McGill study examines global impact of dams on waterways large and small

A new study published by McGill University examines how both existing and planned dam infrastructure impacts people and surrounding ecosystems through river flow and fragmentation.

The report, prepared with assistance from the University of Minnesota’s Institute on the Environment and the university of Wisconsin’s Center for Limnology, relies on what the researchers call the “river fragmentation index” (RFI) and “river regulation index” (RRI) to assess dam effects.

RFI measures the way a river’s natural flow path, or connectivity, has been disrupted by the construction of dams and other barriers.

Meanwhile, RRI measures the proportion of river water that can be stored in reservoirs, thus affecting natural fluctuations and properties of downstream river flows.

By combining the two indices, researches said they can assess the impact of any existing or planned dam.

“Not all dams are equal,” lead author Gunther Grill said. “Our research assumes that it is not only the size of a dam, but also where it is placed along the river that makes a difference.

“So depending on whether a dam is high up in a the mountain headwaters or further down close to the delta, if it is on the main stem of the river or on a small tributary — all of these factors will have varying effects on the rivers and their surrounding ecosystems.”

McGill’s team said the research was only made possible by its development of a new global river map with “unprecedented resolution and detail”. The map includes a cumulative river length of more than 48 million kilometers, with waterways including “small creeks to the largest of rivers.”

The map also includes data of future dam locations, which was assembled by the Leibniz-Institute of Freshwater Ecology and Inland Fisheries in Berlin.

“Over the past 60 years, a myriad of dams have been built either to provide hydroelectric power, or for irrigation purposes, or as flood protection,” McGill professor Bernhard Lehner said.

The study is intended to not only assess what impacts dams have had in the past, but also what their global role might be in the future.

According to the research, 48% of the world’s river volume is currently “moderately or severely” affected by dams, with the figure nearly doubling should all dams planned or under construction ultimately be completed.

The report is available for viewing online here.


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Michael Harris formerly was Editor for

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