IT'S ONLY a matter of time before the great national coming-out party of weekenders and vacationers off to the beaches, backyards, parks, ball fields and other in-the-sun sports. And wherever they may revel, they will be vividly reminded of how billions of throwaway bottles and pop-top cans continue to desecrate the national landscape from the mountains to the prairies. Many good souls will pitch in and pick up what they can, of course - only to have the stuff remade into new throwaways that can be picked up again. slowly but surely, however, Americans are rejecting that wasteful cycle in favor of measures to trim the amount of junk at its source, through such means as depoist laws requiring that beverage containers be returnable.

In the last few weeks, in fact, legislatures in two more states approved deposit legislation by sound margins. The Connecticult Sneate passed a bill, 26 to 10, putting a minimum 5-cent deposit on all soft-drink and beer contianers. The House has already passed it, 87 to 57. And the Iowa legislature overwhelmingly passed a law putting a minumum nickel deposit not only on soft-drink and beer containers but on liquor bottles, as well: 36 to 11 in the Senate and 79 to 12 in the House.

Though more and more consumers and legislators are calling for such laws, the opposition - namely, the industries that manufacture buy-and-toss bottles and cans - is spending heavily to fight the movement. Still, in Iowa the lawmakers rejected an industry alternative proposal, billed as a "litter tax," that would have been levied on manufacturers and distributors to pay for dealing with trash.

That and similar approaches do have some merit; the trouble is, they're being pushed as alterantives to deposit laws. Thus, industry groups have been working on Congress to disregard a sensible uniform deposit proposal that would make a gradual national transition to returnables. The lobby is backing a proposed "resource recovery act" that would provide for a federal excise tax on the manufacturers of consumer products deemed to contribute most heavily to the solid-waste stream. proceeds from the tax would finance state and municipal litter-pickup programs and the construction of resource-recovery facilities.

That's fine, but here's the rub: Under this proposal, which calls for the fund to be set up in Washington and administered by the Environment Protection Agency, no state could benefit if it had a deposit law on the books. Now, if Congress really wants to get into some sort of excise-tax-for-litter program, the tax ought to complement - not prohibit or substitute for - a national deposit law. The most efficient, direct way to cut down on litter is to do it at the source, which is why Oregon Sens. Mark O. Hatfield (R) and Bob Packwood (R), and Vermont Rep. James M. Jeffords (R) - whose states already enjoy deposit laws - are working for passage of a federal deposit law.Their modest, sensible approach still deserves to be the first order of business on that front.