As a precarious 20-story-high mass of volcanic ash and pumice dammed the rising waters of a lake near Mount St. Helens today, this logging town, 60 miles downriver, began bracing for possible floods or flowing mud.

"We just have a trailer sitting in the front yard, all hooked up," said a cheerful-sounding Dorothy Lewis, who lives with her husband in the low-lying area of South Kelso. The three Lewis children have homes in the hills around town, she said, and the minute the radio station or emergency siren warns of approaching floods, the Lewises will simply haul their trailer into one of their children's backyards and settle in for the duration.

Helicopter and airplane patrols kept a close watch n the swelling Spirit Lake, a once-lovely recreation area at the base of the forest blasted into bleached devastation by Mount St. Helens' huge eruption Sunday morning. A hot pyroclastic flow -- broken rock mixed with gases and ash -- boiled over the lip of the volcano Sunday morning, flowing down the north slope to the bank of the lake and forming an immense dam at the lake's natural outlet.

The lake is fed by streams, underground water and the melting snows from Mount St. Helens, and by this morning had risen 200 feet, scientist said. As the water begins to top the mass, said U.S. Geological Survey geologist Dwight Crandell, it may pour safely through the Toutle River valley -- or it may break through all at once, sending torrents of water or mud 40 miles down the valley toward Kelso and nearby Longview.

Mount St. Helens was relatively calm today, while its 3,000-mile-long ash cloud drifted east. As the cloud reached the Eastern Seaboard to night, it had dissipated and could only be seen as a light haze, according to United Press International.

Washington Gov. Dixie Lee Ray asked President Carter to declare her state a disaster area. Federal officials said earlier that they would expedite the process of granting federal aid.

The speed of a flood here, and its potential danger, would depend mostly on its contents, said Cowlitz County emergency services director Nolan Lewis. "Mud probably would not reach here," he said. "Water -- it depends on how much . . . If you get a wall of water, it could come down quite fast, possibly 30 miles an hour."

Since the towns that could be affected by a flood will apparently have at least two hours' warning, and dikes along the Cowlitz River are braced against the flooding that hits Kelso about every seven years, no evacuation plan has been put into effect, Lewis said. Schools remain closed, city employes in Kelso and Longview have been placed on standby alert, emergency vehicle sirens will sound if the floodwaters break through, and residents have been urged to tune in the Longview radio station for regular reports.

The confirmed death toll from Sunday's eruption raised to six -- one family of four identified as Ron and Barbara Siebold and two children, Michelle and Kevin Morris, all of Olympia, Wash., and a Hawthorne, Calif., couple identified as Fred and Margery Rollins. All six were found in their family cars. The cause of death is still undetermined.

Based on telephone calls from worried friends and relatives, officials today raised considerably the number of missing persons to around 100. But they stressed that many of the calls appeared to be from people worried that a vacationing relative or absent friend might have been in the area Sunday.

"I . . . don't think we'll have a death toll of that magnitude," said Cowlitz County sheriff Les Nelson.

But the terrain was difficult to explore, he added, because of mud and ash and intense heat in certain areas.

USGS geologist Dan Miller, who flew in a helicopter over the blast area this afternoon, said he saw footprints leading away from vehicles apparently abandoned in the ash. "It indicates that whoever made those tracks and those footprints did survive the explosion," he said. "Where they are now, I have no idea."

One of the searchers, Nelson said, was a local forester who once could have located a particular clump of trees with no problem. "That country just don't look like his backyard anymore," Nelson said.

As for more eruptions from the volcano, Crandell of the USGS said: "There is no indication that another eruption is imminent." He said he anticipated intermittent "spouting" for weeks.

The volcano's fine, abrasive ash, which was seven inches deep in some areas, disabled hundreds of automobiles.

In Ritzville, Wash., 2,000 motorists trapped in town by six inches of gritty ash revolted and told the sheriff they were leaving town despite his warnings.

Sheriff Ron Snowden sent the cars out one-by-one at 10-minute intervals so that dust from each preceding car would have time to settle. By mid-afternoon, 75 vehicles had driven into the dust but 12 had returned, saying road conditions were too bad to continue.

Officials estimated that 5,000 people were stranded statewide, many of them travelers stuck at closed airports or on roads blocked by drifting ash.

Idaho wheat commission director Dick Rush said northwest rivers were being silted by the ash and that channels would have to be dredged to free shipping lanes.

Washington state officials expressed concern that the ash would clog sewer and water lines, and warned home-owners that accumulated ash on flat roofs might collapse buildings.

At Yellowstone National Park, physical science coordinator Wayne Hamilton said acquatic life could benefit because the ash would add nutrients to water in the park's lakes.

Meanwhile, government geologists warned that a series of major eruptions could seriously threaten food production and surface water supplies in the United States and Canada. They warned that four more eruptions like those of Sunday and Monday could send damaging amounts of acidic sulfur ash and dust into the atmosphere.

The geologists told Agriculture Secretary Bob Bergland that a two-inch layer of volcanic dust now covers the fertile wheat fields of Washington, Montana, Idaho, Wyoming and part of North Dakota.

Four to five inches, they speculated, would destroy this year's wheat crop in those states and almost certainly inflict similar damage on the Canadian provinces of Manitoba and Saskatchewan. CAPTION: Picture 1, Eruption of Mount St. Helens toppled these thousands of fir trees on volcano's north face. They represent enough lumber for 200,000 single-family homes.; Copyright (c) 1980, San Francisco Examiner via United Press International; Maps 1 and 2, no caption, By Dave Cook -- The Washington Post; Picture 2, A Washington National Guard helicopter crewman checks a camper in which two bodies were found north of Spirit Lake; Copyright (c) San Jose Mercury