THE ANCIENT trees cast their shadows against the whitewashed stone of the Abner Cloud House. The canal makes soft swooshing noises against the bank. The miller's house has stood steadfast on this spot since George Washington came here to see how his canal was getting along.

The c&o Canal is sometimes a muddy trickle, other times a swift creek in a hurry on its way to sea. The house has the air of a building that knows its place and intends to keep it, despite the wrecking balls of time.

The Abner Cloud house was built on the banks of the Little Falls Canal somewhere between 1798 and 1801. (Its current address if 4940 Canal Road, next door to Fletcher's Boat House.)

Cloud had a flour mill near by, now long gone, to serve the busy port of Georgetown. George Washington's Potomack Co. had built the canal earlier, and in 1828 the C&O followed the same canal bed.

Cloud built his house from stone, quarried not far away. He had no intention of letting it wash away-the walls are 3 feet thick and rise three stories high. Cloud had a taste for the proper embellishments, as well. After all, he was a prosperous man. So he hired a careful cabinetman to make the wooden mantels and fire-place niches and the handsome staircase that goes all the way to the top. The big windows, neatly shuttered, control a handsome view of the towpath and the Potomac River. The view was not only pleasant, but important to the miller, so he'd know what to expect at his next grinding.

After Cloud, many other millers lived in the house. James B. Frizzell was the most notorious-he was part of a plot to kidnap President Abraham Lincoln.

The mill closed in 1870, when such small operations were no longer economic. The canal was abandoned as a commercial waterway in 1924. The mill and the house sunk first into picturesque disarray and finally ruins.

The National Park Service acquired the site, because of its proximity to the canal, in 1957. The Park Service didn't acquire any money to repair it.

In 1976 Chapter III of the Colonial Dames of America decided to look through the files of the National Trust for Historic Preservation for a Bicentenial project. The Dames petitioned Congress for money to restore the exterior and the first floor to serve as a canal information center. The Dames paid for restoration of the upper two floors to use as a club-house. The Park Service served as restoration architects and historians, winning an award from the Washington Metropolitan Chapter of the American Institute of Architects.

This spring the restored house will open to the public for the first time.

Tomorrow the Colonial Dames chapter and the National Park Service will receive the National Trust's Gordon Gray Award for achievement in preservation. The award and 15 others will be presented tomorrow at the Renwick Gallery, itself a major preservation. (Though, according to testimony this week before Congress, the gallery is in need of about $5 million more on the exterior.)

The Trust annually presents awards during Preservation Week, a time when it stops to congratulate itself on houses, buildings and areas preserved, and vows to revenge those lost. In 1979 there is much good to note. Some 25 years ago, preservation was mostly the province of ancestral societies such as the Colonial Dames, or wealthy women's groups such as the one that presrved Mount Vernon. (The first president's house had been systematically rejected by both the country and the state.) But such presrvation groups were few and scattered, and usually only interested in the preservation of patrotic sites, not in architectural landmarks.

Today there are 589 landmark and historic district commissions in the United States, as opposed to 100 only a decade ago. California, one of the newest states, has 46, compared to 63 in Massachusetts, one of the older. Such commissions study historic and architecturally significant areas and make recommendations. Sometimes the commissions can delay demolition permits or forbid exterior alterations.

The Trust itself, chartered only in 1949, now has more than 155,000 members. The Trust dispenses preservation advice and sometimes funds from matching grants from the federal government and its contributors.

Currently Congress is considering preservation funding. President Carter recommended $45 million. James Biddle, Trust president, testified recently before Congress, asking $60 million, the amount budgeted for this fiscal year. He cited energy savings as well as the revitalization of communities.

The awards this year point out the diversity of preservation projects.

The Trust is giving four David E. Finley awards for outstanding achievements:

The preservation of Louis Sullivan's 1892 Wainwright Building makes good economic sense. The architect's first steel-framed skyscraper is one of the most important architectural monuments of the period. Christopher S. Bond of Kansas City, Mo., a former governor, decreed that the building be used for state offices. Nothing preserves a building like use. Bond also is being honored for his work to preserve the state's governor's mansion, the Thomas Hart Benton Home in Kansas City, and a collection of work by artist George Caleb Bingham.

Vizcaya, the old James Deering estate in Miami, built in the 1914-16 golden age of American palaces, should have been preserved, for surely the likes of this one could never be built again. The 72-room house and its magnificent gardens with cascading fountains were preserved by a group called the Vizcayans.

People are just beginning to recognize today that America's factory architecture is often as worth preserving as residental.Lloyd Thomas Smith, president of the S/V Tool Co. in Newton, Kan., realized that the 1879 Monarch Steam Mills should not only be saved but would make a fine place for his corporate offices as well as for other organizations.

The Camden, S.C., District Heritage Foundation and Historic Camden, Inc., won an award for an historic park.

Receiving the Gordon Gray awards for achievement are five other individuals and groups.

The Corning N.Y. Glass Works paid for architects to demonstrate the feasibility of restoring the late 19th-and 20th-century terra cotta and brick commercial buildings on the main street.

The City of Oakland, Calif., Planning Department published a rehabilitation manual on restoring Oakland houses.

The Old House Journal Corp., Brooklyn, N.Y., publishes a newsletter on renovation.

Franco Scalamandre, chairman of the board of Scalamandre Silks of Long Island City, N.Y., has given advice and materials to more than 500 preservation projects over the past 50 years.

Stewart Title Co., Jouston, has restored the facade of an 1882 Galveston building, revitalizing the Strand area, as well as setting up branch offices in other restored buildings through the country.

The National Trust President's award went to four more projects:

Delaware Trust Co., Wilmington, restored the first floor of the 1853 Wilmington Customhouse, threatened with demolition. The building is now a branch bank.

Junior League of Corpus Christi, Texas, restored the 1893 Sidbury House, the last high Victorian left in the city. The house has wonderful gingerbread. Most of all, its spacious octagonal porches add not only beauty to the house but also are good examples of architecture that responds to the local climatic conditions. The porches must have been delightful places to sip iced tea with mint on hot afternoons. The house now works as a museum with Junior League offices.

Rose Josephine Boylan of East St. Louis, Ill., has worked for 52 years to preserve historical structures, including a log church and mansion in Cahokia, the oldest continuous settlement in the Mississipi Valley.

The Maritime Museum Association of San Diego, Calif., for 52 years has preserved vessels and maritime artifacts.

The Public Service Award will go to:

The City of Evansville, Ind., for its preservation of the 1876-79 Post Office.

Preservation is not always virtuous. Often dilapidated buildings of no known use and little beauty are preserved just because they are old. Often structures are totally rebuilt from the foundation up, in a Disneyland approach that can only be justified for the entertainment value. Sometimes, buildings are so far gone that the cost of restoration would build five new and better ones to serve the purpose. Such preservation efforts are to be deplored.

Row houses are among the best examples of useful preservation. All over the country, people are recognizing the value of these remnants of earlier ages-energy-saving construction, economical of land and utilities. In some cities such as Washington, the great old mansions, the dinosaurs of the past, have new life not only as embassies but as law offices and foundation headquarters. Even old warehouses, their brick neatly cleaned, have proven their worth as shopping centers such as the Foundry in Georgetown. Old goverment buildings such as the Old Post Office on Pennsylvania Avenue can work again as federal offices and shops.

The best preservation is the most economical. Buildings should be preserved when they can live again as working members of the city. Preservation at its best brings the glories of the past to serve the present, invested for the future. CAPTION: Picture 1, A preservation-award winner: The Junior League of Corpus Christi restored this 1893 Victorian.; Pictures 2, 3, and 4, Local winner: The Abner Cloud House stands sturdy and strong again along the banks of the C&O Canal after extensiverestoration to the exterior (left) and interior (above and below).; Picture 5, A preservation-award winner in Miami is Vizcaya, the old James Deering estate