David Treadwell of Central Union Mission watched in amazement this spring as a new warehouse the size of a five-car garage was filled from floor to ceiling, virtually overnight, with donated clothing and household goods.

Recently, Treadwell had to buy a second van and hire a third driver to help pick up the $720,000 worth of goods donated this year--a whopping jump from the $180,000 value of gifts just two years ago.

Used merchandise is hardly a common economic indicator. But Treadwell and other charity executives say economists could chart the country's wealth, growth and confidence by visiting their warehouses to see what is being given away.

Locally, Goodwill, the Salvation Army and other charities that collect used merchandise report that donations have vaulted since the economy took off in 1994. Tens of millions of dollars are being fed into local programs that benefit from store sales, and bargain shoppers are finding a bonanza.

Charities that collect society's leftovers in the Washington area have always been blessed. "With high average incomes, folks in this region are able to buy better quality, so we get quality used items," said Maj. Frank Swimm, of the Salvation Army of Northern Virginia.

And giving is good not only because of the area's affluence, but because of it's transience. When people leave, said Chris Falk, of Davis Memorial Goodwill, "they make hard decisions to leave behind things."

Even people who had planned to stay forever move on: Goodwill gets donated cemetery plots, presumably from people who changed plans about settling in for eternity.

But the generally favorable climate has gotten even sunnier.

The value of items donated to the Salvation Army branches in Maryland and the District has doubled since 1994.

Thrift store sales for the local Goodwill last year were $10.8 million, up 30 percent from 1994. In the last year alone, the value of donations increased 16 percent. Nearly 140,000 people brought donations to the warehouse last year--an 81 percent increase in four years.

In addition to its regular stores, Goodwill has three upscale shops called Best Kept Secrets. Designer clothes and other high quality merchandise are culled from the warehouse to stock these shops in Rockville, Alexandria and Gaithersburg.

Goodwill opened its Best Kept Secrets stores just before the economy took off and has had no trouble stocking them.

"During good times, like now, we get much better clothing," said Larry Ulrich, who picks up donations for the Prevention of Blindness Society. "People are out buying new clothes, then rushing home to clean their closets to make room."

The trend, charities say, is national. A spokeswoman for Goodwill Industries, for example, said the charity collected a billion pounds last year--up from 716 million pounds four years earlier.

Outdated computers and electronic goods are flowing to the secondhand market. And businesses that are doing well have joined ordinary consumers in giving more, and better, items.

"A lot of hotels seem to be refurbishing," said Treadwell, whose warehouse is on 14th Street NW. That helps him get more beds, dressers, lamps and even pictures to poor families. Donations to Central Mission are not sold but routed directly to the poor.

Goodwill thrift shop proceeds support jobs programs for people outside the economic mainstream--the disabled, long-term welfare recipients, former prisoners. Salvation Army shops provide the sole support for adult drug and alcohol rehabilitation programs.

The poor are the intended--but not necessarily the only--beneficiaries of the boom in giveaways.

At Best Kept Secrets in Rockville, Lynn Carver, an administrative assistant, recently sifted through racks hoping to find the clothes that could make her look like someone ready for a promotion.

She approached the counter with a J.G. Hook wool sweater, a jacket by Jones New York, a velour pullover, Calvin Klein pants and--just in case a friend ever had a shower--a Baby Gap sleeper with the tags still on.

The bill: $32 even.

"It's like the ultimate sale, and for a good cause too," she said.

Although the economy is flush with consumer dollars, customers haven't quit coming. Bargain shoppers of all economic classes, of course, are with us always. Some poor people whose boats have risen with the economic tide may choose to switch to new merchandise, but others just buy more and better used items.

In addition, argued Falk, of Goodwill, the stigma of secondhand goods is fading.

"Characters like Kramer [on 'Seinfeld'] have made it trendy to wear thrift store clothes," he said. The charity also has stimulated sales by advertising locally to high school and college markets.

"We were well positioned to do that," Falk said. "There are a lot of for-profit operations selling faded blue jeans, and their prices are steep. We can sell previously enjoyed blue jeans for a couple bucks. That has pulled a new audience into Goodwill stores."

The young's discovery of tie-dyed shirts, bell-bottoms, fringe and even polyester has opened a new market too, said Clara Sachs, who gives classes in finding thrift store bargains at First Class, an adult education center at Dupont Circle.

Charities are always eager to publicize downturns in donations, but some were a little skittish about providing documentation of the upswing. Their needs, they caution, remain great. The increased funds, they say, have been invaluable.

"We were able to increase our in-patient facility from 90 beds to 112 beds," said Capt. Don Smith, of the Salvation Army branch that serves Anne Arundel, Howard and Baltimore counties. The extra money also allowed him to hire professional counselors and to spiff up the dormitories with a paint job, new carpeting, beds and mattresses and lockers.

"It's really made a difference for the men," he said.

The charities will take still more of just about everything and can put it all to good use. With one possible exception: crutches.

Goodwill, for one, is drowning in crutches. Falk said the charity recently unloaded 500 pairs to a medical charity headed to Vietnam, but still has bin after bin.

"I guess the chances of having a friend break their leg just as you are recovering is pretty slim, so the crutches all come to us," Falk said. "The problem is, no one buys them."

But even here, there is a nugget of information for those who would mine charity warehouses for social trends: Despite all the complaints about managed care, crutches, apparently, just aren't a problem.