How changes in river morphology affect their ability to self-purify

Studies conducted on the Shiwuli River in China have shown that morphological changes in the riverbed, especially an increase in its winding, have a significant impact on the self-purification capacity of watercourses. These results can provide a basis for planning the restoration of watercourses in highly urbanized areas.

The authors of the study, the results of which were published June 3 in the journal Water , are researchers from Nanjing Agricultural University in Nanjing and Hohai University, and experts from China’s Ministry of Water Resources.

Impact of urbanization on the self-purification capacity of rivers

Rivers are dynamic ecosystems, encompassing the riverbed, riparian zones and associated floodplain terraces. Their self-purification, resulting from physical, chemical and biological processes, is crucial for maintaining biodiversity and environmental health. As a result of urbanization, rivers are often straightened, disrupting natural processes and reducing their ability to self-purify.

The literature indicates that the shape of a river – especially its meandering (sinuosity factor) – significantly affects treatment efficiency. Previous studies have signaled that effective nitrogen and phosphorus removal occurs at a minimum sinuosity of about 1.42, but so far detailed studies conducted under field conditions have been lacking, especially in the context of urbanized rivers and the role of wetlands in the process.

How was the study conducted?

The authors chose the lower section of the Shiwuli, flowing through the city of Hefei. The morphological transformation of the river due to urbanization has significantly affected the deterioration of its water quality and, as a result, threatened the ecological state of Chaohu Lake downstream. In order to improve the river’s water quality, an artificial wetland was created in the lower section of the river between 2017 and 2023.

Water quality monitoring was conducted at 17 sites along the course of the river, from the mouth to Chaohu Lake. In addition, the effects of treatment at the entrance and exit of the wetland were evaluated. Samples were taken four times a year in 2017 and 2024 (before and after wetland construction). Five key indicators were analyzed: dissolved oxygen (DO), total nitrogen (TN), ammonia nitrogen (NH₃-N), total phosphorus (TP) and chemical oxygen demand (COD).

What the results say, or the crucial importance of sinusoidality

Comparing 2017 and 2024, the authors found a significant seasonal decrease in TP, TN, NH₃-N and COD concentrations, and an increase in DO. The average TP concentration decreased from 0.43 mg/L in 2017 to about 0.13 mg/L in 2024, and TN decreased from 10.19 mg/L to less than 5 mg/L. NH₃-N and COD also decreased, while DO increased from 4.31 mg/l to about 8 mg/l. In the wetlands, even more pronounced effects were observed: TP decreased by as much as 90.9 percent, TN by 72.6 percent, NH₃-N by 83.3 percent, and COD by 79.9 percent. In the riverbed, similar decreases, although smaller, were respectively: 63.3 percent, 25.5 percent, 62.1 percent and 34.8 percent.

The researchers showed a strong correlation between the sinusoidality of the river and the rate of self-purification. The more meandering the section was (sinuosity of 1.0-1.84), the greater the recorded decrease in pollutant concentration and increase in oxygen concentration. The results clearly indicate that greater meandering raises the self-purification capacity.

Mechanism of action of meanders and wetlands

In the discussion, the authors explain that the greater length of the current in meanders increases the residence time of the water, which allows for greater filtration and sedimentation. Increased winding intensifies bank erosion and promotes eddies, which increase water oxygenation (hyporeic exchange with groundwater). This, in turn, accelerates nitrogen oxidation and phosphorus reduction.

In wetlands, an additional benefit is the larger surface area and slower flow, which promotes sedimentation and supports microbial activity. The photosynthesis carried out by plants increases DO, and microorganisms reduce nutrients and organic components more efficiently. In addition, higher summer temperatures promote biological activity.

Key findings

The study clearly showed that meandering sections of rivers – especially those supported by wetlands – show a real higher capacity for self-purification. The results suggest that even with sinuosity reduced from 1.49 to 1.30, wetlands can effectively compensate for this decline.

The authors recommend that river restoration processes should use such solutions as extending the channel, increasing its width, creating wetlands and providing a link to groundwater. All this can significantly improve water quality parameters in urban areas.

The scientific team plans to conduct further physical experiments under model conditions to further investigate the relationship between sinuosity and self-purification. The ultimate goal is to provide practical, scientifically validated guidelines for planners and designers involved in restoring the natural course of rivers and protecting urban water resources.

Bibliography:

Xiao, C.; Chai, Z.; Chen, D.; Luo, Z.; Li, Y.; Ou, Q.; Zhang, Y.. Research on the Influence of River Morphological Changes on Water Self-Purification Capacity: A Case Study of the Shiwuli River in Chaohu Basin. Water 2025, 17, 1694. https://doi.org/10.3390/w17111694