (International Commission on Large Dams, 2011). Since the focus of this article is on large dams, the term ‘dams’ may be used in reference to large dams. Two significant questions have reoccurred in debates on large dams: Should we build more dams or not? Is there a good large dam? Thayer Scudder has expressed a clear opposition to ‘good’ large dams (Scudder, 2017). This is largely due to the evidence that suggests that the persistent, negative socio-economic and environmental costs of large dams exceed their proclaimed economic benefits (Schulz & Adams, 2019; Sovacool & Bulan, 2011). Based on a statistical analysis of 245 large dams in Africa, Asia, Europe, Latin America and North America built between 1934 and 2007, Ansar et al. (2014, p. 2) concluded that ‘even before accounting for negative impacts on human society and the environment, the actual construction costs of large dams are too high to yield a positive return’. Consequently, there have been calls not only to cease the construction of large dams but also to decommission existing ‘unnecessary’ dams (Ansar et al., 2014; Scudder, 2017, 2019). An unnecessary dam is one whose existence is deemed unjustifiable because its negative risks and impacts are considered to outweigh its perceived benefits. Dam decommissioning refers to the process of removing or ceasing the operation of a dam because its negative effects outweigh their perceived benefits. Dam removal is an important tool for river restoration and addressing ageing infrastructure. It is an ongoing process as a large number of ageing dams that are no longer serving their original purposes have become safety liabilities, or represent potential for river system restoration if they are taken down (Duda & Bellmore, 2022). There are two categories of dam removal processes: full and partial decommissioning. Full removal entirely reestablishes the free-flowing conditions in a river because all physical obstacles are abolished, and the river’s continuity is restored (Ayboga, n.d.). Dam removal allows for