Recent boom in dam construction predicted to impact global river water quality

Friday, December 11, 2015

Dams impact more than fish and wildlife habitats, they transform the very chemistry of the water released down stream, according to a new study by the University of Waterloo’s Ecohydrology Research Group.

DamThe Hoover Dam on the Colorado River, USA. Photo credit: Sean Sinclair.

There are currently more than 70,000 large dams worldwide and the proportion of rivers impacted by dams could reach 90 per cent within the next 15 years,” said Taylor Maavara, lead author and a PhD student in the Department of Earth and Environmental Sciences.

Their paper, which presents the first quantitative assessment of the global effects of damming on the nutrient element phosphorus in river systems, appears this week in the prestigious journal, Proceedings of the National Academy of Sciences.

In their study, the Waterloo researchers show the recent surge in global hydroelectric dam construction could double the amount of phosphorus being retained in reservoir sediments. When dams are built, rivers carry less phosphorus, which profoundly changes the water quality delivered to wetlands, lakes, floodplains and costal marine areas.

Phosphorus is an essential nutrient element which regulates the biological productivity of many aquatic environments. The researchers’ model tracks the major chemical forms of phosphorus as it reacts and transforms in dam reservoirs. Model calculations show the total mass of phosphorus retained behind dams in reservoir sediments has doubled between 1970 and 2000. 

By 2030, the Waterloo researchers estimate there will be a further increase in phosphorus retention by up to 41 per cent, mostly in the Yangtze, Mekong and Amazon River basins.

Dams not only modify the natural flow regime of rivers, they also fundamentally change the way nutrients move along the freshwater cycle,” said co-author Philippe Van Cappellen, Canada Excellence Research Chair in Ecohydrology in the Faculty of Science. “This aspect, however, has so far received little attention from the scientific community.”

In their study, the authors point out that dams eliminate phosphorus more efficiently than other key nutrient elements, including nitrogen and silicon. At first glance, this is good news because it reduces the amount of phosphorus released downstream, where it may cause excessive algal growth with severe impacts on water quality, aquatic life and human health.

However, we also have to consider the increasing loading of phosphorus to rivers in many parts of the world,” said Maavara. “As humans, we have doubled the amount of phosphorus reaching our rivers, because of fertilizer use, sewage discharges and soil erosion.”

The researchers conclude that despite the massive dam building activity now and in the coming decades, the global export of bioavailable phosphorus by rivers to the coastal ocean could still be 21 per cent higher in 2030, compared to 2000.

Such a large increase in phosphorus export will further exacerbate the occurrence of harmful algal blooms in the coastal zones of the world,” said Van Cappellen, a member of the Water Institute.   

In a previous study published in the journal Global Biogeochemical Cycles, the researchers used a similar modeling approach to quantify the amount of the nutrient element silicon retained globally in dam reservoirs. They next plan to use their approach to determine how damming is affecting the fate and transport of nitrogen and carbon along river systems.

Given the global scale at which damming is impacting our water resources, there is an absolute need to acquire data on the resulting changes in water chemistry, both up- and downstream of dams,” said Van Cappellen.      

The paper’s authors also include Research Associate Chris Parsons, master’s student Christine Ridenour, PhD student Severin Stojanovic, Research Assistant Professor Hans Dürr, and PhD student Helen Powley.

The Canada Excellence Research Chair program funded the project and Maavara was supported by a Natural Sciences and Engineering Research Council of Canada Postgraduate Scholarship.