NBC Entertainment will announce today it's decided to renew "Saturday Night Live" for another year. What won't be announced is a lot of cast changes. Lorne Michaels remains as producer . . .

National Nielsen ratings for Monday night show that the conclusion of "The Deliberate Stranger" on NBC outscored Part II of "North and South: Book II" on ABC . . .

For the 9-to-11 p.m. period, "Deliberate" averaged a 21.7 rating and a 32 percent audience share. "North and South" did a 20.1/30, while the CBS lineup for those two hours averaged a 13.1/19 . . .

The new Fox Broadcasting Co. -- one of whose six stations is WTTG here -- announced yesterday that it will form a fourth TV network of independent stations around the country . . .

Fox then fired its first popgun in its long-range assault on ABC, CBS and NBC by announcing that Joan Rivers is deserting NBC to host her own late-night talk show on the new network starting this fall -- in direct competition with her old pal, Johnny Carson . . .

Rivers' current contract with NBC to substitute for Carson expires in July . . .

Her Fox show will be the new network's first regular programming effort and will be followed by primetime and other late-night programming starting next March, according to Barry Diller, chairman of Fox Inc. . . .

Diller said Fox is committing more than $100 million to start the new satellite-delivered national program service and is willing to spend "years and years" to see it through . . .

In all, he said, more than $2 billion stands behind the new service, including the 20th Century Fox film studios and the former Metromedia stations purchased by Rupert Murdoch, now consolidated under one management . . .

Those six stations currently reach 22 percent of the national audience, but Diller said the goal is a network of 70 independent stations -- none currently affiliated with a network -- reaching 80 percent of the nation's 85.9 million TV homes . . .

"I want to thank NBC a thousand times," Rivers said. "I am grateful to NBC for the three years I had on 'The Tonight Show,' but they should thank me, too. My ratings were good and they were better than the regular host" (she means Carson) . . .

She said she talked by telephone to NBC Chairman Grant Tinker and Entertainment President Brandon Tartikoff. "I called Johnny Carson twice, but he would not take my call" . . . CBS Cuts

The slow economy and balky advertisers are forcing the CBS Broadcast Group to eliminate several hundred more positions by the end of the year . . .

The cuts in the sports, news, entertainment and owned-and-operated station divisions will be made through attrition and layoffs, according to Group executives . . .

The Broadcast Group currently has about 8,100 employes, down 300 from two years ago. Some 100 positions have been eliminated in the past six months . . .

After a record $408.6 million in 1984, Broadcast Group profits declined to $360.6 million last year . . .

In recent months, four key officials -- Tom Leahy, head of the network and entertainment division; Neal Pilson, in charge of sports, the O&Os and broadcast operations; Van Gordon Sauter, in charge of news, radio and international sales; and Vice President for Finance Tim Reynolds -- have been looking at a variety of cost-cutting measures. So far, CBS insists, no exact head count of positions to be eliminated has been decided on . . .

The task force is expected to make its final recommendations next month . . .

Among cost-cutting measures being considered: the sale of CBS Sports broadcast operation trucks in favor of lease-back deals for weekend events. According to Group Vice President George Schweitzer, CBS will save about $450,000 this year by not replacing two vice presidents who recently left the O&O stations division . . .

Also being considered are more efficient ways for CBS News to cover conventions and elections. CBS News lost 120 employes in the most recent cutbacks. The network's Washington-based late-night news program, "Nightwatch," which loses $4 million annually, is considered a candidate for cancellation, according to News division sources . . .

Schweitzer said that despite the cuts, neither Entertainment nor News program quality will be sacrificed. He pointed out that CBS Entertainment recently ordered a record 31 pilots, at a cost of more than $50 million, despite the belt-tightening . . .

Meanwhile, the talk in Miami is that both CBS and NBC are eyeing CBS affiliate WTVJ there as a prospective owned-and-operated station purchase. Asking price is about $350 million . . .

One CBS official yesterday said the story seems to have come from brokers who recently began contacting possible purchasers. He played down CBS' interest in WTVJ but added that "obviously we're not going to ignore any top-market TV station that comes along" . . .

CBS recently reduced the number of its O&Os to four with the sale of KMOX in St. Louis to Viacom for $122.5 million. NBC currently has five stations but will pick up a sixth, KCNC in Denver, when parent RCA completes its merger with General Electric this summer . . .

NBC News, already in hot water with the State Department yesterday for its Monday night broadcast featuring terrorist Abu Abbas see details on Page A21 , was criticized by Charles Osgood yesterday on his CBS Radio Newsbreak broadcast . . .

Snapped a spokesman for ABC News yesterday on hearing of the CBS editorializing: "We're not in the business of NBC bashing" . . .

Arnold Wallace, general manager of Channel 32 here, has been elected secretary of the board of trustees of the National Association of Public Television Stations, the lobbying arm of PBS . . .

Others named in the NAPTS board election here last week were Chairman Richard Ottinger, executive director of the Georgia Public Telecommunications Commission; Vice Chair Mrs. Mary Levine, citizen fundraiser for KUHT in Houston; and Second Vice Chair Mrs. Sue Keene of the Oregon Commission on Public Broadcasting . . . TV Ratingzzzz

Season winner NBC, which had finished third twice in a row, snapped out of it to finish first for the week ending May 4 . . .

NBC had a 15.4 rating and a 26 percent audience share, compared with CBS at 14.0/23 and ABC at 13.2/22 . . .

If there was any particular surprise for the week, it was registered on the faces of ABC executives yesterday when they learned that Part I of its "North and South" sequel finished with a ho-hum 19.8/30 on Sunday night. . .

Last November, Part I of "North and South" registered a 25.8/37, presaging a big win for the network the next week, one of only two weekly wins all year for ABC . . .

Be that as it may -- and it may, for ABC -- that network's Texas 150th-anniversary special was 46th, CBS' new "Bridges to Cross" tied for 48th, the Disney movie, "Love Leads the Way," tied for 53rd, and the return of the CBS news magazine, "West 57th," was 64th out of 71 shows ranked for the week ending May 4 . . .