There are an estimated 16 million dams worldwide, of which more than 50,000 are large, that is, 15 meters in height or higher. Together large dam reservoirs cover over 400,000 km2, while their volume equals about seven times that of the world’s rivers combined. The first wave of systematic dam building started after the Second World War and peaked between 1960 and 1980. More recently, a second wave of dam building has been taking place, in large part to supply energy to emerging economies. By 2030, it is estimated that at least 3,700 new hydropower dams with generating capacities larger than one megawatt will be added to the global count, thereby increasing global hydroelectricity production by 70%.
Damming may well constitute the largest single anthropogenic alteration of the freshwater hydrological cycle. In 1970 around 17% of the global river catchment area drained into large dams, in 2000 this had increased to 27%, and by 2030 it could be as high as 36%. Following the completion of currently planned dams, by 2030 it is estimated that up to 90% of all rivers will be fragmented by one or more dams.
Rivers act as reactive conduits connecting the continental and oceanic carbon (C) cycles, and all the available evidence suggests that river damming significantly changes the export of organic carbon (OC) to the ocean. Dam construction and closure modify the downstream transfer of OC and essential nutrients, and thus the trophic state of the river system and that of receiving water bodies, including lakes and nearshore marine environments. However, the global-scale changes in riverine OC fluxes due to damming remain poorly quantified. This study therefore assesses the role of damming as a driver of global environmental change, with an emphasis on how damming changes the global biogeochemical cycles of C and nutrients. It presents a worldwide analysis of trends in riverine OC fluxes that explicitly accounts for reservoirs as a compartment of the C cycle on continents.
Methodology
By accounting for the changes over time to the number of dams, as well as the changes in C and nutrient loads to rivers, we can in principle reconstruct the past effects of dams on biogeochemical cycles, and predict future effects using available projections of new dam construction. The paucity of existing data on reservoir biogeochemistry, however, requires a new modelling approach to make statistically meaningful estimations of the global impacts of dams on C and nutrient cycles and fluxes.
A new methodology was developed that builds on the recognition that the processes determining the fate of bioactive elements in human-made reservoirs are the same as those taking place in natural freshwater environments that have received much more attention from the scientific community. These processes include, among others, photosynthesis, respiration, sorption, dissolution, gas exchanges and sedimentation (Figure 1). Based on the existing knowledge we were then able to develop relatively simple mass balance models which account for all the major biogeochemical and physical processes regulating the in-reservoir processing of C and nutrients. Having previously applied this approach to the nutrient elements silicon and phosphorus, we extracted data for dams under operation at selected time points (1970, 2000, 2030 and 2050) from global databases, and applied the mass balance model of in-reservoir OC cycling for each dam.
Figure 1. In-reservoir processes that control changes in the riverine fluxes and elemental speciation of carbon and nutrients similar to those found in natural lakes.
Next, the extensive observational and theoretical knowledge on biogeochemical cycling in freshwater ecosystems enabled the development of models to estimate biogeochemical impacts of damming in river basins. Combining these models with statistical methods, relationships were estimated that allowed for the computation of the net effects of in-reservoir processes from simple reservoir metrics that are readily available from existing dam databases. These global relationships enabled us to scale up process-level understanding to the effects of dams within entire river basins and, ultimately, to make predictions at the scale of entire continents.
Outcomes
Dam reservoirs are hotspots of sediment accumulation, primary productivity (P) and carbon mineralization (R) along the river continuum. Following dam closure, a newly formed reservoir typically acts for a considerable number of years as a net source of carbon dioxide (CO2) and methane (CH4) to the atmosphere due to the mineralization of legacy organic matter in flooded soils and vegetation. Our analysis predicts that, during the period 1970-2030, worldwide reservoir respiration keeps outpacing primary production (P<R) as a result of the construction of new dams, thereby sustaining excess mineralization. The global P/R ratio, however, varies significantly over this time period, from 0.20 to 0.58, because of the changing age distribution of dams. Should dam construction become negligible after 2030, dam reservoirs could globally become net autotrophic (P>R) by 2050.
We further estimated that at the start of the 21st century, in-reservoir burial plus mineralization eliminated 13% of the total riverine export of OC to the oceans. Because of the ongoing boom in dam building, in particular in emerging economies, this value could rise to 19% by 2030. The large reduction in riverine OC export due to damming is one reason why nearshore marine environments are a growing sink for anthropogenic CO2.
Geographically, the model calculations also show that in the coming decades the largest changes in the riverine fluxes of OC and nutrients due to damming will be seen in Southeast Asia and South America, because this is where many of the new dams will be built.
Conclusions
With the upscaling approach used this study, damming is shown to significantly decrease the export of OC to oceans. Through its large impact on the delivery of riverine OC, river damming represents a major anthropogenic forcing on the trophic state and C balance of the coastal ocean. By modifying C cycling and the accompanying greenhouse gas exchanges along the land-to-ocean continuum, dams impact the Earth’s climate. Our analysis indicates that the age distribution of dams exerts an important control on global C sequestration by reservoirs, and therefore on the role of damming in ongoing and future climate change.
Maavara T., R. Lauerwald, P. Regnier, and P. Van Cappellen (2017). Global perturbation of organic carbon cycling by river damming. Nature Communications 8, 15347.
Contact: Philippe Van Cappellen, Faculty of Science
For more information about the Water Institute, contact Amy Geddes.
