The 3.9-foot long, 18.7-pound amphibian was trapped among some twigs at one end of the ditch, which drains water from the Yellow River, China's second largest river, for irrigation.
Kong Haiyu, a farmer in Zhongzhuang Village, Yongjing County, Gansu Province, noticed the twisting amphibian last Tuesday.
Yongjing is located at the upper reaches of the Yellow River.
"There was no way out for it," recalled Kong, "It was too long, and there was not much water there."
Kong mistook it for a catfish and brought it home in a bag.
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"It looks more like a giant salamander," said Yang, "(It) should be a nationally protected animal."
"This is the first time we have ever heard of and found a giant salamander in this area," said Dang Yizhi, an officer from the forest police bureau of Yongjing County.
Dang estimated the giant salamander could be as old as 80 years, according to its length and weight, state-run Xinhua news agency reported .
"The life span of a giant salamander can be 130 years," he said.
"It eats as much as two jin (2.2 pounds) of food in one meal and can survive after two or three years of fasting."
Today the species mainly lives in the mountain creeks of the middle and upper reaches of the Yangtze River, Yellow River and Zhujiang River in China.
It is still unknown how this giant salamander made its way to the trunk stream.
Locals surmised that recent rainfall in the area might have brought the creature away from its usual habitat.
Dang said the appearance of the giant salamander showed the water in the area is very clean, as the species is vulnerable to pollution.