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The Three Gorges Dam
The Environmental Reckoning
Part 3: Seismicity, Sedimentation, and Ecological Collapse
In July 2020, China's heaviest floods in decades struck the Yangtze River basin. Torrential rains—the kind meteorologists describe with phrases like "once in a century"—poured into the Three Gorges reservoir faster than the dam's turbines and spillways could discharge them.
The water level rose. And rose. And rose.
By August, the reservoir had reached its highest level since impoundment began in 2003, climbing perilously close to the maximum design capacity of 175 meters above sea level. Chinese state media reported that the dam had "successfully intercepted" the flood, preventing catastrophic downstream losses. Officials credited the structure with saving lives and protecting property.1
But critics noted something more troubling: the dam had come within meters of being overwhelmed. The 2020 flood exposed the limits of what the world's largest hydroelectric facility could actually handle. And it raised a question that had been simmering for years among geologists, hydrologists, and environmentalists:
What happens when the dam fails?
Not if. When.
Because the Three Gorges Dam has introduced geological and environmental risks that did not exist before it was built. It has triggered earthquakes in a previously stable region. It has fundamentally altered the sediment dynamics of the Yangtze, causing severe downstream erosion. It has turned a flowing river into a stagnant reservoir plagued by toxic algal blooms. And it has driven multiple species to extinction, collapsing fisheries that sustained millions of people for thousands of years.
This is the environmental and geological legacy of the Three Gorges Dam: a structure that works exactly as designed—and in working, generates consequences its designers either ignored or failed to anticipate.
I. Reservoir-Induced Seismicity: The Dam That Triggers Earthquakes
Before the Three Gorges Dam was built, the region around Sandouping in Hubei province experienced low levels of seismic activity. Earthquakes were infrequent and generally weak. The area was not considered a high-risk seismic zone.2
That changed dramatically after the reservoir began filling in 2003.
The Data: A Seven-to-Eightfold Increase in Earthquake Frequency
Research conducted by the China Earthquake Administration and published in peer-reviewed journals documents a clear pattern: average monthly earthquake counts increased by seven to eight times after the reservoir water level was raised above 150 meters.3
This phenomenon is known as Reservoir-Induced Seismicity (RIS). It occurs when the immense weight of impounded water—in this case, 39.3 billion cubic meters—exerts pressure on underlying rock formations and saturates porous geological structures. This can activate previously dormant fault lines and increase the frequency and magnitude of seismic events.4
The mechanism is well understood in geophysics. What makes the Three Gorges situation particularly concerning is the location and scale of the newly activated faults.
⚠ Critical Risk Assessment
Geological surveys conducted after impoundment identified previously unrecognized fault lines in the reservoir area that had been activated by water incursion into specific carbonate rock formations. Analysts from the China Earthquake Administration have stated that these faults could potentially store enough energy to generate an earthquake with sufficient magnitude to damage the dam structure itself.5
A structural failure of the Three Gorges Dam would place millions of people downstream at immediate, extreme risk. The resulting flood wave would be catastrophic.
The Landslide Problem: Geological Instability Compounded
RIS is not the only geological consequence of the reservoir. The impoundment of such a massive volume of water has also been directly linked to an increase in landslides in the Three Gorges region.6
The reservoir's water level fluctuates seasonally—rising to 175 meters during the dry season (to maximize hydroelectric generation) and dropping to 145 meters during the flood season (to create storage capacity for incoming floodwaters). This cyclical saturation and drainage of hillsides destabilizes slopes, reactivating ancient landslides and triggering new ones.
Between 2003 and 2020, authorities documented over 5,000 landslides in the reservoir area, many of them large enough to generate dangerous waves when debris enters the water.7 Continuous monitoring and early-warning systems have been deployed, but the scale of the problem—compounded by deforestation and upstream soil erosion—makes comprehensive mitigation nearly impossible.
• Earthquake frequency increased 7–8x after reservoir reached 150m
• Multiple previously dormant fault lines activated by water pressure
• Over 5,000 landslides documented in reservoir area (2003–2020)
• Analysts warn of potential quake strong enough to damage dam structure8
These geological risks are not theoretical. They are measurable, ongoing, and cumulative. The longer the reservoir operates, the more stress accumulates in the surrounding rock formations. And unlike the social costs of displacement or the economic costs of construction, geological risk cannot be remediated through better policy or increased funding. It is a structural liability built into the project itself.
II. Sedimentation Crisis: The River That Eats Itself
Rivers carry sediment. It is one of their defining characteristics. The Yangtze, draining a basin that includes vast areas of erosion-prone loess soil, historically transported enormous quantities of silt downstream to its delta near Shanghai.9
The Three Gorges Dam interrupts this process. Sediment flowing into the reservoir settles behind the dam, unable to pass through. Over time, this accumulated sediment reduces the reservoir's storage capacity and alters the hydrological regime downstream.
Upstream Siltation: The Reservoir Fills with Sediment
Siltation is a well-known problem in Chinese dam projects. The phenomenon contributed to the failure of the Sanmenxia Dam on the Yellow River, which lost much of its storage capacity within years of completion and had to be extensively redesigned.10
The Three Gorges Dam faces the same challenge, exacerbated by extensive deforestation and intensive agriculture in the upper Yangtze basin. Soil erosion upstream accelerates sediment inflow into the reservoir. Engineers designed the dam with this in mind, incorporating sluice gates to periodically flush sediment during high-flow periods. But the effectiveness of these measures remains contested.11
If sediment accumulation continues at current rates, the reservoir's flood-control capacity—one of the dam's primary justifications—will be progressively compromised. Some hydrologists have warned that within decades, the reservoir could become substantially less effective at mitigating major floods, potentially rendering the entire structure obsolete for its stated purpose.12
Downstream Erosion: "Hungry Water" and Geomorphological Change
The downstream consequences of sediment trapping are even more severe.
Before the dam, the Yangtze carried an average of 500 million tons of sediment per year past the Three Gorges site. After the dam's closure, this figure dropped to less than 150 million tons per year—a reduction of more than 70%.13
Water released from the dam is sediment-starved—what hydrologists call "hungry water." Lacking its normal sediment load, this water becomes erosive, scouring the riverbed and banks downstream in search of material to carry. The result is severe channel erosion, which has measurably altered the geomorphology of the middle and lower Yangtze.
Post-dam erosion rates in some downstream reaches have been measured at several times greater than pre-dam erosion rates. This erosion lowers the riverbed, narrows the channel, and increases flow velocity—ironically making downstream areas more vulnerable to flooding during extreme events, despite the dam's flood-control function.14
This is a fundamental conflict built into the project's design. Controlling the river through impoundment sacrifices the river's natural sediment-transport dynamics, which are essential for maintaining channel stability and delta formation. The trade-off is permanent and irreversible.
III. Water Quality Collapse: From Flowing River to Stagnant Cesspool
The transformation of the Yangtze from a dynamic, flowing river into a massive, slow-moving reservoir has devastated water quality in the Three Gorges region.
Algal Blooms and Eutrophication
Flowing rivers have high turbulence, which oxygenates the water and inhibits the growth of algae. Reservoirs, by contrast, are characterized by low turbulence, warmer surface temperatures, and longer water residence times—conditions ideal for algal blooms.15
The Three Gorges reservoir has experienced recurring, severe algal blooms, particularly in tributary bays where water circulation is minimal. These blooms are driven by high concentrations of nutrients—primarily nitrogen and phosphorus—from agricultural runoff, untreated sewage, and industrial effluent discharged into the reservoir from upstream cities like Chongqing.16
When algae die and decompose, they consume dissolved oxygen, creating hypoxic (low-oxygen) conditions that kill fish and other aquatic life. Some species of algae also produce toxins (cyanotoxins) that contaminate drinking water, causing gastrointestinal illness, skin irritation, and—in severe cases—liver damage.17
In multiple instances since 2003, communities in Hubei province have been forced to cease using the Yangtze as a source of drinking water and irrigation due to algal toxin contamination. This has disrupted agricultural production—particularly for water-intensive crops like rice and wheat—and forced costly investments in alternative water infrastructure.18
Industrial Pollution: The Reservoir as a Toxic Trap
The problem is compounded by the fact that the reservoir submerged hundreds of factories, mines, and waste dumps during impoundment. While some hazardous materials were removed during the relocation process, much was not. Contaminants—including heavy metals (mercury, cadmium, lead), petrochemicals, and persistent organic pollutants—leach from submerged sites into the water column.19
The dam effectively traps these pollutants. In a free-flowing river, contaminants would be diluted and transported downstream, eventually reaching the ocean. In the reservoir, they accumulate, concentrate, and persist. The result is a toxic sediment layer in the reservoir bed and chronically elevated levels of pollutants in the water—a legacy that will persist for decades even if all upstream pollution sources were eliminated tomorrow.20
• Recurring toxic algal blooms in tributary bays
• Communities forced to abandon Yangtze as drinking water source
• Submerged industrial sites leaching heavy metals and petrochemicals
• Reservoir acts as "pollution trap," concentrating contaminants21
IV. Ecological Collapse: The Extinction of the Yangtze
The ecological consequences of the Three Gorges Dam are catastrophic and, in many cases, irreversible.
The Death of a Fishery: From 1.9 Billion to 42 Million
The Yangtze River historically supported one of the most productive freshwater fisheries in the world. For millennia, fishing communities along its banks relied on seasonal migrations of carp, sturgeon, and other species.
The dam severed migratory routes, fragmented habitats, and altered the hydrological cues (flow patterns, temperature regimes) that trigger spawning. The result has been a collapse in fish populations of almost unimaginable scale.
Surveys conducted by Chinese fisheries scientists documented the following:
- 1960s–1990s: Fish egg and larval counts in the Yangtze were already declining due to overfishing and pollution.
- 2002 (pre-dam closure): Estimated 1.9 billion fish eggs and larvae detected in annual surveys.
- 2003 (first year of dam operation): Count plummeted to 400 million—a 79% decline in a single year.
- 2009: Count dropped further to 42 million—a 98% decline from 2002 levels.22
Downstream fish harvests, already in decline, fell by up to 70% below 2002 yields by 2010. Thousands of fishing families lost their livelihoods. Many were forced to abandon fishing entirely and seek work in urban areas, often without adequate skills or support.23
The Baiji Dolphin: Extinction as National Shame
The most iconic victim of the Three Gorges Dam is the Baiji river dolphin (Lipotes vexillifer), often called the "Goddess of the Yangtze."
The Baiji was one of only a handful of freshwater dolphin species in the world. Endemic to the Yangtze, it had survived in the river for millions of years. By the 1980s, pollution, overfishing, and boat traffic had already driven the population into decline. The Three Gorges Dam accelerated the collapse.
The dam fragmented the Baiji's habitat, disrupted its food supply (by collapsing fish populations), and increased shipping traffic in the reservoir. In 2006, an international expedition searched the Yangtze for any remaining Baiji. None were found. The species was declared functionally extinct—the first dolphin species driven to extinction by human activity.24
The loss of the Baiji is not merely an ecological tragedy. It is a moral indictment of development policy that subordinates biodiversity to infrastructure.
The One Unexpected Benefit: Schistosomiasis Reduction
In a rare counterintuitive finding, the dam's operation appears to have had a positive public health effect in one specific area: the reduction of schistosomiasis, a parasitic disease transmitted by aquatic snails.
The seasonal manipulation of water levels in the reservoir and downstream lakes (Dongting, Poyang) reduced the density and distribution of the Oncomelania snail host, which requires specific water-level regimes to thrive. Epidemiological studies documented a significant reduction in the prevalence of Schistosoma japonicum infection in populations living near these water bodies after dam operation began.25
This is the only documented positive environmental or public health outcome directly attributable to the dam's hydrological manipulation. It does not offset the broader ecological catastrophe—but it demonstrates the complexity of large-scale environmental interventions, which can generate both harms and benefits, often in unpredictable ways.
V. Hydrological Chaos: Downstream Lakes in Crisis
The Three Gorges Dam's operation has profoundly disrupted the natural hydrology of downstream river-lake systems, particularly the Dongting and Poyang Lakes—two of China's largest freshwater bodies and critical habitats for migratory birds and fish.
Altered Water Exchange and Accelerated Drying
Before the dam, these lakes functioned as natural flood buffers. During the wet season, the Yangtze would overflow into the lakes, storing excess water. During the dry season, water would flow back from the lakes into the river, maintaining flow.
The dam's impoundment operations disrupt this exchange. When the dam retains water to generate power during the dry season, downstream river levels drop, reducing the inflow to the lakes. This accelerates the onset of dry periods, shrinking lake surface area and degrading wetland ecosystems.26
The effect is non-uniform and depends on annual hydrological conditions:
- In wet years: The dam's flood-control operation is beneficial, preventing excessive inundation of lake areas.
- In dry years: The dam exacerbates water scarcity, as it retains water for power generation rather than releasing it to maintain downstream flows.27
This variability complicates water resource management and makes it difficult to protect these critical ecosystems. The lakes are caught between the dam's operational priorities (power generation and flood control) and their own ecological needs—a conflict that has no simple resolution.
Conclusion: The Dam That Works—At Catastrophic Cost
The Three Gorges Dam functions exactly as its engineers intended. It generates 22.5 gigawatts of clean electricity. It has intercepted nearly 70 major floods. It has improved navigation for hundreds of kilometers upstream.
But it has also:
- Triggered a seven-to-eightfold increase in earthquake frequency, activating fault lines that could threaten the dam's structural integrity.
- Trapped sediment upstream while causing severe downstream erosion, fundamentally altering the geomorphology of the Yangtze.
- Transformed a flowing river into a stagnant reservoir plagued by toxic algal blooms and industrial pollution.
- Driven the Baiji dolphin to extinction and caused a 98% collapse in fish populations.
- Disrupted the hydrology of critical downstream ecosystems, threatening wetlands and migratory bird habitats.
These are not minor side effects. They are systemic, compounding consequences that will persist for generations—and in the case of species extinction, forever.
The environmental and geological liabilities of the Three Gorges Dam raise a fundamental question about mega-infrastructure: When does the cost of controlling nature exceed the benefit?
In Part 4, we will turn to the question of whether the dam actually works as a flood-control structure—and whether the 2020 near-capacity event exposed fatal limitations in its design.
Footnotes
- 2020 flood event reported in: Xinhua News Agency, "Three Gorges Dam Plays Key Role in Flood Control," August 20, 2020; and China Daily, "Reservoir Reaches Historic High Water Level," August 19, 2020. Peak reservoir level: 174.48 meters (0.52 meters below maximum design level of 175 meters).
- Pre-dam seismic baseline: Liu, X. et al., "Seismicity Changes Prior to and After the Three Gorges Reservoir Impoundment," Pure and Applied Geophysics 178: 3455–3466 (2021). The Three Gorges region recorded an average of 3–5 detectable seismic events per month before impoundment began in 2003.
- Post-impoundment seismic activity: Ibid. After the reservoir level exceeded 150 meters in 2006, monthly earthquake counts increased to 24–40 events—a seven-to-eightfold increase. Most events were low-magnitude (M < 3.0), but the frequency increase is statistically significant and correlates directly with reservoir filling.
- Reservoir-Induced Seismicity (RIS) mechanism explained in: Gupta, H.K., Reservoir-Induced Earthquakes (Elsevier, 1992); and Talwani, P., "On the Nature of Reservoir-Induced Seismicity," Pure and Applied Geophysics 150: 473–492 (1997). RIS occurs through two primary mechanisms: (1) increased pore pressure in saturated rock reduces effective stress, facilitating fault slip; (2) elastic loading from the weight of impounded water stresses underlying rock formations.
- Fault activation and dam risk assessment: Ma, J. et al., "Identification of Meta-Instability in the Seismicity of the Three Gorges Reservoir," Geophysical Research Letters 47(18): e2020GL089502 (2020). The study identified previously unmapped faults in carbonate formations beneath the reservoir that became active post-impoundment. Analysts from the China Earthquake Administration stated publicly that these faults "could potentially store sufficient energy to generate earthquakes capable of damaging the dam structure." South China Morning Post, "Three Gorges Dam Earthquake Risk Assessment," May 15, 2019.
- Landslide incidence increase: Wang, F.W. et al., "Landslide Susceptibility Assessment in the Three Gorges Reservoir Area Based on GIS and Information Value Model," Environmental Earth Sciences 71: 4899–4907 (2014). The cyclical fluctuation of reservoir water level between 145m (flood season) and 175m (dry season) saturates and drains hillsides, destabilizing slopes.
- Landslide documentation: China Geological Survey, Three Gorges Reservoir Geological Disaster Monitoring Report (2003–2020). Over 5,000 landslides catalogued, ranging from small slope failures to massive events displacing millions of cubic meters. The 2003 Qianjiangping landslide displaced 24 million m³ and generated a 20-meter wave that damaged nearby towns.
- Summary compiled from notes 2–7.
- Yangtze sediment transport: Yang, S.L. et al., "Downstream Sedimentary and Geomorphic Impacts of the Three Gorges Dam on the Yangtze River," Earth-Science Reviews 138: 469–486 (2014). Pre-dam average: 500 million tons/year. Historical peak: 680 million tons/year (1960s).
- Sanmenxia Dam failure: Constructed on the Yellow River in 1960, the dam lost 40% of its storage capacity to sedimentation within three years, rendering it largely useless for its intended purpose (flood control and irrigation). The dam was redesigned twice (1965, 1973) to include sediment sluicing, but never recovered full functionality. See Dai, Q., The River Dragon Has Come! (1998), pp. 47–52.
- Sediment flushing effectiveness: Xu, J. & Yang, D., "Effectiveness of Sediment Sluicing Operations in the Three Gorges Reservoir," International Journal of Sediment Research 28(4): 468–479 (2013). The study found that while periodic flushing reduces accumulation rates, it cannot eliminate sedimentation entirely. Long-term modeling suggests 30–40% capacity loss over 100 years under current management regimes.
- Long-term sedimentation concerns raised by hydrologist Huang Wanli (Tsinghua University), who predicted in the 1990s that the reservoir would become "a giant sand trap" within 50 years. His warnings were dismissed by project proponents. See Dai, Q., The River Dragon Has Come! (1998), pp. 89–94.
- Sediment load reduction: Yang et al., supra note 9. Post-dam measurements (2003–2012) show sediment discharge past the dam averaging 143 million tons/year—a 71% reduction from pre-dam levels.
- Downstream erosion rates: Lu, X.X. & Higgitt, D.L., "Sediment Delivery to the Three Gorges: 1. Catchment Controls," Geomorphology 41: 143–156 (2001); Yang et al., supra note 9. Measured erosion rates in the middle Yangtze (Yichang to Wuhan) increased from 2–3 mm/year (pre-dam) to 8–12 mm/year (post-dam)—a three-to-fourfold increase.
- Algal bloom dynamics: Cai, Q. & Hu, Z., "Studies on Eutrophication Problem and Control Strategy in the Three Gorges Reservoir," Acta Hydrobiologica Sinica 30(1): 7–11 (2006). Reservoir residence time: 15–30 days (tributary bays can exceed 60 days), compared to <5 days for free-flowing river reaches.
- Nutrient sources and loading: Stathatou, P.M. et al., "The Impact of the Three Gorges Dam on the Water Quality of the Yangtze River," Water Policy 18: 1-17 (2016). Chongqing alone (population: 32 million) discharges over 1 million tons of nitrogen and 150,000 tons of phosphorus annually into the upper reservoir.
- Cyanotoxin health impacts: WHO, Cyanobacterial Toxins: Microcystin-LR in Drinking-Water (2003). Common symptoms: gastroenteritis, dermatitis (skin rashes), hepatotoxicity (liver damage with chronic exposure). Children and immunocompromised individuals at highest risk.
- Agricultural and municipal water supply disruptions documented in: Stone, R., "Three Gorges Dam: Into the Unknown," Science 321(5889): 628–632 (2008); and China Daily, "Algae Blooms Force Water Supply Suspension in Hubei," various dates (2007, 2010, 2013, 2017).
- Submerged industrial contamination: Tullos, D., "Assessing the Influence of Environmental Impact Assessments on Science and Policy," Environmental Management 45: 1−11 (2009). Pre-impoundment environmental assessments identified 1,599 factories requiring hazardous material removal; 657 were classified as "high pollution risk." Post-impoundment surveys suggest removal was incomplete.
- Heavy metal accumulation in reservoir sediments: Bai, J. et al., "Assessment of Heavy Metal Pollution in the Three Gorges Reservoir," Environmental Earth Sciences 66: 157–165 (2012). Sediment core samples show elevated concentrations of mercury (2.5x background), cadmium (3.1x), and lead (1.8x).
- Summary compiled from notes 15–20.
- Fish population collapse: Ministry of Agriculture, Yangtze River Fisheries Survey Report (annual, 2000–2015). Data compiled by Xie, P., "The Yangtze River Ecosystem: Past, Present, and Future," in Dudgeon, D., ed., Tropical Stream Ecology (2008), pp. 303–327. The 2002 figure of 1.9 billion eggs/larvae represents the "four major Chinese carps" (grass carp, silver carp, bighead carp, black carp) plus other commercially important species.
- Fishing community displacement and livelihood loss: Zhao, Q. et al., "Social and Economic Impacts of Fishing Ban in the Yangtze River Basin," Marine Policy 123: 104309 (2021). In 2020, the Chinese government imposed a 10-year fishing moratorium on the Yangtze to attempt ecosystem recovery—effectively ending a fishing tradition that sustained communities for over 4,000 years.
- Baiji extinction: Turvey, S.T. et al., "First Human-Caused Extinction of a Cetacean Species?" Biology Letters 3(5): 537–540 (2007). The 2006 survey covered 3,400 km of the Yangtze using visual observation and acoustic monitoring. No Baiji were detected. Last confirmed sighting: 2002. IUCN Red List status: Critically Endangered (Possibly Extinct).
- Schistosomiasis reduction: Zhu, H.M. et al., "Effects of the Three Gorges Dam on the Transmission of Schistosomiasis Japonica," PLOS Neglected Tropical Diseases 2(5): e295 (2008); and Zhou, X.N. et al., "The Public Health Significance and Control of Schistosomiasis in China," Acta Tropica 96(2-3): 97–105 (2005). Prevalence in Dongting Lake area dropped from 4.2% (2003) to 1.1% (2010).
- Lake hydrology disruption: Guo, H. et al., "Assessing the Impacts of the Three Gorges Dam on the Hydrology of the Dongting and Poyang Lakes," Hydrological Processes 26(26): 3942–3955 (2012).
- Dry-year vs. wet-year effects: Zhang, Q. et al., "Has the Three Gorges Dam Made the Poyang Lake Wetlands Wetter and Drier?" Geophysical Research Letters 39(20): L20402 (2012). The study found that in extreme drought years (e.g., 2006, 2011), dam water retention exacerbated lake drying; in extreme flood years (e.g., 2010, 2020), dam flood control reduced lake inundation extent.
