By Tyne Eckmyn, Publishing Associate: Researcher and Writer at Save the Water™ | June 30, 2026

Edited by Joshua Awolaye, Publishing Associate: Editor at Save the Water™

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Rivers depend on a delicate balance of chemicals to support life. One of the most important is dissolved oxygen (DO), which fish and other aquatic animals need to survive. But oxygen levels in rivers around the world are falling. Rising temperatures, pollution, and dam construction are all contributing to this decline.

A Brief Explanation of Dissolved Oxygen Chemistry

The oxygen that aquatic aerobic organisms breathe is not the oxygen that makes up the water molecules themselves. Instead, they breathe oxygen that has dissolved into the water. This dissolved oxygen comes from the atmosphere and aquatic plants that release oxygen during photosynthesis.

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The concentration of DO in a given water body depends on multiple factors:

• Water Temperature: Warmer water can hold less DO than colder water can. This is because when a gas (oxygen) dissolves in a solvent (water), the reaction gives off heat. Thus, colder water can hold more dissolved oxygen. But when the water is warmer, it “pushes” the reaction in the other direction, allowing less gas to dissolve.

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• Air Pressure: If the undissolved gas is under higher pressure, it will “push” more gas into dissolving in the solvent. While air pressure does fluctuate, changes in air pressure are not a driving factor in river deoxygenation.

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• Nutrients: The respiration of aquatic life consumes oxygen and gives off carbon dioxide. Photosynthesis, on the other hand, consumes carbon dioxide and produces oxygen, which can increase DO concentration in water bodies. If there is an excess of nutrients, such as nitrogen however, it can lead to algal blooms. These can block sunlight for other plants. When the algae decomposes, it also consumes oxygen and releases carbon dioxide.

Global Trends in Fluvial Dissolved Oxygen Concentrations

A recent study analyzed the DO concentration of 21,439 rivers from 1985 to 2023. It found that 78.8% of them experienced declining DO concentrations at a rate of −0.045 mg liter⁻¹ decade⁻¹. Rising global temperatures drove most of this decline by reducing oxygen solubility (OS). Changes in ecosystem metabolism (e.g. respiration and photosynthesis) and individual heat wave events also contributed.

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Another major factor is that of dam impoundment of river systems. In general, a higher flow rate increases DO concentration by mixing the air with the water. Still water is also more vulnerable to warming. Damming river systems therefore generally decrease DO concentrations. In large and deep enough dam reservoirs, there can be an increase in local evaporation rates. This has a cooling effect that can help to offset the other factors.

What Can Be Done to Reverse the Decline in Dissolved Oxygen?

Lowering global average temperatures is the best long-term solution, but other approaches have also improved river health. Nutrient management programs have helped raise the DO levels in China’s ten largest rivers between 2006 and 2020. Another success story is that of Puerto Rico. After implementing the Clean Water Act in 1974, they have made progress in boosting DO levels. Many rivers are still highly polluted, but these success stories prove that improvement is possible. 

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