The Santa Cruz River isn't a river anymore. Its empty bed twists like a parched tongue through the desert. Along its banks, dead mesquite trees cast shadows on barren fields.

It wasn't always so.

"Originally, this was lush land," says Cecil Williams, the burly tribal chief who governs 14,000 Papago Indians on reservations south of Tucson. "The river ran most of the year. For as long as anyone can remember, the Papagos have been farming. We grew corn, watermelon, beans. We raised cattle."

But whites have taken the Indians' water. Farmers, large copper mines and the booming city of Tucson drilled wells around the reservation, sinking the water table hundreds of feet. The river ran dry. The Papagos are still farming several hundred acres, but soon. Williams says, "We won't be able to farm anymore."

So the Papagos, joined by the federal government, have filed suit. They are among more than 50 tribes around the nation now asserting ancient water rights. In the West, where increasingly scarce water is the basis of all economic activity, the suits are threatening the agriculture, industry and urban growth of vast regions.

In the Missouri Basin, Indians are claiming huge amounts of water which energy companies had purchased for future projects. In California, Nevada and Washington, they are suing farmers over irrigation rights. In the Southwest, they are demanding water needed by cities such as Phoenix, Tucson and Alburquerque.

The suits have inflamed the politically sensitive issues of private property and states' rights, drawing Congress and the Carter administration into the fray. As the Indians' trustee, the federal government is bringing the suits on their behalf. States are fighting back, suing to retain their traditional control over water allocation.

Indians blame the federal government for allowing white settlers to "steal" their water through groundwater pumping and upstream diversions. In many cases, the federal Reclamation Bureau built dams and sold the water to whites. Now the tribes are demanding millions of dollars in damages.

"In the past, Indian water rights have been largely ignored from the standpoint of water resources development," says Interior Secretary Cecil D. Andrus. "Today the legal entitlement of Indian tribes can no longer be overlooked. The nation faces a conflict between the legitimate right of Indians to develop their resources and the impairment of enormous capital investments already made by non-Indians in the same water supply."

Unless a solution is found, federal officers predict an avalanche of lengthy, acrimonious lawsuits which the government has insufficient manpower and funds to prosecute. Already Congress is considering several bills ranging from dismissal of Indian rights to condemnation of white farmers' water rights.

The Indian issue is a key part of President Carter's new water policy, to be announced late this month. One option under consideration is to provide Indians with federally built irrigation projects in exchange for limiting their water claims.

The Indians are willing to bargain. For example, the Ak-Chin, a group of 350 Indian farmers south of Phoenix, saw their water table drop from 50 to 400 feet because of pumping by wealthy farmers around the reservation. To head off a suit, Congress is about to give the Ak-Chin a $43 million system delivering water from nearby federal lands. In exchange, the tribe will waive damages and future claims.

In another settlement, the Kennecott Copper Corp. is paying Arizona's Gila River Indians $1.5 million for past use of Gila River and $50 per acre-foot for future water (an acre-foot is an acre of land covered by one foot of water). In exchange, the Indians agreed not to sue Kennecott, but are still pressing claims against farmers, other companies and possibly the City of Phoenix.

The Papages, too, are willing to negotiate. They claim rights to Santa Cruz basin's entire replenishable supply, and are asking the court to stop Tucson, four mining companies, a large pecan farm and 1,500 area landowners from pumping. Since January, attorneys for all sides have been bargaining intensively.

"We have been pretty bitter," Williams said. "The white man takes your water and anything else that comes along. In spite of that, the Papagos have always been peaceful - not like the Apaches. If we win the suit, it's going to hurt everyone. So we're sitting down to see what we can do instead of fighting each other."

The latest idea is for the federal government to clean up Tucson's sewage effluent and pump it to the reservation. But the $100 million plan could run into trouble from Pima County, which claims to own the effluent.

And, as in most Indian-white relations, centuries of mistrust play a role. Farming is "a pretense" for the Indians, said Keith Walden, manager of the 5,000-acre pecan farm. "They just want us to pay them a handsome sum for their water rights."

Walden, who pumps roughly a third of Santa Cruz Basin water, said, "The Indians are no more entitled to that water than they are to Manhattan Island. They once owned it, but they didn't develop it. The white men did."

The miners and city officials - who were fighting Walden for the water long before the Papagos claimed it - are more tolerant. "The Indians have rights," said Charles Stott, who runs Twin Buttes Mine one of the nation's largest. "They want to develop that reservation so they can make a living."

The Papago suit, he said, is "a serious threat" to the billion-dollar investment of Anaconda Co., American Smelting and Refining Co. and other area miners who employ thousands. "If we want to continue to live, we've got to negotiate."

Tucson is one of the few cities in the nation to depend entirely on groundwater. Its supply has been dwindling at more than double the rate nature replaces it. The Indians say Tucsonians must conserve water more - a key element of Carter's voters recalled four city councilmen - water policy. But when the city tried to increase water rates last year, [WORD ILLEGIBLE] a vivid testimonial to the emotional politics of western water.

Meanwhile, the Papagos have claimed about a third of the city's water supply. They are on the verge of filing another suit in the Avra Valley, north of Tucson, where the city bought 12,000 acres of farmland to secure water rights. Another supply, east of Tucson, is threatened by Gila River Indians.

"It looks like we're surrounded," said Hugh Holub, a city attorney. "We're trying not to have another Indian war, so we're using lawyers instead of horses and soldiers."