A new, huge surge of mountain water bore down on besieged Hoover Dam today, exposing again the federal government's inability to predict and react quickly to record snow runoffs and forcing plans for more intentional flooding of two states and Mexico.

Like an inland tidal wave lumbering downhill at about 14 mph, the water glut--dumped into the Colorado River by heavy weekend storms in the Rocky Mountains--today forced officials to increase the water release at Glen Canyon Dam, which is more than 300 miles upriver from Hoover Dam, by 31 percent, to 92,000 cubic feet per second.

At Parker Dam, about 160 miles south of Las Vegas, the flow was increased 15 percent, to 38,000 cubic feet per second, to lower the level of Lake Havasu in Arizona.

The surge down the Colorado has inundated parts of Grand Junction, Colo., where 1,000 evacuees today were told they could return home.

Increased releases from Glen Canyon Dam are likely to worsen damage in the Grand Canyon, a summertime vactioners' mecca where one person has already drowned in a flood-related accident.

For the first time in four decades, water is expected to begin cascading over the top spillways of the 726-foot-high Hoover Dam on Saturday.

The U.S. Bureau of Reclamation announced today that the dam's current outflow of 40,000 cubic feet per second may have to be increased by 12 to 25 percent next week. Increases also were planned downstream at the Parker and Davis dams, threatening further serious flood damage on the California-Arizona border and in Mexico, where five persons have died from Colorado River flooding.

"There is just no way we can catch this stuff," said Gerald Williams, chief National Weather Service hydrologist at Salt Lake City, referring to the sudden rapid snow melt in early June that caught federal flood control officials unaware.

Williams said the government does not have enough snow and runoff gauges in the Rocky Mountains--and not enough experience with them in unusual weather conditions--to quickly predict a sudden and serious snow melt.

Further complicating the problem, Bureau of Reclamation officials said, was the need to keep enough water in the reservoirs for normal use in five western states legally entitled to tap the upper Colorado River. Even if some forecasters had suspected a damaging runoff, federal officials could not have emptied the reservoirs to prepare for it simply as a precaution.

"We would have been in severe straits if we had drawn down Lake Powell above Glen Canyon Dam and it had turned out to be a normal year," said Kathy Loveless, speaking for the Bureau of Reclamation's upper Colorado regional headquarters.

Confusion and controversy continue to grow along the swollen river and in Washington over what federal officials knew and when they knew it.

In some western mountains, such as the Sierra Nevada range in California and the Wasatch range above Salt Lake City, heavy snow fell early enough in the season for local officials to empty reservoirs and make other preparations for a big runoff.

Rep. Jerry Lewis (R-Calif.), who represents Needles and other riverside communities damaged by the recent floods, said today he would ask Interior Secretary James G. Watt to make a full-scale investigation of the federal failure to anticipate the runoff. He said he would accompany Watt on a tour of flood-ravaged areas Friday and said there could also be congressional investigations.

Lewis said federal officials did not consult soon enough with leaders of downstream communities affected by the increased water releases from federal dams to relieve pressure on the reservoirs and forestall even more serious flooding later in the summer. Before the crisis began, he said, "any fool looking at that snow pack would have been able to see this might require some emergency measures."

But Williams said winter snows in the Rocky Mountains feeding the Colorado River were not as great as in other parts of the West.

On May 1, the bureau predicted only 17 percent more runoff than average, but late spring snows increased that prediction to 30 percent by June 1. Loveless said that could have been accommodated by Lake Powell and the Glen Canyon Dam until more rain and a heat wave in early June triggered a massive runoff.

Last week, the bureau predicted a runoff 90 percent above average; this week, the figure jumped to 112 percent.

The four major downstream river dams--Glen Canyon, Hoover, Davis and Parker--began the rapid water releases that officials along the California-Nevada border estimate have caused at least $20 million in damage and forced Mexican officials to warn 25,000 people in two communities near Mexicali to prepare for evacuation.

The water levels in Lake Powell behind Glen Canyon Dam are now 6 feet above the usual limit.