Have you ever wondered that a river’s “health” depends not only on the clarity of its water but also on its ability to sustain countless tiny organisms? Today, from a scientific perspective, we’ll delve into the intricate relationship between river dredging and ecological balance, unlocking the “aquatic ecological code” of rivers.

 

The river channel is an “underwater forest.”

The river ecosystem consists of three components: water body, bottom sediment, and biota. The microbial biofilm, benthic organisms, and aquatic plants in the bottom sediment together form an “underwater forest,” where elements such as carbon, nitrogen, and phosphorus undergo continuous cycling.

Among these, microbial biofilms serve as the “digestive organs” of aquatic ecosystems, responsible for breaking down organic matter; benthic animals (such as snails and aquatic insects) act as “scavengers,” facilitating the cycling of nutrients; and submerged plants, through photosynthesis, release oxygen, thereby maintaining adequate dissolved oxygen levels in the water.

 

If dredging is excessive—just as if you were to remove all the soil from a forest—though it may seem tidy, it would lose its vitality. Research shows that when the thickness of bottom sediment falls below 10 centimeters, the number of benthic species declines by more than 50%, and the water body’s self-purification capacity is significantly weakened.

The technology has shifted from “thorough dredging” to “precise washing and removal.”

In the mid-20th century, dredging techniques primarily relied on mechanical excavation, aiming to "expose the riverbed to the bottom." However, with the advancement of ecology, people came to realize the destructive nature of this approach—it not only drove bottom-dwelling organisms to extinction but also disrupted the microbial biofilm at the sediment-water interface.

Starting in the 1980s, Japan proposed the “partial dredging method,” which preserves the anaerobic sludge at the bottom layer to avoid disrupting the denitrification function.

 

In recent years, ultra-nano-aerobic reoxygenation technology has emerged as an innovative hotspot in in-situ sediment remediation techniques. This technology involves injecting high-concentration dissolved oxygen into the bottom of water bodies, rapidly improving the redox environment of the water and accelerating the recovery of aerobic microbial communities in both the water column and the sediment. By doing so, it speeds up the degradation of organic pollutants without disturbing the sediment itself. After being applied to a certain canal in the Dongxihu District of Wuhan City, the water body’s condition improved dramatically within just one week: its turbidity dropped from black and odorous to a transparency of 70 centimeters, and benthic organisms began to recover—demonstrating remarkable effectiveness.

Aquatic organisms are telling us about the health of the river.

Scientists assess the health of river ecosystems through “biomonitoring.” The four major groups commonly used for identification include: benthic organisms—where the presence of aquatic earthworms indicates pollution, while the appearance of mayflies suggests relatively clean water quality; fish—such as crucian carp (indicating pollution) and mandarin fish (indicating clean water quality); aquatic plants—where the presence of blue-green algae (indicating pollution) and eelgrass (indicating clean water quality); and microbial biofilms, whose thickness reflects the efficiency of organic matter decomposition.

 

As long as you can distinguish these organisms, you’ll be able to intuitively assess the health of the river—just like a “medical checkup.”

Why can’t rivers be “too clean”?

Dead branches, fallen leaves, algae, and zooplankton in river channels form the foundation of the food chain. If these are eliminated using ultraviolet light or chemical disinfectants, filter-feeding fish such as silver carp and bighead carp will lose their food source, disrupting the entire food chain.

Moreover, although the anaerobic layer in the sediment may not be aesthetically pleasing, it performs denitrification, converting nitrate into nitrogen gas and thereby alleviating eutrophication. Studies have shown that retaining a sediment layer 5 to 10 centimeters thick can increase the total nitrogen removal rate in water bodies by more than 20%.

Smart dredging and ecological intelligence are the directions of the future.

Modern dredging is no longer a one-size-fits-all approach; instead, it integrates remote sensing, the Internet of Things, and biological monitoring to achieve precise, segmented, and timely dredging. For example, Guangdong Province, in its “Implementation Plan for Building Happy Rivers and Lakes,” proposes establishing a “River and Lake Ecological Flow Management Platform” that integrates functions such as data collection and monitoring command. At the same time, through measures like constructed wetlands and vegetation restoration, the self-purification capacity of river channels is enhanced, creating a “breathing” aquatic ecosystem.

 

The course of history shows that the health of a river lies not in how “clean it appears,” but in how many lives it can sustain. Only by embracing an appropriate degree of “messiness” and respecting nature can we turn the vision of “clear waters, green banks, and fish swimming in shallow bottoms” into a tangible reality.