Hurricanes Katrina and Rita reminded us that cities can be changed suddenly and adversely by natural forces. Yet forces unleashed by humans also can radically alter the physical form and character of a city, although more gradually and, one hopes, constructively.

Such forces operate on metropolitan Washington, where significant urban mutations are underway or are likely. All promise to improve the economic, social, cultural and aesthetic health of the region. Consider just a few.

* The District's Southeast waterfront. The Anacostia River waterfront, from the Navy Yard to South Capitol Street, will never be the same. As the Navy fixes up its yard, improving facilities for visitors and employees, the General Services Administration is developing the adjacent Southeast Federal Center. A complex of new office buildings, stores and public open spaces will replace the cracked asphalt and obsolete industrial buildings that dominated this riverfront real estate for so many decades.

* The District's Southwest waterfront. The Southwest waterfront along the Washington Channel, parallel to the Potomac River, is slated for a complete redo. The city's Anacostia Waterfront Initiative envisions replacing the existing, suburban-style restaurants and hotels with a series of new, multistory, multiuse buildings. Maine Avenue will remain, but Water Street, the adjacent service road and surface parking lots will be eliminated. A key design goal is to enhance views of the river from city streets.

* Walter Reed Army Medical Center. The Defense Base Realignment and Closure Commission recommended shutting the Walter Reed complex. If it is closed, it could provide Washington an extraordinary opportunity to transform a large piece of real estate. Decommissioning Walter Reed poses technical challenges and economic hardships in the short run. But in the long run this 113-acre site between 16th Street and Georgia Avenue NW could be a model of urban redevelopment if issues of transportation, land use, density, open space and public amenities are wisely addressed.

* Crystal City and Rosslyn. Arlington's accessible and visible mini-downtowns across the Potomac River from the District are undergoing makeovers. Conceived in the 1960s and built out mostly during the 1970s, both were designed primarily to accommodate government-related office workers. Although both had a few hotels and apartment buildings, neither had enough residents, street-level shopping, or other commercial and entertainment activities to relieve their intrinsic visual and functional sterility. Now property owners, developers, occupants and the county are trying valiantly to make Crystal City and Rosslyn much more urbane by enhancing the quality of their streetscapes and diversifying uses, tenancy and architecture.

* Alexandria. In just 15 years, an entirely new downtown, "Carlyle," has risen on land owned and previously used by a railroad company. Situated between Duke Street and Eisenhower Avenue, west of Old Town and a few hundred feet south of the King Street Metro station, Carlyle is a new grid of blocks and streets that encompasses the U.S. Patent and Trademark Office complex, the federal courthouse, multiple office and apartment buildings, townhouses, hotels, parks and plazas, and an increasing amount of street-level retail activity coming as Carlyle's resident and employment population increases. Immediately west of Carlyle, more high-density commercial and residential development is underway along the Eisenhower Avenue corridor.

* Silver Spring. Downtown Silver Spring is among the most impressive urban transformations in the region. Still a work in progress, it occurred relatively quickly and entailed a challenging combination of new construction, demolition and preservation. This formerly lackluster commercial crossroads has been rejuvenated. As in other parts of the region, redevelopment has been intense and dense, with a vital mixture of commercial, residential, cultural, civic, recreational and institutional uses. Today, downtown Silver Spring offers theaters, cinemas, extensive shopping and an expanding array of places to eat, along with thousands of jobs. Also like other planned urban mutations, this one was enabled by existing infrastructure, particularly the Metro station and regional road network.

* Rockville. The new Rockville Town Center is under construction in the heart of the city. However, instead of piecemeal development stretched out over many years, the new town center is being built all at once by the city and developers. Grading, excavation and foundations are underway on a large, recently cleared site bounded by Rockville Pike, East Middle Lane, North Washington Street and Beall Avenue. When finished, the town center's three blocks will be occupied by a new Montgomery County library, Rockville Cultural Arts Building, offices and apartments above street-level retail stores, restaurants and parking garages. At the heart of the complex is the Town Plaza, a square accommodating public events as well as routine pedestrian activities, outdoor dining and impromptu gatherings. With both paved and planted areas, the plaza is lined by eateries, shops and civic buildings and is overlooked by apartments.

All these places and transformations involve rational planning based on changing needs and circumstances. In each one, something previously created is evolving into something new and different. And each can significantly affect people and places beyond their boundaries.

Nature can alter the evolution of a city, but so can man, which is when evolution and intelligent design ideally go hand in hand.

Roger K. Lewis is a practicing architect and a professor of architecture at the University of Maryland.