Network news as America has known it for the last decade is on the decline, according to TV journalist and talk show host Geraldo Rivera.

"The monopoly is broken," Rivera told members of the Academy of Television Arts & Sciences at a luncheon here recently. Viewers who once slavishly tuned to the networks for their daily dose of news are no longer doing so, he said."A network is only a syndication outfit that got there first and got the most stations," said the 44-year-old host of "Geraldo!" Non-network syndication companies have developed their own stable of stations and enough popular programs in recent years to become "ad hoc networks," he said.

His own ad hoc network, Tribune Broadcasting, helped break the stranglehold of the three traditional networks two years ago with the first of Rivera's series of six semi-live prime-time documentaries, he said. That two-hour special in the spring of 1986 on the opening of Al Capone's vault demonstrated that a slick, if gimmick-laden, documentary could actually trounce the networks in the ratings.

Rivera's subsequent prime-time specials dealt with modern love, wayward children, the present-day Mafia, the national drug epidemic and, in his most recent documentary that aired two weeks ago, murder in America.

"People will watch a valuable program regardless of where it appears," he said.

As more syndicators, cable operators and others enter the television field, Rivera predicted, the only way that ABC, CBS and NBC will be able to compete effectively is through reality-based programming comparable to Rivera's specials. He cited CBS News' "48 Hours" and Ted Koppel's remote broadcast of ABC's "Nightline" from Israel last week as examples of such reality-based programming.

"Has anyone noticed how aggressive the networks are being these days?" he asked. "You're not going to see anchors just sitting in the studio anymore."