Large pools of glowing lava were seen for the first time on the floor of the volcano at Mount St. Helens yesterday, raising the risks of mudslides and floods as the hot material pours out from the earth's crust in the weeks ahead.

"Aerial observers saw several bright red glows as large as 30 feet in diameter on the floor of the crater," Dr. Robert Tilling, chief of the Office of Geochemistry and Geophysics for the U.S. Geological Survey, said yesterday. "Based on what we know about lava coloring and temperature, I'd have to say that this rock is anywhere from 1,200 to 1,300 degrees Fahrenheit."

Chunks of rock as big as four feet in diameter were sighted shooting up from the brilliant red "lava dome" swelling inside the steaming volcanic crater.

Geologist Mary Hill of the U.S. Geological Survey said she saw gravel-sized rocks "and some really big ones -- maybe four feet in diameter" shooting up from the force of steam explosions during her afternoon flight over the mountain.

She said the big rocks fell back into the crater. Small fragments of rock rose higher and showered down outside the crater. Relatively small puffs of ash were caught by prevailing winds and blown to the west and south.

The first sign of lava pushing up through the floor of the volcano is viewed by geologists as a good and a bad sign. The sight of lava decreases the risk of huge ash eruptions like the one that took place May 18, but it increases the danger of mudflows, flooding and forest fires because of the heat involved.

"The molten material opens up conduits to the surface that weren't there before," Tilling said. "This releases some of the internal pressures that have been building up and were a main cause of those explosive ash eruptions the mountain had."

Tilling said there is no way to predict how long it will take the lava pooling up through cracks on the crater floor to flow out through vents on the flanks of the mountain. He said he expects the lava to fill the floor of the crater before it flows out through the sides of the mountain.

Mudslides and floods have already wreaked havoc on the flanks of the volcano. The south and north forks of the Toutle River have flooded their banks, and the Cowlitz River has flooded its banks upstream of the town of Castle Rock, Wash.

"If they have any heavy rainfalls anytime soon, the downstream side of that river could flood too," said Harry Barnes, chief of the surface water branch of the U.S. Geological Survey. "The potential for flooding is very great right now."

So much mud and ash from the volcano has fallen into the surrounding rivers and lakes that it has choked them, displacing huge volumes of water. The water level in Spirit Lake just north of the base of the volcano has risen as much as 200 feet, largely because of the debris that has slid into the lake.

The explosive ash eruptions on May 18 and May 25 completely melted the glaciers on the north slope of the mountain, releasing an estimated 46 billion gallons of water down the north flank and into Spirit Lake and the Toutle River, which feeds into the Cowlitz.

The Geological Survey said that sections of the Cowlitz River are now filled with as much as 15 feet of mud, which alone has raised the water level to flood crests and has reduced the water flow to a snail's pace in some spots.

"If the channel capacity is not restored," the survey said, "the most severe flood potential will probably occur during next winter's wet months."

If lava rushes out from the flanks of the volcano, it raises the risk of far more serious flooding. There is still deep snow on parts of the mountain, which lava would melt fast enough to trigger flash floods.

One of the greatest potential hazards involves Swift Reservoir at the foot of the south flank of the mountain.A mud flow triggered by an outpouring of lava could raise the water level in the reservoir faster than the water could be drained. Any breaching of the dam that blocks the reservoir could result in what the Geological Survey calls a "catastrophic flood."

Meanwhile, the Cowlitz County corroner's office said it would begin issuing "presumptive" death certificates for the 55 people missing since the volcano's major eruption May 18. The air search for the missing was called off Thursday after officials conceded that the chance of finding survivors was nil." The official death toll stands at 22.