An obviously thorough and detailed study into the causes of the Jan. 1 collapse of Quail Creek Dike near St. George has proved to be satisfying for two reasons: (1) It showed the fault did not lie in the dike itself, and (2) there is no reason why a replacement dam cannot be built on the same site.

Beyond that, however, there is not much to cheer about. The failure of the dike was caused by significant seepage beneath the dike because the bedrock was badly fractured in places. That weakness apparently was not discovered by engineers who originally planned the dam.The panel of outside experts who examined the dike failure refrained from pointing fingers, but several of them clearly felt that not enough was done in checking out the bedrock at the start of construction.

This is not to say that engineers and designers were careless. The bedrock fractures were a vertical type that are hard to detect, even when some drilling is done. Yet vertical cracks were visible in the bedrock both upstream and downstream from the dam site.

The task force investigating the dike disaster said that "foundation exploration was not designed or complete enough to fully detect seepage problems . . ."

It seems that once they scraped away overburden and reached bedrock, engineers assumed that it was safe enough to build a dam on. Many times, when things go wrong, the problem can be traced back to some kind of assumption. All this, of course, is said with the benefit of hindsight - always unfailingly accurate.

In any case, the investigating panel's job was not to place blame but to pinpoint the cause of the Quail Creek disaster. Simply stated, the cause was seepage through the cracked bedrock that slowly ate away the earthen fill of the dike, eventually causing the collapse of a 300-foot section. The root cause of the collapse was not the dam structure itself.

If those bedrock cracks had been discovered as part of the site preparation, a concrete or gravel base could have been built as a base for the dike, acting as a seal or filter to prevent eroding of dike material.

With proper preparation, a replacement dam can be built on the same site, although it is certain to be more expensive. Merely repairing what is left of the dike is not an adequate answer. But the Quail Creek reservoir is badly needed in the St. George area and should be replaced.

What to do to make sure something like this doesn't happen again? One step has already been taken. State officials have adopted a policy of having independent experts review all future dam proposals.

That ought to go a long way toward making sure that "assumptions" don't slip through without being thoroughly examined.