The Platte is a homely river. A mile wide and an inch deep - so the pioneers described it. From the Colorado Rockies, it winds through the endless flat fields by Wyoming and Nebraska with little scenic relief.

But each year when the snow melts from the fields, hundreds of thousands of migrating birds swoop down on the Platte's barren sandbars: ducks, whitefronted geese, songbirds, bald eagles, sandhill cranes and, among the babbling army, 75 members of North America's most celebrated endangered species - the whitefeathered whooping cranes. Therein lies a story. For the homely Platte is not only a popular waterfowl resort, but also an economic lifeline of the Great Plains. Its water irrigates farms, electrifies cities, and powers are draglines of giant western coal companies.

On a tributary of the Platte, near Wheatland, Wyo., a group of utilities is building a massive $1.6 billion coalfired power plant to serve eight states. The plant, which includes the large Grayrocks dam and reservoir, would reduce the river's flow and according to environmentalists, possibly wipe out the whoopers downstream.

The situation has aroused national controversy, not only because of whoopers' fame and beauty and the magnitude of the economic interests at stake, but also becuase it has become a test of the new endangered species act.

The National Wildlife Federation and the state of Nebraska filed suit and succeeded in temporarily stopping the project last month when, on the last frenzied day of Congress, retiring Rep. Teno Roncalio (D-Wyo.) got an amendment adopted that exempted Grayrocks from the act.

A conference with the Senate lifted the exemption, but in final form it required the situation to be resolved within 90 days by a Cabinet-level committee. The committee was set up by the new act to choose, in the event of irresolvable conflicts, between development and an endangered species.

The situation is also entangled in the volatile politics of western water. The Wyoming plant would use water that heretofore has flowed into Nebraska. Nebraskans want that water for economic development but are pragmatically teaming up with environmentalists who want it for wildlife.

Meanwhile, President Carter is trying to revise federal water policy by slowing down the dam building and fierce competition for western water. He wants to conserve water which would mean more for wildlife and whooping cranes, a constitutency with limited popularity in the development-minded western establishment.

As Patrick Parenteau, a National Wildlife Federation attorney, sees it, the cranes have been going to the Platte River for 10 million years to perform their graceful mating dances before the long flight to Arctic nesting grounds. What right has man to interfere?

"In the last half-century or so, the Platte's stream flow has been reduced by 70 percent because of 43 dams and innumerable irrigation ditches," Parenteau said. "We don't know how much further we can reduce it before the river can no longer scour itself."

Absent the scouring of spring floods, vegetation will grow on the sandbars and the cranes won't stop there for fear of predators. This has already occurred. Parenteau said in an area around Brady. Neb. Once heavily used by cranes it now is not used at all.

Today a 60-mile stretch of river between Lexington and Denham, Neb., has been designated "critical habitat" for whooping cranes by the U.S. Fish and Wildlife service. It is also a crucial staging area for 90 percent of the world's sandhill cranes, a smaller, gray cousin of the whoopers, and 90 percent of North of America's white-frented geese, Parenteau said.

For now, he acknowledges, the area has a "pretty good" waterflow of 820,000 acre feet a year. But the project, and an associated irrigation program, could reduce it by 60,000 acre feet a year. The cranes left Brady when the flow went down to 420,000 acre feet.

"Somewhere, you reach the point of no return," Parenteau said. "There are a lot of proposed developments for the Platte. Where do you draw the line?"

As Edward Weinberg, attorney for the Missouri Basin Power Project, sees it, you won't draw the line at a project that is 50 percent complete and will provide electricity to two million consumers.

"It's highly unfair," he said. The federal Rural Electrification Administration, which is guaranteeing loans for the project, published an environmental impact statement "But in 12 single-spaced pages of comments, Interior didn't say tiddleyboo about the whooping crane," Weinberg said.

"Planning for this project started in 1971, and it wasn't until after construction began that the Fish and Wildlife Service weighed in with the whooper," he added.

Grayrocks is 275 miles north of the cranes' habitat, and, Weinberg contended, other dams and diversions, including Nebraska's immense Kingsley dam, can be operated to compensate for the water loss.

While Weinberg uses the measured arguments of an attorney, the debate on the House floor reflected the high emotions of the issue.

Roncalio pleaded, "Do you want to send me back to Wyoming, after 10 years as your friend and colleague, to face 2,000 unemployed people in Wheatland on account of a totally unjustified thing like this, the Endangered Species Act?"

Referring to the arduous, 40-year-effort to rebuild the species, Roncalio said, "The whooping crane is now up to 75 in population. There were only 20 when I was a kid. They are not being endangered or diminished if they have gone from 20 to 75. I have seen the whooping cranes in Yellowstone Park, and I love them."

The Grayrocks case, he contended, "has much more to do with an old water fight between Nebraska and Wyoming, and environmental law is being used as a ruse."

On other side, Rep. Gerry Studds (D-Mass) attacked the Roncalio amendment as "a repeal of the heart and soul of this statute. Either we intend to continue with some respect for other forms of life and to continue our determination that we will not take the ultimate arrogant step of deliberately exterminating other forms of life, or we do not."

But he added, "I have a feeling that I know the result of this vote, partially I think, because there are 70 cranes left and one one Teno Roncalio. The affections of the House are quite clear."

However, the situation may not require an all-or-nothing choice between the whoopers and the power plant. Parenteau and Weinberg are working on a settlement that could involve the purchase of replacement water and the monitoring of vegetation on the sandbars. A team of biologists under auspices of the Fish and Wildlife Service is preparing a biological opinion, due within two weeks, on how the cranes might be affected by the plant. Also, the Cabinet-level committee, to D. Andrus, is expected to examine all be chaired by Interior Secretary Cecil ternatives.