Throughout the world, dams provide us with essential water supplies needed for drinking, crop irrigation, hydropower and industry. There are around 58,000 large dams, exceeding 15 meters in height, built on rivers around the world. Yet, while these dams provide us with water essential to our survival, and hydropower is seen as a green energy source, the construction of dams on our waterways comes at a significant cost to the environment.
But managing rivers so that they meet both the needs of human society and that of aquatic ecosystems is a complex challenge. Communities need water as well as power, but building dams on rivers disrupts ecosystem functions and the services these ecosystems provide.
A recent study conducted by researchers from Utah and Colorado State Universities at Glen Canyon Dam on the Colorado River provides some insight into how to best mitigate the negative impact that dams have on the environment, including proposing a new management method to reduce the impact of hydropeaking — a practice that is commonly used by hydropower dams, which has a negative impact on aquatic food webs further downstream.
The researchers discuss their findings under the context of increasing global pressure to construct more dams, in a paper that was recently published in the scientific journal Science.
"Dams change rivers by creating artificial lakes, fragmenting river networks and distorting natural patterns of sediment transport and seasonal variations in water temperature and stream flow," says co-author Jack Schmidt, a professor in the Department of Watershed Sciences at Utah State University, who served as chief of the U.S. Geological Survey's Grand Canyon Monitoring and Research Center from 2011 to 2014.
Hydropeaking, is the practice commonly used by hydroelectric dams whereby river flows are increased during times of peak electricity demand by consumers. Aquatic insects, which form an essential part of river food webs, are particular vulnerable to the effects of hydropeaking. Insects lay their eggs on submerged aquatic vegetation near the shoreline, but drastic fluctuations in water levels can expose the eggs and/or larvae, causing them to dry out and die.
"Hydropeaking creates a fluctuating daily pattern of water flows that can severely impair productive shoreline habitats through repeated wetting and drying. A conundrum for river scientists and managers is how to counter these negative effects in a cost-effective manner," says Schmidt. "Managers have to meet customer demand so total elimination of hydropeaking isn't an option. However, we assert that even small adjustments to river flow regimes might help to restore river ecosystems."
The authors reviewed recent studies that focus on the impact of dams and dam operations on downstream ecosystems, which show how small changes in dam management can have a big positive impact further downstream. For example in a previous hydropeaking study looking at the impact on ecosystems further downstream, the authors suggest "giving aquatic insects the weekend off." Schmidt agrees; by giving insects a two day break from hydropeaking activities, may give them time to recover, which may allow a more natural aquatic food web to re-establish in the river, benefitting fish in the river ecosystem.
While restoration efforts at existing hydroelectric dams is a good start to addressing the issue, the authors recommend that any new proposed dam project in South America, Asia and Africa should only go ahead after cautious planning and careful consideration is given to their design, location, overall number, and how the proposed new dam will be managed.
While hydropower is considered a renewable source of energy, it is not always 'green' unless careful consideration is given to the location and operation of those dams to mitigate negative ecological effects.
"In a world of growing demand for water and energy, we face an increasingly uncertain hydrological future," says Schmidt. "We have to balance economic gain against environmental degradation."
N. LeRoy Poff & John C. Schmidt. How dams can go with the flow. Science; 09 Sep 2016: Vol. 353, Issue 6304, pp. 1099-1100. DOI: 10.1126/science.aah4926