You my be surprised to hear me claim that sometime in the next 10 years the District of Columbia will no longer be the most important jurisdication in this region, That distinction will belong to Fairfax County, which has been adding 25,000 residents a year while the District has lost 83,000, or 11 percent of its population since 1970. By the 1995 the nation's capital won't even be second (that's Prince George's County) or third (that's Montgomery County) in size. Nor will roosters crow in rural Loudoun and Prince William Counties, because by then they will be as suburban as the inner counties are urbanized.

Travel to Nassau County, Long Island, and see Nothern Virginia 10 years hence - with a population now larger than three of New York City's five boroughs, including Manhattan, taxing and spending more than the District to keep afloat. For with increased density of population come shopping malls and neon lights, roads and traffic, pollution and sewage disposal problems, poorer housing, overcrowded schools, pockets of the poor and unemployed, rising welfare, crime and taxes - and underfunded governments shrinking under the load. It is awesome to reflect that the separation of the races, and the myth that blacks are responsible for urban decay, are a fundamental dynamic of population shifts in this and other U.S. regions.

It is no coincidence that most of Fairfax County's new white population is moving to its outer borders, or that inner Fairfax had a 60 percent jump in its black population from 1970 to 1975. It's no coincidence that as 91,000 blacks moved to Prince George's County during the same period, 73,000 whites left, spilling out into Anne Arundel County, Baltimore suburb. It's no coincidence that white arrivals to Montgomery County have slowed since 25,000 blacks moved in. Nor is it surprising that for the first time in a quarter century, for the last two years the number of whites moving into the District increased (by 10,000 since 1970), while white flight tapered off. . . since from 1970 to 1975, 53,000 blacks left D.C. for the deteriorating suburbs.

Not counting prejudice, the reasons for white District in-migration and the black exodus to the suburbs are different - and the differnece is the legacy of racism. Since the sharpest decline in District blacks has occurred among children under 15, some think we are moving to better schools. But that's only a partial answer. Obviously, those of us who are newly or marginally middle income (needing two wage earners per household) can now afford to move, otherwise we would have sought out "cleaner, safer" suburbs sooner. Let us be grateful for small blessings. But we are moving 30 years late - perhaps too late, we might consider.

The reasons why affluent young white married couples are thought to be moving into the District should give us pause. The newspapers report them to be middle class, post "baby boom" adults. . . products of women's lib, with more women holding professional jobs and having fewer children. . . two-wage-earner, high-income households with tax reasons for equity and "shelters". . . cultural activists who want to be near the Kennedy Center. Their reasons for opting out of the suburbs include: scarce energy, rising utility and transportation costs. . . housing more expensive than it's worth. . . disinterest in public education. . . a "sterile" environment. . . skyrocketing taxes. . . and not having a commute to D.C. where the top jobs are.

The time is right in Washington to look forward to a better residential mix of races and income levels, and thus to some neighborhood improvement and business reinvestment, some new jobs, some restored tax dollars for badly needed municipal services. The District also has the special promise of a committed new administration, which wants to pay its dues and understands both needs - to encourage investment and to distribute new resources to the black poor.

But managing urban change in the District or suburbs cannot mean removing or containing poor people. Whether jurisdiction are riding a crest of business investment, just opening up cheap new residential land or getting their first taste of revitalization, the great temptation is to plan against poverty, or to throw new money at old problems.

SOS'78 - Speak Out for Survival!, a recent Urban League survey of nine District census tracts in transition, found that from a third to a half of neighborhood residents were being displaced because of revitalization, most of them low-income renters forced out by rent increases, evictions and urban renewal.

But where were they being evicted? To neighborhoods where conditions were as bad as or worse than those they left. And what was the upshot of investments revitalizating Northeast Capitol Hill, Adams Morgan, Mount Pleasant, the 14th Street Corridor? They were facilitating enclaves for middle income and well-to-do citizens who will better articulate demands for improved services.

Such racial decline and renewal dynamics are operating in education and employment also. Over in Alexandria and Arlington, the Urban League's Northern Virginia branch has been wrestling with what happens to inner suburban schools when middle-income white taxpayer/parents note overcrowding and declining academic achievements, and move farther out ot new school districts. They leave behind communities increasingly composed of black suburban newcomers and low-income citizens, families without children, single or older persons. They also leave behind schools disproportionately composed of students suffering the effects of earlier disadvantage, in communities whose resources for educational improvement have been reduced by their departure.

Basically there are three Washingtons not working well together now: low-income black central Washington, federal Washington, and affluent white suburban Washington. Needed is the recognition that it is efficient for each, while tackling its own problems, to influence the others' problem-solving constructively. CAPTION: Illustration, no caption