A new study published in Nature Communications has revealed that floods, traditionally viewed as natural "oxygen boosters" for rivers, are instead causing sudden oxygen depletion in waterways across China. The research, led by Prof. Yongqiang Zhou from the State Key Laboratory of Lake and Watershed Science for Water Security, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, exposes the hidden link between flood-induced organic pollutants and river hypoxia, posing a dire threat to aquatic ecosystems.  

Under global warming, extreme rainfall events are becoming more frequent, exacerbating pollution during flood seasons. While conventional wisdom suggests that turbulent floodwaters increase dissolved oxygen (DO) levels, the study found the opposite: 80.1% of the 1,156 rivers monitored across China experienced a 19.7% drop in DO during floods, with 69.4% showing a 16.2% decline in oxygen saturation (DO%sat). Small to medium-sized rivers in heavily agricultural and urbanized areas were the most severely affected.  

The research team introduced the concept of "sudden deoxygenation shock," demonstrating that floods can plunge healthy rivers into hypoxic (≤2 mg L^-1) or even anoxic (≤0.5 mg L^-1) states within hours. This phenomenon is driven by massive inflows of ammonium nitrogen and chemical oxygen demand (COD) from agricultural runoff and urban drainage. In hotspots like the North China Plain and the Yangtze River Basin, ammonium nitrogen concentrations during floods exceeded non-flood levels by more than 1 mg N L^-1.  

The study quantified the relationship between land use and oxygen depletion, showing that every 10% increase in agricultural or urban land within a 30 km buffer zone led to a 6.1% drop in DO and a 4.9% decline in DO%sat during floods. "This highlights the urgent need for pollution interception measures, such as wetland buffers and stormwater management systems, to mitigate hypoxia risks," said Prof. Zhou.  

Interestingly, urban rivers recovered faster from hypoxia (up to 0.5 mg L^-1 per day), likely due to efficient drainage systems and natural reaeration in larger waterways. However, this recovery does not offset the cumulative stress on sensitive species or the potential invasion of hypoxia-tolerant invasive species, which could disrupt entire food webs.  

Under the high-emission scenario (SSP5-8.5), intense rainfall events in China are projected to increase by 30–50% by the end of the century, making flood-induced hypoxia more frequent and severe. The researchers urge a shift in river management from water quality standards to "hypoxia risk" control, emphasizing nature-based solutions like wetland restoration, precision agriculture, and sponge city development. 

The full text is available at: https://www.nature.com/articles/s41467-025-62236-5