The Colorado pikeminnow was once a prized catch. The toothless, torpedo-shaped fish had dark, tender, flavorful meat and came in absurdly large sizes, especially for a minnow. In the early 1900s, fishers reported catching some up to six feet long, using mice, frogs and cottontail rabbit heads to lure in the voracious eaters. They were so plentiful that people could catch them using pitchforks.

Today, these sightings are recounted as if from a history book. In the 1960s, the fish were federally listed as endangered — in part because of the construction of several dams.

Hydropower dams blocked the fish’s migrations for spawning, altered river flow and churned cooler water downstream. The Colorado pikeminnows, which were not accustomed to the cooler waters, were soon outcompeted for food by nonnative fish. Now, most Colorado pikeminnows reach only two to three feet long.

“Those fish are now endangered and have been replaced with cold-water-adapted trout,” said Gordon Holtgrieve, a fish ecologist at the University of Washington. “The river doesn’t look anything like it used to, and Native Americans of the region, who traditionally used these fish, have lost part of their culture.” He said the trout are part of highly prized recreational fishing in the area.

The ubiquitous dams around the world are built to guard against extreme flooding, meet steadily increasing water demands and provide hydroelectric power. They also alter river ecosystems — such as by changing temperatures downstream — and can substantially change nearby fish populations.

In China, the Xinanjiang and Danjiangkou hydroelectric dams caused the peak summer temperature to decrease 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 the local extinction of many of the warm-water fish. The Keepit Dam in Australia also reduced temperatures in the Namoi River, disrupting thermal spawning cues for many native fish.

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

Now, in a recent study, researchers have created a first-of-its-kind machine learning model that can predict temperature changes as a result of dams planned around the globe and could help planners and engineers mitigate the environmental impact. Analyzing future dams worldwide, the team found 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 downstream temperatures.

)“The Congo, Amazon and Mekong basins are going to have a large number of dams, and that is inevitable,” said Shahryar Ahmad, the lead author of the study. “We don’t want to repeat the same mistakes, or at least some of the disadvantages, that we are seeing from the dams that have been built over the past century.”
Cooler in the summer, warmer in the winter

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

Hydropower dams generally operate by drawing water from the deeper layers of a reservoir into a turbine for energy. This brings colder waters downstream and causes a cooling effect in the summer; the effect reverses in the winter. Some also draw water from the surface or have shallower reservoirs, which could create warmer downstream temperatures.