Three years ago, Mount St. Helens blew her snow-capped top with the force of a hydrogen bomb, splintering giant forests, killing at least 59 persons, wreaking more than $1 billion in damage and capturing the imagination of a nation.

In the election year of 1980, the volcanic explosion also captivated politicians.

President Carter swept in by helicopter. Washington Gov. Dixy Lee Ray (D), pounded a table and demanded federal money. Sen. Warren G. Magnuson (D-Wash.), then chairman of the Senate Appropriations Committee, produced almost $1 billion in special disaster aid.

Now, with the mountain bubbling only occasionally and all three of those politicians removed by the Democratic natural disaster of 1980, the 45,000 people in the valleys below the mountain are living with a time bomb potentially more catastrophic than the volcano.

But, in an era of budget cuts and short memories, they cannot get anyone to listen.

"What no one seems to understand is that we still could lose everything," said Van A. Youngquist, a Republican county commissioner from Cowlitz County who made his fifth fund-seeking trip here last week. "No one in the Reagan administration seems to understand the magnitude of the problem."

To anyone living in the volcanic-gray valley west of Mount St. Helens, the problem is ominous. Above them looms a giant reservoir of water and mud precariously held back by an eroding wall of silt and debris dumped by St. Helens into the only outlet of once-pristine Spirit Lake.

The U.S. Geological Survey estimates that a break in the Spirit Lake volanic dam could send 300,000 acre-feet of water and 2.4 billion cubic yards of sediment into the valley.

The USGS says that mud flows, common after ancient Pacific Northwest volcanic eruptions, could inundate already battered hamlets such as Castle Rock and Lexington with 60 feet of mud. The Columbia River port towns of Longview and Kelso could be buried 30 to 40 feet deep.

"We would cease to exist," Youngquist said, noting that is just the beginning of the problem.

Such a catastrophic mud flow, he said, would probably cut off Columbia River shipping lanes, isolating the busy port at Portland, Ore., from the sea. Backed-up flood waters could also threaten the Trojan nuclear-power plant on the bank of the Columbia and cut off the north-south interstate highway connecting Seattle and Portland.

Youngquist's constituents, living beneath 21 new warning sirens strung along the Toutle River up toward Spirit Lake, are not simply frightened. They are angry about federal promises that seem to have disappeared into a maw of bureaucracy and politics.

Of the $951 million appropriated in 1980 for volcano relief, $435 million has been spent on volcano damage. The rest of the money is long gone--spent by federal agencies on other disasters after a House-Senate conference committee failed to include the words "Mount St. Helens" in the relief bill.

Of the largest portion of the funds--$430 million for low-interest Small Business Administration loans--about $70 million has found its way to volcano victims.

Property owners in the region have bought almost $900 million in federally backed flood insurance since the eruption. Erosion along riverbanks is still dragging homes into the water. And frustrated homeowners have found that erosion losses are not covered by the federal program.

In the first year after the eruption, the Army Corps of Engineers spent $215 million dredging mud-filled riverbeds and building levees to protect the valley against floods from Spirit Lake. But the corps, hampered by budget cuts, ran out of money in September, 1981.

Silt is again filling the rivers. In some places, river bottoms are almost as full--and as threatening--as on the day after the eruption.

"You can't imagine the emotional suffering the people have endured out there for the past three years," says William Paris, an aide to Gov. John D. Spellman (R-Wash.).

Three years ago, riverfront farmer Sam Hornstra watched the first mud flow race down the Toutle, with salmon and steelhead trout leaping out of the volcano-heated waters and landing on their backs, dead.

That flood left his 18-acre filbert orchard under 10 feet of mud. A year later, all but five of his 800 filbert trees were dead. Today, most of his farm is covered with levees excavated from the Toutle, and he has received no federal aid.

Meanwhile, Youngquist, Paris and other local officials are continuing hat-in-hand trips here, pleading with congressional committees and trying to get the White House to support a $48.1 million special appropriation to allow the Corps of Engineers to start dredging again so that, if the worst happens, riverbeds can carry most of the mud past threatened towns.

"I guess I'll never understand Washington," Youngquist said as he headed home last week. "We've got people scared to death. But the federal government also has $900 million in flood insurance at stake. If Spirit Lake goes, they'll have to pay off. Isn't $48 million a better investment?"

At Spirit Lake, high above Youngquist's county commission office, the average annual precipitation is 105 inches. The area around the lake is a moonscape, stripped of timber and ground cover by the 1980 eruption. Rainwater has no place to flow, except into Spirit Lake.

The Corps of Engineers used the last of its money installing 20 pumps to take some of the water around the shaky dam, stablizing the size of the lake at 275,000 to 300,000 acre-feet. But USGS is not sure that the dam will hold a lake of that volume, more than twice what it was after the eruption.

Youngquist said the government is taking a "tremendous risk" if it does not provide money in time for dredging this summer.

The region is noted for dry summers and mild, wet winters when heavy mountain snowfalls are often followed by mild Pacific rainstorms that cause flooding even under normal circumstances.

"Since the eruption, we've been blessed by dry winters," Youngquist said. "We can't stay that lucky. We had three winters in the 1970s that could have breached that dam. One more like that, and we're done for. There would be nothing of value left."