THIS SEASON has seen two landmarks in the history of the old-fashioned pork barrel. One was the opening in Columbus, Miss., of the Tennessee-Tombigbee waterway, a project that was always difficult to justify (since the Gulf of Mexico is already connected to the Tennessee Valley by something called the Mississippi River) but which had the advantage of running through the district of House Appropriations Committee Chairman Jamie Whitten. Its completion represented a triumph for old-fashioned pork barrel politics.

But, it may be one of the last. For the other landmark was the rejection Thursday by the House of a water projects appropriations bill by a 203-202 vote. This was not, admittedly, a straight up-or- down vote, nor is it likely to be the last verdict on the water projects in question. But you never would have seen anything like this big a no vote on a pork barrel measure 30 or 20 or even 12 years ago. Mr. Whitten professed a certain bafflement at the result: "When did it get to be bad to look after your own country?" Pork barrel politics goes back at least to the days when DeWitt Clinton created a career around building the Erie Canal and Henry Clay proposed a federal program of internal improvements. Today pork barrel politics seems to be in decline -- a slow decline, to be sure, but one which is probably irreversible.

True, politicians are still in the business of, in Mr. Whitten's words, looking after their own country. But the landscape of their countries has changed since Mr. Whitten was first elected to the House the week before Pearl Harbor. Most congressmen today represent districts that are mostly suburban and, by any historic standard, exceedingly affluent. In Henry Clay's time most Americans lived on farms unconnected by road or navigable river with the market for their produce. Today in most congressional districts few voters feel themselves or their communities in need of a dam or a post office, a breakwater or a newly deepened barge lock.

This doesn't mean that we've seen the end of pork barrel bills. The network of politicians, bureaucrats, construction companies and civic boosters that sustain water projects will keep working with some success for many years, even without much backing from voters. And water projects are not the only kind of pork there is. Politicians "looking after their country" forge alliances to protect locally vital farm subsidies, defense installations and urban aid programs. Pork barrel politics is perhaps one of the tolerable inefficiencies of democracy, something that can't be entirely avoided. The slow decline of the old-fashioned pork barrel -- of preposterously expensive and useless water projects -- is still something to be cheered and if possible hurried along.