The projects are part of a frenetic and much-criticized rush into hydroelectric power by the Chinese government, which, with 26,000 such dams, already has more than any nation in the world. At 1,760 megawatts, the Xiaonanhai project is comparatively small by Yangtze standards, but still three-quarters the size of the Hoover Dam, Scientific American reported in 2009.

Critics say the project makes little economic sense except as a temporary job creator. The reservoir will flood 18 square miles of prime farmland and displace 400,000 people, driving the cost of every kilowatt of generating capacity to $2,144 — triple that of the Three Gorges dam, according to Fan Xiao, a geologist who has fought the project for years.

The national reserve that critics say will be destroyed by the dam was, in fact, established to address concerns that the Three Gorges dam would endanger the fish population. Of the Yangtze’s 338 freshwater fish species, 189 live in the reserve — and many of those are found in no other river basin in China.

Opponents had staved off the project in past years by bombarding public officials with letters and reports documenting what they saw as the dam’s environmental and economic flaws. Chongqing’s response was to address the major concern — the destruction of the rare-fish reserve — by moving the reserve farther from the dam site.

When first established in the 1990s, the reserve covered about 500 fast-flowing miles of the Yangtze. Officials sliced about 95 miles away in 2005 to support construction of another dam. The latest change cuts an additional 62 miles.

“The conservation zone is the last stretch of free-flowing water body on the Yangtze that is absolutely essential for the reproduction of many rare fishes,” Li Bo, the head of the group Friends of Nature, said in an interview. “Once the border of the conservation zone is moved, those fish would not have enough space to reproduce.”

Mr. Li has played a main role in pushing the project. The South China Morning Post reported this week that the environmental and agriculture ministries, which have authority over the reserve, had refused to release important documents about the reduction of the reserve.