Steamboating in Style on the Mississippi River : Paddle-wheel cruises are so popular some are sold out six months in advance.

The mighty, majestic Mississippi River: Mark Twain popularized it, and the Delta Queen Steamboat Co. has packaged it in convenient, captivating cruises that have made its two paddle-wheel steamboats favorites with mature travelers.

According to the company, “active retirees” make up the majority of its passengers, with an average age of about 56. Mature travelers have helped make the boats so popular that some special event cruises, such as the Kentucky Derby or Great Steamboat Race, are sold out as many as six months to even a year in advance.

Cruises are offered year-round on the venerable Delta Queen and newer Mississippi Queen, the only overnight paddle-wheel steamboats still in operation.

No senior discounts are offered, which is not surprising when you consider that only two or three of the major cruise lines offer senior discounts of any kind.

Unlike the cut rates or deep discounts frequently offered by Caribbean cruise lines to fill berths in a very competitive market, the Delta Queen Steamboat Co. seldom needs them.

Cost-conscious mature travelers can still find good deals through group tours offered by senior clubs and organizations that use lower group rates and other incentives to put together riverboat cruise tours.

The Delta Queen Steamboat Co. does not need to rely on low prices to lure individual mature travelers. On a typical four-night cruise (one of the most popular), the basic cabin rate per person is $650. There are seven upgrades available to a top of $2,280 for a suite.

An average of about $154 per night for a week’s cruise is a bit higher than most cruise ships sailing the Caribbean, but most people who have taken a river cruise on either the Delta Queen or the Mississippi Queen agree that the value is good.

Stateroom size and decor, service and meals compare favorably on both boats, though minimum-price cabins on the 176-passenger Delta Queen are not appropriate for maximum-size people.

The smaller Delta Queen has a legion of fans who love its intimate ambience and couldn’t care less about such things as a swimming pool, deck tennis or night club-style evening shows.

What both boats offer that other cruise liners do not is motion-free river cruising where no waves even ripple your mint juleps or iced tea.

During the day, there is mile after mile of changing scenery to view instead of just water--through the heartland of America, up and down the Mississippi, the Ohio and, on occasion, the Tennessee and Cumberland rivers as well. Plus ports of call where the history and attractions are fascinating Americana. Then again, the nearest thing to duty-free shopping may be the local K-Mart.

While New Orleans is home base for both riverboats and sees the majority of departures, you can also find two-day to one-week-or-longer cruises leaving from Memphis, Nashville and Chattanooga, Tenn.; St. Paul, Minn.; St. Louis; Mobile, Ala.; Cincinnati, and Pittsburgh, depending on the time of year and your choice of riverboat.

Generally, both boats operate out of New Orleans from November to about mid-April, when both start a series of cruises up the Mississippi as weather warms. They reach St. Paul just in time for some fall foliage cruises in September and October, then gradually work downriver back to New Orleans.

There are a variety of possible side tours, including a combination riverboat cruise and motorcoach tour to the Ozarks, plus others to Kentucky horse farms, fall foliage tours, the Grand Ole Opry in Nashville and Civil War battlefields. There are also pre- and post-departure package stays in port cities available.

Special event cruises include the Big Band Vacations aboard the Mississippi Queen, now through March 1. Travelers can enjoy the music of the Les Elgart, Sammy Kaye and Harry James orchestras. The Delta Queen offers Dixie Fest Vacations, now through March 7, with either Pete Fountain or the Preservation Hall Jazz Band.

Neither the Delta Queen nor Mississippi Queen offers gambling, nor does the line anticipate that it might do so n the near future.

Given the number of dates, schedules, options and other cruise variations of paddle-wheel steamboat vacations, travelers should consult the free Delta Queen brochure, available from your travel or cruise agent or by contacting the Delta Queen Steamboat Co., Robin Street Wharf, New Orleans, La. 70130-1890, (800) 543-1949.

Among the tour and travel specialists running group tours this year with the Delta Queen Steamboat Co. (all prices are per person, double occupancy, at the minimum cabin rate):

--Calico Tours: Offers an excursion aboard the Mississippi Queen, Nov. 30-Dec. 7. Three nights in New Orleans, a four-night cruise, from $1,392 plus round-trip air fare from Los Angeles (currently $446). For more information, call (310) 478-5865.

--Trieloff Tours: A 10-day escorted “Dixie Tour” featuring three days aboard the Mississippi Queen, an overnight in New Orleans and a five-day motorcoach tour to Natchez, Vicksburg and Biloxi, Miss., and Bellingrath Gardens near Mobile, Ala. Departures from Los Angeles on March 22, April 12, May 13 and Nov. 12 and 19. Prices start at $1,599. Information: (800) 432-7125.

--Great Western Tours: The nine-day “Heart of Dixie Tour” consists of a three-night cruise on the Mississippi Queen before or after a four-night motorcoach tour through Louisiana Cajun Country plus two days in New Orleans. Price: $1,995, including air from Los Angeles. Information: (800) 344-7090.

--Westward Travel: A four-night cruise on the Mississippi Queen out of New Orleans, Dec. 3-7. Price: from $1,350, including air from Los Angeles. (An earlier seven-night cruise, May 22-29, sold out, but travelers can ask to be put on the wait list.) Information: (800) 537-4602.