THE POTOMAC RIVER'S water, city officials now finally agree, is clean enough above Key Bridge to be safe for swimming. It's a remarkable achievement -- and a reminder that sometimes things go right, given enough years, enough money and sustained public support.

There was general skepticism in 1965 when, along with a lot of other lavish promises, Lyndon Johnson undertook to make the murky and smelly Potomac safe for swimmers. The river was to become "a model of scenic and recreational values for the entire country," the president declared. The skepticism deepened as the fish kills went on, and the stench of rotting algae seemed to have become a permanent addition to summer life along the river. Part of it was improving the Blue Plains sewage-treatment plant. Part was the arduous political labor of constructing cooperative agreements among all the contending local jurisdictions.Part was tightening the restrictions on untreated sewage and industrial wastes from the towns upstream. It was all enormously expensive, and one crucial element was the steady flow of federal funds.

Before you pick up your towel and head for the water, a word of caution: while you won't catch typhoid from the water, that isn't the only danger of swimming in the river. There are powerful and eccentric currents in that part of the Potomac. It would be reckless to dive in now before the Park Service can establish beaches protecting swimmers against being pulled away from shore. But the present rate of progress means that there's a real chance of safe and very pleasant swimming there next summer.

The purity of the Potomac's water -- the city's main water supply -- is visibly improving. The pollution in the air is slowly declining. Metro's subway service is expanding, and the first Sunday service attracted far more riders than anyone had expected. Each of these examples is an improvement in the terms of daily life here. Each is the result of long and close collaboration among local and federal politicians, and each represents a lot of money. At a time when politicians' status seems to be evern lower than usual, and when there is a loud cry to cut budgets, it's worth noting that these public policies, steadfastly pursued for many years, are now bringing important benefits to the people of the Washington metropolitan region.