The retreat of the delta: a geomorphological history of the Po river basin during the twentieth century

Water History (2021) 13:117–136 https://doi.org/10.1007/s12685-021-00279-3 The retreat of the delta: a geomorphological history of the Po river basin during the twentieth century Giacomo Parrinello1 · Simone Bizzi2 · Nicola Surian2 Received: 29 April 2020 / Accepted: 8 April 2021 / Published online: 21 June 2021 © The Author(s) 2021 Abstract The morphology of rivers and deltas, like many features of the Earth’s physical geography, is today subject to dramatic and rapid changes due to human actions. Deprived of sediment from their basins and besieged by rising sea levels, many deltas are at risk of complete disappearance. Despite a rich historical scholarship on rivers, we know little about the history of these important geomorphological processes. This paper sheds light on the geomorphological history of rivers by investigating the case of the Po River basin and its delta during the twentieth century. By combining the insights of fluvial geomorphology and a historical methodology, the paper analyses three main drivers of geomorphic alterations in the catchment that had an impact on the delta: hydroelectricity, sand and gravel mining, and methane extraction. In each case, it focuses on how experts, policy-makers, and overseers understood and regulated (or not) these geomorphic alterations. During much of the twentieth century, engineers and hydrologists monitored geomorphic processes with increasing detail, while state and business actors practiced multiple forms of sediment management. For most of the twentieth century, however, experts did not acknowledge the scale and nature of human-induced geomorphic alteration. Sediment management, moreover, did not take into account sediment scarcity until late in the century, and remained exclusively motivated by local concerns. Through this particular case, this paper offers insights on the historical limits to environmental expertise and policy when facing long term and largescale geomorphic processes, and encourages a more sustained incorporation of fluvial geomorphology into the history of water systems. Keywords Delta · Sediment · fluvial geomorphology · Po River · environmental expertise · environmental policy · Anthropocene * Simone Bizzi Giacomo Parrinello Nicola Surian 1 Center for History at Sciences Po (CHSP), Paris, France 2 Department of Geosciences, University of Padua, Padua, Italy 13 Vol.:(0123456789) 118 G. Parrinello et al. Introduction After centuries in which they grew farther out into the sea—a process called progradation—in the second half of the twentieth century many deltas of the world started to sink and retreat. This historic reversal is part of a large-scale alteration of sediment fluxes, which is transforming the morphology and ecology of river systems around the world (Bravard 2018). The alteration of sediment fluxes is also one of the most conspicuous signs of human impact on natural processes and is often evoked in relation to the Anthropocene, the debated new epoch of the Earth’s geological history characterized by human geophysical agency at a planetary scale (Crutzen and Stoermer 2000; Crutzen 2002; Steffen et al. 2007, 2011; on sediment see Syvitski and Kettner 2011; Cooper et al. 2018). Sediment flux alteration and delta retreat put the lives of millions of people at risk in places like the Mekong, the Yangtze, or the Mississippi deltas, especially when combined with localized subsidence and the effects of climate change-induced sea-level rise (Kondolf et al. 2018). This paper investigates the history of one such case: the Po River delta in Italy during the twentieth century. The paper analyses geomorphic alterations at the watershed scale that had an impact on the delta, focusing on how historical actors understood and regulated (or not) these alterations. It shows that river experts and regulators, while aware of human influence on geomorphic processes at the catchment scale, failed until the late twentieth century to take into account the possibility of sediment scarcity and the anthropogenic retreat of the Po delta. Present-day fluvial geomorphology has produced a sophisticated understanding of the links between upstream basins and deltas and their anthropogenic alteration, including the impact of dams on sediment fluxes, the consequences of sand and gravel mining, and the construction of embankments (Kondolf and Piégay 2011). This literature has shown how modifications in upstream portions of a watershed can have significant consequences on downstream sediment deposition, influencing the morphology of the river channel as well as delta build-up or retreat (Wohl et al. 2017; Wohl 2017). While fluvial geomorphology has been sensitive to the consequence of human intervention, its focus remains the understanding of geomorphological processes themselves rather than their social and historical drivers. During the past few years, interdisciplinary social sciences and political ecology have begun integrating geomorphic processes into our understanding of water systems and society (see for instance Micheaux et al. 2018; Carse and Lewis 2017; Cortesi 2018). Scholars have argued that sediments are essential components of more-than-human watery assemblages; their presence (or absence), their composition, and movement shape and are shaped by social inequalities and uneven power relations. While these contributions offer important conceptual and methodological tools to analyze the social and political dimensions of geomorphic processes, they say little about how these dimensions have evolved over time. Geomorphic alterations play out on at large spatial scales and over the long term, with significant spatial and temporal lags between causes and effects (Kondolf and Podolak 2014). Historians can offer a unique contribution to understand the way human societies have influenced these processes. Surprisingly, environmental and water historians have thus far paid scant attention to the geomorphic dimension of river histories. They have mostly studied river development, bringing into focus its massive ecological consequences, from the large-scale alteration of the hydrology of the American West (Worster 1985) to the massive pollution of the Rhine River during two centuries of river development (Cioc 2002). Historians of the Columbia (White 1995) and the Rhône rivers (Pritchard 2011) maintained that hydrological and 13 The retreat of the delta: a geomorphological history of the Po… 119 ecological transformations should not be viewed as departures from a natural state but as new hybrid configurations of nature and technology. These contributions offer indispensable insights on the deep nexus that exists between the ecological transformation of rivers and the historical processes of economic growth, technological innovation, the making of scientific knowledge, and institutional policy-making. These works, however, invariably show a strong water bias in their understanding of river systems, (...truncated)