THE BIG news this fall is that Washington art galleries have clearly become a T leading force in the rebirth of the downtown area. Within a year nearly a dozen commercial establishments have been successfully transplanted from locations all over the city to the 7th Street strip (between Pennsylvania Avenue and F Street NW). Their success can be measured by the way other galleries have rushed to join them as fast as spaces can be renovated.

Next Saturday, the new season really gets under way when the seven dealers who pioneered the building at 406 7th St. NW celebrate their first anniversary with a bevy of new shows. Also on that day, the Washington Project for the Arts will open next door in a building at 404 7th St. NW, which will include not only a new downtown branch of Middendorf/Lane and Slavin Gallery (a newcomer), but also Washington's first California hot tub spa, Making Waves, where frustrated artists and gallery owners can rent hot tubs by the half hour. Cranes and scaffolding across the street from them reveal that, after years of delay, construction has finally begun on the $7 million Gallery Row project, due to open January 1983 with 10 more galleries.

The Lansburgh Project, still in the process of being transformed, will eventually house several galleries and arts organizations. Olshonsky, formerly of 18th Street, will reopen in October at 443 7th St. NW, on the second floor of the building occupied by dc space. Diane Brown is working with architect David Schwarz on the redesign of the basement and first floor of 421 7th St. NW, currently occupied by Bargaintown.

Despite the loss of several dealers to downtown, galleries are still moving into the Dupont Circle area and two new galleries will open there soon. Meanwhile, Georgetown galleries continue to thrive and suburban galleries continue to multiply. Alternative spaces have become epidemic, and it is hard to find a blank wall in any federal lobby, private office building or bank that hasn't been taken over by Washington's increasingly aggressive art community.

Here is a sampling of what's being offered through the end of the year:

September

Jack Rasmussen has already opened his first fall offering, recent paintings by William Willis and Nancy Palmer. And under way at the Hom Gallery is "Prints About Prints," a delightful survey of a subject that will fascinate any print lover. On Tuesday, Gallery K, launches the new season with a group show of recent work by gallery artists, and on the same day Middendorf/Lane, will show the complete prints of eye-dazzling realist Richard Estes. On Wednesday Viennese photo realist Gottfried Helnwein will debut at Baumgartner Galleries, with a show of watercolors and drawings.

Highlights among the seven new shows opening Sept. 19 will include new and old portraits by famed photographer Yousef Karsh at Lunn, the unusual figurative bronze sculpture of Martin Silverman at Diane Brown, the work of sculptor John Van Alstine at Osuna, and new silk-screen prints by Richard Estes at Barbara Kornblatt. Next door, WPA will open with a show of photographs by Linda Wheeler and Arnold Kramer relating to the history of the building they occupy. Middendorf/Lane Downtown will take advantage of their new space by showing large-scale works by William Christenberry, Sam Gilliam and Rockne Krebs.

On Sept. 22, Barbara Fiedler will open a show of work by Hilda Thorpe, and on Sept. 23 the new Addison/Ripley Gallery Ltd. will open in a handsomely re-converted stable at 9 Hillyer Court, behind the Phillips Gallery and the Cosmos Club. The venture is jointly owned by artist Christopher Addison, an exhibition designer at the Renwick Gallery, and artist Sylvia Ripley, daughter of Smithsonian Institution Secretary S. Dillon Ripley. The opening show will feature geometric abstractions by Washington artist Michael Smallwood.

October

It's hard to anticipate which will be the most interesting of the 135 shows opening in galleries in October, but sure to attract attention will be the show of abstract bronze sculpture by William Noland, opening Oct. 22 at Addison/Ripley. Noland is the 25-year-old son of Washington color painter Kenneth Noland.

The Franz Bader Gallery will show "The Worlds of Pietro Lazzari" on Oct. 7, the first retrospective exhibition of this Washington sculptor's work since his death two years ago. On Oct. 10, Diane Brown will show the unusual "lampworks" of R.M. Fischer, and on Oct. 13, Olshonsky will open with a two-man show featuring virtuoso draftsman Paul Martyka and printmaker Mark Nelson.

In mid-October, Baumgartner Galleries will introduce the American Indian painter Randy Lee Whitehorse from Taos, N.M. On Oct. 27 Jane Haslem will begin a series of four shows focusing on innovations in American printmaking since 1920.

November

The first major show opens in November at WPA, entitled "Emerging Washington Photographers," with curator John McIntosh. Paintings by Robert McCurdy will be featured at the same time. On Nov. 22, WPA will sponsor a work by performance artist Alan Kaprow, father of "the happening."

Middendorf/Lane Downtown will feature a major exhibition of contemporary folk art by six artists from the Southeast. The show, whose opening date with curator George Hemphill. Opening date to be announced.

December

Group shows focusing on small works will fill the galleries, as they always do, with the Bader group show of gallery artists opening Dec.8.

For their Christmas show, Fendrick Gallery, will show the wrought iron work of Albert Paley including the grates and benches he's been commissioned to design for Pennsylvania Avenue as a gift to the people of Washington.