A New Mexico dam break that loosed thousands of tons of radioactive water and residue onto Navajo Indian lands last July could easily have been prevented, a House sub-committee was told yesterday.

In addition, Rep. Morris K. 1dall (D-Ari) said that at least three federal and state regulatory agencies had "ample opportunity" to conclude that such an accident would happen.

A question the Interior oversight hearing left unanswered was why, in light of various preditions several years ago that the dam eventually would break, regulators allowed it to continue in operation.

The dam break released an estimated 93 million gallons of liquid contaminated with low-level radiation and some 1,100 tons of similarly polluted solid waste into the stream. Contamination spread almost 100 miles downstream, in Arizona. The point for Udall was how poorly the various responsible parties performed in failing to prevent the dam break, which happened at Church Rock, N.M. But the point for many of yesterday's witnesses was how well they responded after the break occurred.

United Nuclear Corp., owner of the holding pond for the waste left after uranium is milled, downplayed the health hazards from the spill, insisted all possible precautions had been taken and siad nearby residents were given quick assistance.

Officials of the Nuclear Regulatory Commission, the Environmental Protection Agency and the New Mexico Environmental Improvement Division talked about their own quick responses to the dam break.

One subcommittee member, Rep. Manuel Lujan JR. (R-N.M.), praised state officials and urged that the basic regulations of uranium tailings be left with the states.

But spokesmen for the Navajos, whose language has no word for "radioactivity," expressed skepticism about health assurances they have received and called for stringent new controls on the uranium mining industry.

Tribal vice chairman Frank E. Paul said the Navajos are confused and bitter about seemingly cavalier government and industry policies that permit their land to be polluted and their lives threatened.

Another witness, William P. Robinson of the Southwest Research and Information Center at Albuquerque, said that total decontamination of the spill areas -- assurances notwithstanding -- is not likely to be achieved.

"There are more United Nuclear employes in this room today than there were on the cleanup during the first month at Church Rock," he said, alluding to the firm's dispatch of half a dozen workers with shovels and barrels to scoop up spilled waste.