Lake Tefé Reached 41°C — Hundreds of Amazon River Dolphins Found Dead as Heatwave Turned Lake into a "Steam Bath"

Extreme heat and drought killed hundreds of Amazon river dolphins

When dozens of dolphins began washing ashore on Lake Tefé in Brazil's Amazonas state in September 2023, hydrologist Ayan Fleischmann and his team were sent to investigate. What they found was startling: an extreme drought and heatwave had transformed parts of the lake into a near-steam bath, with surface temperatures reaching about 41°C (105.8°F) — hotter than most home hot tubs.

The team's results, published in Science, underline how rising global temperatures are already affecting tropical freshwater ecosystems and the communities that depend on them. The findings were released as world leaders gathered for the UN's COP30 climate talks in Brazil.

"You couldn't put your finger in the water," said Fleischmann of the Mamirauá Institute for Sustainable Development, describing the scene and the psychological impact of encountering carcasses of Amazon river dolphins (pink dolphins) and tucuxis.

Key findings

Widespread overheating: Researchers sampled 10 central Amazonian lakes and found daytime water temperatures above 37°C at five sites. Lake Tefé had the most extreme reading and its surface area had shrunk by roughly 75%.

Mass mortality: Conservation groups reported at least 153 dolphins found dead during the week of Sept. 23, 2023 — including 130 pink (Amazon) dolphins and 23 tucuxis. Media reports later cited more than 200 dolphin deaths recorded over two months; fish kills were also widespread.

Unusual thermal structure: In Tefé the extreme warmth was detected throughout the entire two-meter water column rather than only at the surface, placing additional physiological stress on aquatic life adapted to much narrower temperature ranges.

Drivers identified: Using models, the team concluded four interacting factors amplified heating: strong solar radiation, shallow waters, low wind speeds and high turbidity (water cloudiness). Shallow, turbid basins trap heat, while weak winds limit cooling.

Other ecosystem impacts: Researchers documented mass fish deaths and a stressed phytoplankton bloom that turned parts of the lake red — a phenomenon the team will describe in a separate paper.

Broader context

Analysis of NASA satellite data back to 1990 shows Amazonian lakes have been warming at roughly 0.6°C per decade, a rate higher than the global average. Scientists warn that human-caused climate change is increasing the frequency, intensity and duration of droughts and heat extremes in the region.

Research in Hydrological Processes supports these findings, noting that droughts intensify radiation exposure, lower water levels and slow flows — all of which raise river and lake temperatures and harm aquatic life.

Fleischmann said he will advocate at COP30 for sustained monitoring of Amazon lakes and for greater inclusion of Indigenous peoples, riverine communities and Afro-descendant groups in developing locally appropriate solutions.

Why it matters: Tropical lakes are crucial for food security and livelihoods across the Amazon, yet they have been understudied compared with lakes in temperate regions. This study highlights an urgent, locally devastating consequence of global warming that also has wider implications for biodiversity and human communities.