Rivers Around the World Are Losing Oxygen — and Tropical Systems Are Hit Hardest

A global picture of river oxygen loss

To examine long‑term trends, researchers analyzed satellite observations and environmental data using a machine‑learning model covering roughly 21,439 river reaches worldwide from 1985 to 2023.

Across these river segments, dissolved oxygen concentrations showed a consistent downward trend, declining at an average rate of about −0.045 milligrams per liter per decade.

Other reporting on the same dataset estimates that average oxygen levels in global rivers have fallen roughly 2.1% since 1985.

That drop might appear small, but oxygen availability in water is tightly constrained. Even modest declines can push ecosystems closer to thresholds where fish, insects, and other organisms struggle to survive.

Why warming rivers lose oxygen

The mechanism behind the decline is largely physical. Warmer water holds less dissolved oxygen than colder water, meaning rising temperatures directly reduce the oxygen capacity of rivers.

Climate change compounds this effect in several ways:

• Higher average water temperatures reduce oxygen solubility.

• Heatwaves cause short bursts of extreme warming that can quickly push rivers toward oxygen stress.

• Biological activity increases in warmer water, which can consume more oxygen through respiration and decomposition.

Together, these processes reduce oxygen supply while increasing oxygen demand, accelerating the risk of hypoxic conditions.

Tropical rivers are the most vulnerable

The global analysis highlights tropical river systems as the biggest emerging hotspots of oxygen loss.

There are two main reasons for this:

First, tropical rivers are already warm. Because oxygen solubility decreases rapidly as temperature rises, even small additional warming can significantly reduce oxygen concentrations.

Second, many tropical ecosystems already operate close to their thermal tolerance limits. That leaves less ecological buffer when temperatures rise further.

As a result, regions across the tropics—including parts of South Asia, Africa, and South America—could experience some of the most pronounced future oxygen stress in freshwater systems.

Growing risk of freshwater “dead zones”

When dissolved oxygen drops below critical levels, rivers can experience hypoxia, sometimes called aquatic “dead zones.” These conditions can suffocate fish, disrupt food webs, and trigger mass mortality events.

Scientists warn that climate change could increase the frequency and duration of low‑oxygen events in rivers worldwide during the coming decades.

Previous river studies have already found that warming rivers frequently show simultaneous oxygen declines. In one large analysis of hundreds of rivers in North America and Europe, 70% showed decreasing oxygen levels alongside widespread warming trends.

If warming continues, some rivers could periodically reach oxygen levels capable of causing acute stress—or even death—for sensitive aquatic species.

Other human pressures amplify the problem

Climate warming is not acting alone. Several other human influences can intensify river deoxygenation.

Land‑use change, agriculture, and urban runoff can add nutrients and organic matter to waterways. When this material decomposes, microbes consume oxygen, further lowering dissolved oxygen levels.

Human activity has already altered the global oxygen cycle in inland waters, reshaping how oxygen is produced and consumed across freshwater ecosystems.

Infrastructure such as dams and altered river flows can also influence oxygen dynamics by slowing water movement or changing mixing patterns, although the extent of those effects varies widely between river systems.

What the research suggests for the future

The new global analysis reinforces a growing scientific consensus: deoxygenation is becoming a widespread feature of the world’s freshwater systems.

Rivers have historically received less attention than oceans and lakes in oxygen research, but they are crucial ecological corridors that support biodiversity, fisheries, and drinking‑water supplies for billions of people.

If global temperatures continue to rise, scientists expect river ecosystems—especially those in the tropics—to face increasing episodes of low oxygen that could reshape freshwater biodiversity and ecosystem stability in the decades ahead.