February 7, 2003
A huge dam near the Grand Canyon in the United States, which has killed off half the native fish species that once thrived downstream, holds lessons for the Three Gorges project.
As the environmental impacts of the Three Gorges dam cause increasing concern in China, one of the issues that has emerged as particularly worrying is sedimentation and its role in devastating fisheries below a big dam.
At least 60 per cent of the silt carried by the turbid Yangtze River will be trapped behind the Three Gorges dam once it begins holding back water in June, according to optimistic official estimates. Yet it is precisely this nutrient-rich sediment, flushing downriver and spreading over a floodplain, that forms the sandbars and transports the organic matter that fish need to breed and feed. Ultimately, without this sediment, the entire aquatic food chain is disrupted.
The world’s biggest dam, being built on the Yangtze River, will change other conditions downstream to which local species have adapted, such as water levels, temperature and velocity, as well as physically blocking fish migration routes.
“The Three Gorges project will change the flow of water on the river, with floods controlled according to human will,” journalist Tang Jianguang wrote recently in News Weekly (Xinwen zhoukan), a magazine published by China News Service (Zhongguo xinwen she). “But it is these same floods that provide a paradise for fish, which spawn in the floods – the bigger the better.”
Mr. Tang wrote that the floods that are regarded as disasters for humans are vital for the Yangtze fishery, and that Chinese scientists are struggling to resolve these conflicting interests.
“One suggestion has been to create artificial floods in the middle section of the Yangtze, to promote fish production by regulating the water volume in summer and recreating the conditions they need,” he wrote. “But this proposal raises concerns about whether such floods would threaten the safety of the dam and the effectiveness of flood-control efforts aimed at protecting humans.”
Whether simulated floods can be effective even for the intended purpose of moving sediment, and recreating hospitable conditions for fish in a dammed river, has been called into question by similar experiments elsewhere.
One such case is the Glen Canyon dam in southern Utah, built 40 years ago on the Colorado River upstream of a U.S. national icon, the Grand Canyon. A popular Chinese publication recently ran a feature article on the controversial American dam, focusing on its role in the extinction of half of the native fish species that once flourished downstream, and on various unsuccessful attempts since 1996 to restore the fishery.
Though the Dec. 19 article in the Guangzhou-based South Weekend (Nanfang zhoumo) newspaper did not mention by name the dam currently being built in China’s own scenic grand canyons, readers were clearly meant to draw the parallels between the Glen Canyon and Three Gorges projects.
Virtually all of the sediment that flows into the Glen Canyon reservoir (known as Lake Powell reservoir) is trapped behind the huge 216-metre-high (710-foot) dam. As a result, the sandbars and sandy banks where fish spawned and grew downstream have disappeared or been severely eroded. (With a reduced silt load, a river below a dam flows more quickly and has a greatly increased “scouring” or erosive capacity.)
Four local species – the bonytail, roundtail cub, Colorado pikeminnow and razorback sucker – have become extinct. The population of a fifth species, the humpback chub, a fish found nowhere else, is shrinking rapidly, down in just two decades from 8,000 to 1,000 adult fish.
Owen Lammers, executive director of Living Rivers – a group based in Moab, Utah, that is campaigning to have the Glen Canyon dam removed – says river otters, muskrats and every native insect species that once formed the base of the local food chain have also disappeared.
|The Colorado River in the Grand Canyon|
Lake Powell reservoir and the Colorado River have over the years been stocked with trout and other game fish for sport fishers, but this has helped push out local varieties that were once freely available to native communities and others along the river. Trout are “sight-feeders” that thrive in the clearer waters below the dam, and prey on the native species, which need turbid water.
The Flagstaff, Ariz.-based Glen Canyon Institute, which also wants to see the restoration of a free-flowing Colorado, says the river downstream of the dam “is practically devoid of nutrients, because most of these are bound to sediments, and therefore they never leave the reservoir.”
Meanwhile, in the 300-kilometre-long (186-mile) reservoir behind the dam, “harmless trace metals and salts, such as selenium, arsenic and mercury, when trapped in stationary sediments, can be converted into detrimental forms. These then become incorporated into plankton and zooplankton, and are distributed throughout the food web.”
Mr. Lammers says that while the demise of Glen Canyon was obvious and dramatic, the loss of the Grand Canyon ecosystem downstream has been more subtle and less noticeable. “Many river rafters are unaware they’re floating on a sterile, regulated canal – on a river in crisis,” he says.
In 1992, Congress passed the Grand Canyon Protection Act, which gave the secretary of the interior the power to protect the national landmark. That authority was used in 1996, when the floodgates were opened in an effort to flush sediment downstream and return the river to its natural state, temporarily, for the sake of the fish.
The attempt to replicate a seasonal flood produced only minor, short-term results. Some sandbars and beaches returned, but did not last. Canyon experts agree that all the churning water did was move sand from one beach to another.
“These experiments just push around a few grains of sand,” Mr. Lammers says. “They do nothing to bring the much-needed sediment from Lake Powell reservoir into the Grand Canyon. For that, a tractor-trailer load of sediment needs to go around the dam every minute of every day – 100 million tons a year.”
Last month, the U.S. Interior Department again began releasing water from the dam, this time in a three-month program of fluctuating flows to try and restore fish-friendly conditions in the river. To comply with the 1973 Endangered Species Act, which requires all federal agencies to use their authority to help preserve listed species, other plans call for stocking the Grand Canyon with the fingerlings of indigenous fish, including the vanishing humpback chub.
This is not a restored fishery, Mr. Lammers insists. “Recovery implies self-reproducing native populations, but what they’re doing is placing fish from a hatchery into the Grand Canyon, fish that cannot reproduce because their habitat has been so degraded. In particular, the water is too cold for them to spawn. So the fish are put in there, and they die. And when the numbers go down, they just add more. It’s a horrible arrangement.”
Living Rivers believes the short-lived attempts to mimic the river’s natural flows are futile, and that only dismantling the dam and draining its reservoir can save the Colorado.
“If the Grand Canyon ecosystem is to be saved, and the Endangered Species Act adhered to, natural flows must be mimicked year-round, sediment must be moved from around the dam, and the water temperature must be increased dramatically,” Mr. Lammers says. “The cost of achieving this far outweighs the benefits of maintaining the dam, which is a key reason that Living Rivers and a network of 150 advocacy groups across the United States are calling for the dam to be decommissioned.”
|Living Rivers’ Sedimental Journey protest, September 2002|
Regardless of the mounting crisis in the Grand Canyon, he says, “it is not a question of if, but when, Lake Powell reservoir will be drained. Sediment is filling it in, as will also happen with Three Gorges, and the Glen Canyon dam will have to be decommissioned in 50 years – if nature doesn’t do it first through drought or dam failure. But we’re making major headway on the political front to make sure the dam is dismantled as soon as possible.”
Grassroots campaigns to remove other dams and repair damaged river ecosystems have been successful elsewhere in the United States. About 300 of the country’s 74,000 dams have been decommissioned in recent years, and more are slated for demolition. For instance, the U.S. Interior Department has agreed to tear down two small dams on the Elwha River in Washington state, which have virtually wiped out the steelhead trout and salmon fisheries that a local native group had been granted “in perpetuity” by a 19th-century treaty.
Meanwhile, back on the Yangtze, the important fishery below the Three Gorges dam is already in decline, a fact underscored by a fishing ban that has just been imposed on China’s longest river and its main tributaries. The Ministry of Agriculture has ordered all fishing boats out of the water in the upper reaches (from Deqin county in Yunnan province downriver to the Gezhouba dam in Hubei) from Feb. 1 to April 30, while in the lower reaches (from Gezhouba to the river’s estuary in Shanghai) the ban will extend from April 1 to June 30.
The Yangtze produces 60 per cent of China’s annual freshwater fish supply, China Daily reported on Jan. 11, citing Ministry of Agriculture statistics that probably refer only to wild-caught fish and exclude aquaculture output. “Because of overfishing and pollution, the annual catch has dropped to 100,000 tons in recent years, less than one-fourth of what Yangtze fishermen routinely caught in 1954, when the haul reached 427,000 tons,” said Guo Wei, an official with the ministry’s fishery bureau.
In addition to the Yangtze’s main channel, the fishing ban will cover major tributaries such as the Min, Jiang, Wu and Han, and large lakes in the river valley, including Dongting and Poyang. The Chongqing Morning Post (Chongqing chenbao) reported that if the moratorium goes smoothly, it could be imposed every year.
The ban will throw an estimated 50,000 fishermen out of work, the Xinhua news agency reported Feb. 1. In addition, the local buying and selling of fish will be prohibited, and fish must be taken off restaurant menus during the ban.
The prohibition will be in force during the breeding season of key species in the Yangtze, such as herring and chub. Millions of fish fry will also be released into the river during this period to help boost dwindling stocks, China Daily said.
Even before the Three Gorges dam begins disrupting their downstream habitats, many of the Yangtze’s estimated 180 fish species are becoming hard to find. “Catfish and soft-shelled turtles are barely seen any more, while white sturgeon and dashi sturgeon have not been caught for a number of years. Experts believe these species are headed for extinction,” the Chongqing Morning Post said in its Jan. 18 report.
“Since the 1980s, too many water conservancy projects have been built on the Yangtze, while pollution became severe,” Xinhua explained.