Predict how dams will alter river temperatures and disrupt fish populations

Today, these sightings are recounted like in a history book. In the 1960s, the fish were federally listed as endangered, in part due to the construction of several dams.

Hydroelectric dams have blocked spawning fish migrations, altered river flow, and churned cooler water downstream. The Colorado pikeminnows, which were unaccustomed to cooler waters, soon came under competition for food from non-native fish. Now, most pikeminnows in Colorado only reach two to three feet in length.

“These fish are now endangered and have been replaced by cold-water adapted trout,” said University of Washington fish ecologist Gordon Holtgrieve. “The river doesn’t look like it used to, and the Native Americans in the area, who traditionally used these fish, have lost part of their culture.” He said trout is a popular recreational fishery in the area.

The world’s ubiquitous dams are built to guard against extreme flooding, meet ever-increasing water demand, and provide hydroelectric power. They also alter river ecosystems – for example by altering temperatures downstream – and can significantly alter nearby fish populations.

In China, the Xinanjiang and Danjiangkou hydroelectric dams caused the maximum summer temperature to drop by 7.2 to 10.8 degrees (4 to 6 degrees Celsius) in the downstream reaches of nearby rivers. Fish spawning was delayed by three to eight weeks, causing many warm-water fish to become locally extinct. The Keepit Dam in Australia has also reduced temperatures in the Namoi River, disrupting thermal spawning signals for many native fish.

Worldwide, at least 3,700 medium and large hydroelectric dams are planned in the coming decades or under construction, highly concentrated in South America, Africa and South and East Asia. Hundreds of millions of people in the major river basins of these regions depend heavily on the river for their livelihoods, Holtgrieve said. For example, Cambodians get about 80 percent of their animal protein primarily from wild freshwater fish caught in the Mekong.

Now, in a recent study, researchers have created a first-of-its-kind machine learning model that can predict temperature changes resulting from planned dams around the world and could help planners and engineers mitigate environmental impact. . By analyzing future dams around the world, the team found that some dams changed downstream temperatures by as much as 10.8 degrees Fahrenheit (6 degrees Celsius).

Based on the research, the team created a public tool that allows people to plug in the dimensions of a future dam and learn how it will affect temperatures downstream.

“The Congo, Amazon and Mekong basins are going to have a large number of dams, and that’s inevitable,” said Shahryar Ahmad, the study’s lead author. “We don’t want to repeat the same mistakes, or at least some of the drawbacks, that we see with dams that have been built over the last century.”

Cooler in summer, warmer in winter

Like layers in a cake, large bodies of water typically have different temperatures at different depths, known as thermal stratification. Cooler, denser layers gravitate downward, while a relatively warmer layer heated by the sun sits near the surface.

Hydroelectric dams generally work by drawing water from deep layers of a reservoir into a turbine to generate power. This brings cooler waters downstream and causes a cooling effect in the summer; the effect is reversed in winter. Some also draw water from the surface or have shallower reservoirs, which could create warmer temperatures downstream.

“These kinds of things are known, but in developing countries where you have so little data, it’s hard to track,” said Faisal Hossain, a professor at the University of Washington who, with Holtgrieve, was co- author of the study. “It was the amount of cooling or warming that we could detect that was surprising, sometimes 6 degrees, which can be quite substantial.”

The team first analyzed the thermal impact of more than 100 existing dams of various sizes, depths and characteristics in the United States, South Asia, Europe and South America. In the United States, the team examined historical records from upstream and downstream temperature monitoring stations near dams. In places without ground observations, they used satellite thermal observations to study temperature differences.

They found that most US dams produced a downstream cooling effect in the summer and a warming effect in the winter.

The magnitude of cooling or warming is related to the size and depth of a dam’s reservoir. Dams connected to larger and deeper storage basins, which had stronger thermal stratification, had the greatest temperature differences from upstream to downstream. Smaller storage tanks had weaker stratified water layers, which could be easily mixed by winds, and resulted in weaker downstream cooling effects, or even warming.

“Depending on the storage capacity of the reservoir and the ability to stratify the water, that’s where the thermal cooling and the thermal heating come from,” said Ahmad, who conducted this research as a doctoral student at the University of Washington and is now a researcher at NASA.

The location of the dams also mattered. Dams in hot, arid regions with hotter summers, and therefore hotter surface layers, tended to have more downstream cooling. Dams in humid and snowy climates, and therefore cooler air temperatures, did not show strong cooling impact and sometimes produced warming downstream.

Using this information, the team trained their machine learning model to predict the impact of future dams in summer and winter.

The future of dams and fish

Of the 216 future dams studied, about 73% would potentially cool downstream rivers during the summer by up to 11.9 degrees (6.6 degrees Celsius) compared to upstream. About 25% is predicted to warm downstream rivers by up to 8.5 degrees (4.7 degrees Celsius). In winter, most dams warmed rivers downstream by up to 3.6 degrees (2 degrees Celsius).

Some of the areas most deeply affected by dams have appeared in the biodiverse basins of the Amazon, Paraná, Niger and Mekong. The Amazon basins will experience moderate cooling and warming in various dams. Dams in the Niger Basin are likely to cause warming downstream. The Paraná Basin will experience moderate cooling in summer.

The team also looked at how river temperatures near dams would change with climate change if humans continued to emit greenhouse gases. In the event of increased warming, the warmer temperatures of the rivers downstream would warm and the cooler temperatures would also warm in both summer and winter.

“I was most surprised by the magnitude, where a lot of these planned dams come in, and just the magnitude of a problem,” said Lindsey Bruckerhoff, a fish ecologist at the US Geological Survey who was not involved. looking. “In many basins in South America and Asia where they want to build some of these dams, there are still important fisheries that people actually have a livelihood on.”

The team designed the machine learning model as a publicly available tool so people can model the effect of their future dams on river temperature.

“Somebody can just plug in the size of a dam, and then they can know, ‘If my dam is that big and that wide and has that massive capacity, it’s going to cause a thermal change of that magnitude,'” a Ahmad said. “This is where our study may be very helpful.”

Hossain said the monitoring tool could help inform dam operations so temperatures downstream stay within a tolerable range for wildlife. He said some dams may be able to draw less water from deeper, cooler layers and release more into warmer waters near the surface. Previous work by the researchers has also shown that hydroelectric dams can be more efficient by taking weather forecasts into account.

Ahmad said replacing dam power with smaller turbines, or multiple smaller ones, can also reduce the environmental footprint in some cases.

Perhaps, Hossain said, one solution is to not build the dam at all, if the environmental effects are too disruptive in that area.

“Maybe we can look at what the temperature changes might be, bring the community together and explore other alternatives in a cost-effective way,” Hossain said, “and then minimize the negative impacts on Mother Nature.”


An earlier version of this article incorrectly identified Bruckerhoff as Bruckeroff. The article has been corrected.

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