THE RIVER ahead of us turns and disappears, a deep green tunnel lost in the splashes of red, yellow, bronze and gold of the surrounding mountains.

At the bend, a great old tulip poplar leans over the water, limbs spread as if ready for flight, and catches a shaft of sunlight in its yellow leaves. The river takes on the tarnished glow of ancient coins.

An aluminum canoe floats into the scene and picks up the glow, a golden barge on a golden river. Even the bright red leaves of a downstream dogwood are gilt-edged.

As if the sight alone were not breathtaking enough, the river mirrors it, each line and nuance of color.

Like Clark Gable as Rhett Butler, fall on the river is unmatched.

Certainly, the traditional drive into the mountains can offer a stunning panoramic view, but sometimes the awe and pleasure get lost in a haze of exhaust emissions and a line of traffic longer than the season. And that's when a river can be a great friend.

Canoe, rowboat, raft, sailboat, cruiser or paddle wheel -- it doesn't really matter what kind of craft as long as it's suitable to the trip, comfortable and safe. All you'll miss is the crowds.

Every color on the season's palette is represented in the maples, sycamores, dogwood, beech, hickory, oaks, sweetgum and poplars that line the banks of this region's waterways.

The rivers of the Potomac watershed showcase the colors particularly well because they cut so deeply into the Appalachians and the Piedmont Plateau: The reds and rusts, yellows and browns are hung, gallery style, vertically against steeply rising ridges.

This normally is a dry time of the year, so there is less sediment washed into the rivers, letting you see the rocks and ledges of the streambed and providing a superb mirror to double the image and double the fun of Mother Nature's annual extravaganza.

Sometimes the doubled image is as detailed and precise as a Kodachrome. Sometimes, with a bit of wind wrinkling the surface, it is more impressionistic, colors flowing into each other as in a Monet.

The view from water level puts you within the scenery, rather than outside and detached. You can almost hear the exhalation of the trees, feel the leathery texture of the changing leaves. Of course, it helps if you are something of a romantic as well.

And it helps that a river tour can give you an extra advantage in gauging the right moment for a trip. The colors tend to change a little later and remain a little longer along the larger rivers, says Dr. Stanley Krugman of the U.S. Forest Service, because there is more moisture in the soil along the river bank.

So come on along for a quick tour of some favorite fall streams:

WHITEWATER RAMBLES The first is a section of the Potomac that George Washington mentioned in writing about his 1748 trip into the wilderness. Near Romney, West Virginia, it's called the Trough because it cuts some 2,000 feet deep between two "ledges of mountains -- impassable," as Washington put it.

The spectacular rock formations and the soaring mountains are the backdrop for the trees and make this the most popular fall canoe trip year in and year out.

I remember best a day when the leaves were dropping all around us, making small dents in the water and drifting downstream with our flotilla.

The Trough is mostly flat, slow water, and we floated lazily with a slight breeze at our backs. It was enough, though, to snap off the leaves and send them into spins and curlicues and swings, choreographed by a micrometeorology we could neither see nor feel. They landed with soft plop-plops, sometimes in the boats. Once on a paddle in mid air.

We stopped for lunch, tying the canoes to a tree, and when we returned, leaves had piled up on the water between them, completely covering the patch in a multilored, crazy quilt.

Camp Wapacomo Campground, west of Romney off U.S. 50, is a handy site to check conditions, camp overnight and take out your canoe at the end of a day's paddling.

Another, easy canoe trip with mountains on both sides is the Paw Paw section of the Potomac. This usually is an overnight trip, and the nights are cool even in summertime. The river is wide and flat, with switchbacks for about nine miles.

The Sorrel Ridge Hiker Biker campground on the C&O Canal is at the south end of the Paw Paw Tunnel, which carries the canal through the mountain that the river has snaked around all day. It's fun to walk back through the tunnel and marvel at man's achievement as well as nature's.

The second day's paddle, from the campground to Little Orleans, is straighter and shorter.

The Cacapon River in West Virginia is my particular favorite. The scene described at the beginning is from a section that runs from Capon Bridge to Route 45.

Its as isolated as the Trough and Paw Paw and has brilliant colors combined with some fascinating rock formations right at the edge of the river. One is a cross-section of a wooden ship's hull, or so it seems to me, that really illustrates the compression and folding of the Appalachians. Another is creased and peaked like a chapel and, naturally, is called Chapel Rock.

The most memorable view, though, is Caudy's Castle, a ragged pinnacle of rock rising above the surrounding hills and forests just at a bend in the river. On any but a rainy day, the rock, the trees and the sky are mirrored in the river along with the shapes and colors of the forest.

The disadvantage of this section of the Cacapon is that it should not be attempted by novice paddlers, especially at this time of year. It's best to visit this stretch in company with an outfitter guide service or one of the canoe clubs.

A particularly beautiful fall whitewater run accessible by commercial raft is the Staircase of the ShenandoahRiver at Harpers Ferry.

Here, where the Shenandoah and Potomac merge, the mountains converge also. From the Staircase, so called because the riverbed is a series of diagonal ledges, mountains appear to surround you, as though you are on the stage of an amphitheater.

"With migrating flocks overhead and the brilliant colors of fall foliage on the Blue Ridge Mountains," this stretch offers "the perfect setting in which to savor autumn," says Lee Baihly, whose River and Trail Outfitters runs raft trips on the Staircase.

Baihly and two other companies, Blue Ridge Outfitters and River Riders, also offer canoe trips along this section for those who have the experience to handle them.

Like Baihly, though, Mark Grimes of River Riders recommends raft trips for people who know nothing about canoeing. "Canoeing whitewater is not something people should do without previous experience," he stresses.

A milder section of the Shenandoah, between U.S. 50 and U.S. 7, is also worth a trip. While there is more farmland than on the Staircase, there are long stretches where hardwoods rise around you.

The Rappahannock River, on the Kelly's Ford section, is also highly recommended for a fall colors trip. It's mostly flatwater, with one whitewater rapid. The Rapidan, a tributary of the Rappahannock, is also recommended.

One expert, canoeist-author Ed Gertler, recommends the milder-mannered sections of two of the region's popular wild- water rivers: the Cheat and the Youghiogheny.

On the Cheat, Gertler suggests the section between Parsons and Rowlesburg, West Virginia (the river gets very nasty very quickly below Rowlesburg): "It is awfully beautiful. It has birch, beech, maple, magnolia, oak, all the eastern hardwoods and it is a pretty easy stretch of river."

On the Yough (pronounced Yock), as paddlers call this powerful river, Gertler recommends the middle Yough, which is the mildest section and ends at Ohiopyle, Pennsylvania.

My year-round favorite setion of river is closer to the city. It's Mather Gorge, a spectacular, steep-sided channel below Great Falls, where the Potomac runs through an ancient fault line.

At times it can be heavy whitewater. But, again, it is served by commercial rafters. Both Blue Ridge Outfitters and Potomac River Tours run trips through the gorge and on down the Potomac to Lock 10, which is well above the danger of Brookmont dam.

The attraction of Mather Gorge lies not in the density of the trees, but in their scarcity. There are trees high above along the rim and some on the rock face, clinging to little more than imagination. But they are few and, for that reason, more striking in their solitude.

The walls of the gorge are a combination of bedrock metagraywacke and mica schist, plus intrusion rock such as quartz, and themselves offer a spectacle of textures and colors that change with the angle of the sun and the moisture in the air. It is a place of eons, really, not seasons.

Yet, against this blue-gray background, small shrubs and trees strut and preen like rock stars on a floodlit stage.

There is one small red maple that is hardly worth noticing for most of the year except for the tenacity of its toehold in a crevice.

But come the fall, it turns a brilliant crimson and, for a while, dominates its piece of eternity.

FLATWATER FLOATING Downriver, below Fletchers Boat House, there's an easier- to-reach and flatter section of the river that has a semblance of the Gorge's steep rock wall and a wider range of trees and shrubs.

The Virginia Palisades are high and sheer where it was quarried, but the Maryland shore between the river and the C&O Canal is thick with deciduous trees and flowering shrubs and, a couple of weeks ago, even a few wildflowers.

You can rent canoes and rowboats as Fletchers, but do not head upstream. There lie the unspent, treacherous and swirling currents left from the river's tumultuous journey over Little Falls.

In town, Theodore Roosevelt Island offers a Joseph's coat of color from the river, and it, too, is easy to reach. Canoes and rowboats, sailboards and even small sailboats are available from Thompson Boat Center at the mouth of Rock Creek or from Jack's Boat House at the foot of K street in Georgetown (under the Key Bridge.)

Below the city, the river is again heavily wooded, thanks to the scenic easement that protects the setting of Mount Vernon. Sailboats, motorboats, tour boats, yachts and an occasional ocean vessel cruise here. The variety of trees, the number of creeks that notch the shoreline and the profusion of birds make this section especially beautiful.

Sailboats are available for rent from the Washington Sailing Marina and Belle Haven Marina, both south of National airport along the George Washington Memorial Parkway. Several tour boats, including those to Mount Vernon and a paddle wheeler, cruise the lower river. The Dandy offers lunch and dinner cruises year round.

COASTAL PLAIN CRUISES There's much to be said for fall colors on the gentle marshland streams of the Coastal Plain.

"Most people don't think of the Coastal Plain for fall colors," says author Gertler, "but it can be just as beautiful. The Pine Barrens in New Jersey, for instance, are just fantastic."

Deciding between the coastal plain and the mountains streams, the Forest Service's Krugman agrees, "is not an either/or choice. You get the same colors, just different species, and there will be more conifers, evergreens" on the plain. "The reason people tend to head for the mountains is that the change there is more abrupt," he adds.

The Pine Barrens are really spectacular. Their small rivers, flat and fast-moving, are the color of strong tea and lined with a combination of evergreens and eastern hardwoods. And yet the barrens are eerily empty, considering how close they are to Atlantic City and the New York megalopolis.

There is a fine campground at Batsto State Park, and a reconstructed village of the same name. The Wading River flows through the campground, making it easy to begin or end a trip at home base. If you ask around, you can have the special treat of watching cranberries harvested by underwater thrasher.

Accommodations other than the campground are slim through here, though.

The Pocomoke Riveron Maryland's Eastern Shore is another coastal stream that Gertler recommends: "Wildlife and waterfowl abound," he reports, and the banks are heavily wooded, though the original cypress swamp has been severely damaged by lumbering and fire. Time has brought back the trees along the river, and Gertler says, "It has all the right kinds of trees for brilliant fall colors."

It also is easily accessible to canoes even near its headwaters.

A FINAL FLING A word about one last spot, the C&O Canal. While it doesn't have the full wide-screen impact of a river, it's nearby, easy to manage and pretty, especially the section called "Widewater," which is a shrt distance north of the access spot near the Old Angler's Inn off MacArthur Boulevard. MAKING YOUR WAY ON THE WATER

If you're thinking of a river tour, here are some outfits that offer raft or boat trips or canoe rentals. Many also provide advice on water levels.

THE TROUGH

EAGLE'S NEST OUTFITTERS -- Petersburg, W. Va., 304/257-2393.

PAW PAW

LAZY RIVER CANOE COMPANY -- Little Orleans, Md., 301/478-2701.

CACAPON RIVER

DON WALFORD -- Capon Bridge, W. Va., 304/856-2629.

STAIRCASE

RIVER AND TRAIL OUTFITTERS -- Knoxville, Md., 301/834-9950.

RIVER RIDERS -- Knoxville, Md., 301/834-8051.

BLUE RIDGE OUTFITTERS -- Harpers Ferry, W. Va., toll-free 554-2560.

ELSEWHERE ON THE SHENANDOAH

FRONT ROYAL CANOE BASE -- Front Royal, Va., 703/635-2741.

SHENANDOAH RIVER OUTFITTERS -- Luray, Va., 703/743-4159.

RAPPAHANNOCK AND RAPIDAN

CLARK BROTHERS -- Warrenton, Va., 703/439-8988. CLORE BROTHERS OUTFITTERS -- Fredericksburg, Va., 703/786-7749.

MIDDLE YOUGHIOGHENY

RIVERSPORT -- Confluence, Pa., 814/395-5807.

CHEAT RIVER

TRANSMONTANE -- Davis, W. Va., 304/259-5117.

MATHER GORGE

BLUE RIDGE OUTFITTERS -- Harpers Ferry, W. Va., toll-free 554-2560.

POTOMAC RIVER TOURS -- P.O. Box 30061, Bethesda, Md. 20814; phone 530-8733.

LOWER POTOMAC

FLETCHERS BOAT HOUSE -- 4940 Canal Road NW, 244-0461.

POTOMAC PARTY CRUISES -- Alexandria, has the "Dandy" lunch and dinner cruises, 683-6076.

SWAINS -- 10700 Swains Lock Road, Potomac, Md., 299-9006.

THOMPSON BOAT CENTER -- Rock Creek Parkway and Virginia Avenue NW, 333-9711.

WASHINGTON BOATLINES -- Pier 4, Sixth and Water streets SW. Has a paddle wheeler, "The First Lady," and other tour boats, 554-8000.

PINE BARRENS

MICKS' CANOE RENTALS -- Route 563, Jenkins, N.J., 609/726-1380.

POCOMOKE RIVER

POCOMOKE RIVER CANOE -- Snow Hill, Md., 301/732-3971.