Amazon Floodplain Lakes Reached 41.0 °C in 2023 Drought, Killing Hundreds of Dolphins and Thousands of Fish

Extreme 2023 Drought Heated Amazon Lakes to Hot-Tub Temperatures, Causing Mass Die-Offs

In 2023 a severe drought pushed several floodplain lakes in the central Amazon to unprecedented temperatures. Brazil’s Lake Tefé reached 41.0 °C (105.8 °F), and five of the ten lakes studied exceeded 37 °C at times. Those extreme conditions led to the deaths of hundreds of dolphins and thousands of fish and other aquatic animals across the affected floodplain lakes.

Hydrologist Ayan Fleischmann of the Mamirauá Institute for Sustainable Development, who helped investigate the event, said:

“The climate emergency is here, there is no doubt about it.”

Fleischmann and colleagues combined satellite observations with on-the-ground measurements taken during the 2023 drought to reconstruct the event and identify the factors that created lethal lake conditions.

Why the lakes became so hot

• Very low wind speeds and shallow water depths limited mixing and cooling.
• Eleven consecutive cloudless days produced intense solar heating.
• Increased turbidity (haze) in the water caused it to absorb more sunlight.
• Large short-term temperature swings — up to 13 °C — imposed lethal thermal stress on wildlife.

In their paper, the researchers link the unprecedented drought intensity to climate change: widespread ocean warming (notably in the North Atlantic) combined with a moderate-to-strong El Niño. As University of Greenwich ecologist Adrian Barnett, who was not involved in the study, put it:

“A 10 °C increase in water temperature is unparalleled. The volume of energy needed to achieve this in such huge volumes of water is jaw‑dropping.”

Death toll and physiological effects

The World Wildlife Fund initially reported a loss of roughly 10% of the local dolphin population within one week, including about 130 pink river dolphins (Inia geoffrensis) and 23 tucuxi dolphins (Sotalia fluviatilis), both species of conservation concern. Subsequent counts found approximately 330 dolphin carcasses around the lakes, with thousands of fish and other aquatic animals also dying.

Biologist Adalberto Val of the Brazilian Amazon Research Institute explained the physiology driving the die-offs:

“When the water reaches 41 °C, fish enzymes and metabolic processes fail and they can no longer survive.”

Broader implications

The central Amazon floodplain contains the largest intact tract of tropical rainforest and stores roughly one-fifth of the planet’s freshwater. Fleischmann and colleagues report that average water temperatures in these floodplain lakes have risen about 0.6 °C per decade since 1990, a trend that raises concern about the increasing frequency of similar events.

UC Santa Barbara ecologist John Melack warned:

“We are concerned that these conditions are becoming more common. The implications for biodiversity and local communities are profound.”

Researchers emphasize that this is not solely a regional problem: avoiding more catastrophic wildlife losses will require global reductions in fossil-fuel emissions and coordinated climate action. The study documenting these findings was published in the journal Science.