Why fish are disappearing: Satellite survey of 287 US dams finds 71% alter river temperatures

Fish tell time by temperature. When the water warms to a certain point, they know it is time to spawn.

But that biological clock is being thrown off. Dams alter river temperatures — and even a difference of barely one degree can determine when fish lay eggs, how fast those eggs hatch, and whether the young survive.

A satellite survey of 287 large dams across the United States found that in seven out of 10 cases, water temperatures downstream differed from those upstream.

According to a paper published in volume 12, issue 28 of the international journal Science Advances by a research team led by Emily Ellis of Virginia Tech, the phenomenon of dams altering downstream water temperatures is not confined to specific regions or seasons — it is occurring broadly across the entire United States.

The fact that dams change river temperatures is not new. Previous research had been limited to individual dams, single rivers, or specific seasons. Placing thermometers in rivers made it impossible to survey large areas all at once.

The research team turned to thermal infrared imagery captured by the US Geological Survey's Landsat 8 satellite, which detects heat emitted by objects to measure surface temperatures.

The study covered 287 dams on rivers at least 100 meters wide. The team selected cloud-free and ice-free images from 2013 to 2024, stitching together surface water temperatures upstream and downstream of each dam to produce 11,868 temperature profiles.

The team also verified the reliability of the satellite readings by comparing them against measurements from 87 in-river water temperature gauges. The direction and magnitude of temperature differences detected by satellite matched the actual sensor readings.

The analysis found that 71 percent of all temperature profiles showed a difference between upstream and downstream water temperatures.

Of those, 60 percent showed warmer water downstream, while the remaining 40 percent showed cooling. The average magnitude of change ranged from 0.7 to 1.4 degrees Celsius.

A difference of around one degree may seem small, but not for aquatic life. The timing of fish spawning, the speed of egg hatching, and the survival of juvenile fish all depend on water temperature. The gap was widest in spring and summer — precisely the seasons most critical for fish reproduction and growth.

Warmer water also holds less dissolved oxygen, since water absorbs less oxygen as it heats up. It also creates favorable conditions for harmful algae to proliferate.

Perhaps the most unexpected finding was that the temperature difference did not disappear downstream.

Conventional wisdom holds that while temperatures may shift sharply just below a dam, the river should gradually exchange heat with the surrounding air and return to its natural temperature as it flows on.

That did not happen. At 2 kilometers below the dam, the temperature difference measured 0.4 to 0.9 degrees Celsius. At 18 to 20 kilometers downstream, it had actually widened to 0.6 to 1.5 degrees. Rivers that had warmed kept warming; rivers that had cooled kept cooling.

The research team said this was the first time the phenomenon had been confirmed at such a broad scale — meaning the temperature change caused by a dam persists for at least 20 kilometers downstream without recovering.

Not all dams behaved the same. The difference between dams with reservoirs and those without was stark.

Of all cases where temperatures shifted by more than 4 degrees Celsius, 91 percent involved dams with reservoirs. The same was true for 75 percent of cases with changes between 2 and 4 degrees.

The research team attributed this to thermal stratification inside reservoirs. In summer, the upper layer of reservoir water warms in the sun while the bottom layer stays cold. When a dam releases water from the bottom, the downstream river cools; when water flows out from near the surface, the river warms.

The research team suggested the findings could point toward new solutions for protecting ecosystems in an era of climate change.

As rising global temperatures push river water warmer and cold-water fish species lose their habitat, stretches of river cooled by dam releases could serve as refuges for those species.

By identifying cold-water zones using satellite data and strategically adjusting dam operations, the team said, it would be possible to deliberately create habitat for cold-water species.

However, the team acknowledged limitations: satellite imagery captures only surface water temperatures, making it difficult to assess conditions at depth, and the daytime-only nature of the observations means daily temperature fluctuations outside those hours are not fully reflected.