ABOARD "EAST IS RED" RIVERBOAT NUMBER 42 - Dynamite charges boom and ugly dirt barriers jut into the wide serene Yangtze as China for the first time bottles up the world's third-longest river into a potential source of power.

Inspired by an old poem of Mao Tse-tung, this 1.6-mile-wide dam being built near Yichang is expected to flood the ancient and treacherous Yangtze River gorges and turn generators adding 6 to 9 percent to China's total power output. That is three times the total electrical output of China in 1949.

It is a colossal project and something of a risk. Efforts to dam China's Yellow River, and this project itself, have been crippled in the past by an old enemy - massive amounts of silt churning in these deep-brown waters.

The 3,914-mile river carries 300 million tons of silt a year, enough to push its delta 1,000 miles downstream, another 82 feet a year toward Japan and enough to clog ship channels and foul hydroelectric plants if not discharged.

The Gezhouba project,a s the Chinese call it, is designed to add 13.8 billion kilowatts of power a year to the industrial boom Peking has planned for the central China by the projected completion date, 1985. The dam will create an artificial lake extending more than 60 miles upstream. It is to include shipping locks easing navigation of the beautiful, narrow upstream gorges, a key hazard for a river that carries 80 percent of China's inland water freight.

"I don't know how much higher the water will be," said river boat captain Chew Yungtong, "but it will make the rapids easier and we'll have boating locks to take the ships through the dam."

Here, just above the river town of Yichang, two islets divide the river into three channels, known as the Great Stream, Second Stream and Third Stream. Thousands of workers dynamiting tons of rock off the banks upstream and floating them down in low barges have already dammed the Second and Third Streams, closing off the northern side of the river course.

On top of this huge brown hump jutting into the river, a forest of cranes has sprouted to carry on the construction of an initial 965,000 kilowatt-capacity hydroelectric station, two shipping locks, a silt discharge gate and a 27-arch flood discharge gate.

In the next stage of construction, the rock barges will dump a coffer dam across the southern stream, leaving the river to roar down the flood discharge gate in the northern side while the rest of the construction is completed.

After marvelling for a full day at the steep-walled beauties and rushing waters of the gorges, a boat traveler finds the dam construction site a squalid mess of dirt, trucks, low banks and masses of workers in blue padded clothing.

Dynamite blasts echo regularly off the dwingling gorge walls. Dozens of dump trucks speed back and forth along a road on the northern edge of the river to bring more fill. Big blue dredgers, to deepen the channels and help build the locks, are tied up along the northern shore.

The Chinese call the Yangtze the Changjiang, or "Long River." Only the Nile and Amazon are longer. For centuries it has provided irrigation and cheap, long-range transport, becoming the backbone of the Chinese empire. Yet, it has never been successfully dammed on its main course.

Mao described this old stream in a 1956 poem, Composed after swimming the Yangtze at Wuhan, downstream from here.

Walls of stone will stand upstream to the west

To hold back Mt. Wu's clouds and rain

Till a smooth lake rises in the narrow gorges.

The mountain goddess, if still alive

Will marvel at a changed world.

The man responsible for putting Mao's musing into action, the late premier Chou En-lai, came here in a riverboat in 1958 with China's current fourth-ranking leader, Li Xiannian. Ignoring a snowstorm, they climbed up riverside peaks and checked out the area, then returned to the boat for long arguments over how to proceed. The dam is only the beginning of a number of ambigitous projects that would not only dam the river but send tons of river water north to irrigate arid northern Chinese plains, which the feeble Yellow River cannot satisfy.

Once completed, the dam and its electrical plants would provide power to four provinces and greatly accelerate river traffic. It is the largest hydroelectric project in China, and although only one of 120 major programs in China's 10-year plan, still one of the few where substantial progress can be seen.

At night, the top of the half-completed dam is ablaze with lights as concrete is mixed and moved to the site. "Tens of thousands of builders are working on the project round the clock," said China's news agency. The riverboat does not stop, its Western passengers seeing no scenic virtue in the huge brown clump slowly pinching the river shut.