Stream of Consciousness Excursions in Europe : The Elbe, Danube and Rhone are among the major rivers plied by small luxury vessels.

It is so quiet as we move along the river, we can hear the birds singing in the nearby trees when our cabin’s French doors are open. There’s no deck or balcony outside, just waist-high metal bars, but the open doors provide fresh air and a closer rapport with the scenery.

From our mooring in the village of Decin, we can see a fairy-tale castle perched atop the hill. An oompah brass band in bright costumes greeted us on our return from a day in Prague with some loud and lively tunes.

Along the river we have seen ducks, swans, geese, a few storks, even a blue heron, and once we glanced a clutch of deer standing stock-still in the rich green meadows.

The boat glides along so smoothly and soundlessly that the motion is more akin to floating in a hot-air balloon than sailing on the water. There’s not a chance of seasickness, not a single vibration. On a narrow shelf by the French windows, a bottle of German wine and two glasses--part of the welcome amenities placed in every cabin along with a dish of fresh fruit and a vase of flowers--have not so much as clinked together once.

We are aboard the 140-passenger Prussian Princess, one of three Cunard EuropAmerica river vessels, and we are sailing along the Elbe between Hamburg and Decin.

The other two boats in the trio, which also make seven-day cruises, are the 144-passenger Princess de Provence, which plies France’s Rhone River round trip from Lyon, and the 200-passenger Danube Princess, which sails from Passau, Germany, along the Danube into Austria and Hungary.

While there are a number of river vessels sailing in Europe this summer and fall, the ones most actively marketed in the United States are the three Cunard boats, French Cruise Lines and KD River Cruises of Europe.

The Prussian Princess is the most lavish river vessel we’ve sailed aboard, with its antique-style furnishings, oil paintings, Tiffany stained glass and big vases of fresh flowers. Passengers sit at sociably small tables set with crisp white linen. Service is swift and sophisticated and the cuisine is continental, with elaborate evening menus that often include a sorbet between the fish course and the meat course. Desserts from the Viennese chefs, not surprisingly, are delectable.

The vessel’s sister ship Princess de Provence also has elegant furnishings and cabins with full-length French doors opening to the scenery. The Danube Princess, a bit more modern in style, offers television sets in each cabin and a small swimming pool on deck.

When we sailed with French Cruise Lines’ 100-passenger Normandie along the Seine several years ago, the cabins were trim and comfortable but somewhat simpler, and we dined on set meals at a large, communal table. The vessel sails between Paris and Honfleur, calling at Monet’s house and gardens in Giverny. French Cruise Lines also sails the 100-passenger Arlene along the Rhone and Saone between Macon and Avignon.

The two KD boats we have been playing tag with along the Elbe--the Clara Schumann and the Theodor Fontane--are also more modern in decor, with a large bar and lounge and bigger rectangular tables for four or six, set close together in the dining room. The cabins aboard are about the same size as those on the Prussian Princess, with the same pull-down second berth, but they don’t have the full-length French doors that are so appealing.

One of the interesting sidelights of this cruise is that most of the staff are from the former East Germany or the Czech Republic.

Our captain, Jaroslav Drozdik, is Czech and has spent 35 years on the river, first on his father’s ship and then in his own vessel. During the days when Germany was divided, only Czech shipping was permitted on the Elbe, so the Czech captains are the most experienced on the river.

The 2,600-ton Prussian Princess was built in England with a three-foot draft specifically to cruise the Elbe, and at 363 feet long is the largest boat on the river.

Capt. Drozdik says the Elbe is about 750 miles long and from 54 to 600 feet wide. We travel at six to nine miles per hour.

English-speakers are still fairly rare in the former East Germany, more common in the Czech Republic. Our cabin steward Hans speaks only a smattering of English, and on shore excursions we notice that most of our English-speaking guides are over the age of 60.

Breakfasts on board begin with a vast cold buffet that runs from smoked fish to steak tartare to cold cuts and cheese, plus fresh fruit, cereals, yogurt and freshly baked rolls and croissants. In addition, bacon and eggs are prepared to order, along with a daily breakfast special that may be German apple pancakes one morning, a hearty farmer’s omelet with potatoes and onions another morning.

The Prussian Princess cruises between Hamburg and Dresden with a call further upriver in Decin for an optional all-day excursion to Prague, capital of the Czech Republic. Most tours in the old cities are walking tours, from the walled city of Tangermunde to Martin Luther’s historic Wittenberg. Meissen, where a visit to the famous porcelain factory is available, and Dresden, where a performance at the spectacular opera house may often be booked, are particularly outstanding.

Fares for the Deck A cabins with French doors aboard the Prussian Princess are $1,840-$2,340 per person, double occupancy, depending on season, with the cheapest fares set aside for the last sailing of the season, Oct. 30. Lower deck outside cabins without the French doors are $1,595-$2,115 per person, double. Transfers within Germany are included, but transatlantic air fare is extra. Air packages from 33 gateway cities are available from Cunard.

The 1994 season aboard the ship runs March 26 through Oct. 29.

The 200-passenger Danube Princess sails round trip from Passau, Germany, along the Danube to Grein, Melk, Durnstein and Vienna, Austria; Bratislava, Slovakia, and Esztergom and Budapest, Hungary. Fares range from $1,435 (for an inside cabin) to $2,340 per person, double, for seven-day sailings. Some 11-day cruises into the Black Sea are also available at fares from $3,185 to $4,025 per person, double.

And the 144-passenger Princess de Provence plies France’s Rhone and Saone rivers round trip from Lyon with calls at Chalon-sur-Saone, Macon, Trevoux, Vienne, Tournon, Avignon and Arles. Fares range from $1,595 to $2,340 per person, double occupancy, depending on accommodations and season. Last sailing of the season is Nov. 13, and the 1994 season runs from March 26 through Nov. 12.

KD’s Elbe River seven-night cruises are priced from $1,050 to $1,750 per person, double, depending on the season and accommodation. Air fare is extra.

French Cruise Lines’ Normandie and Arlene are priced from $1,500 to $1,995 per person, double, depending on season and accommodation. Air fare is extra.

To get free color brochures, call Cunard at (800) 221-4770, and French Cruise Lines and KD River Cruises at (800) 858-8587.