BENTONVILLE, VA. -- Folks who regularly fish the upper Potomac and Susquehanna rivers complained of a shortage of keeper-size smallmouth bass this year. Some said it marked a region-wide, general downturn in smallmouth angling.

If so, someone forgot to send word to the ageless Shenandoah, where the rocky shallows are as chock full of bass as ever.

"Remember, it's bad luck to catch anything on your first cast," said Jim Clay of Winchester, Va., as we scrambled down the bank to the South Fork Wednesday evening. But I couldn't stop that fat bluegill from speeding out from under a rock ledge to gobble my little white tube lure.

Nor did it prove bad luck. In the next two hours, as darkness gathered, Clay and I set the hook on scores of Shenandoah bass and sunfish. Many of the smallmouths were 11- and 12-inchers that smashed at our topwater lures, then leaped and tail-walked till we could bring them to hand and set them free.

"Leaping Lana from Texarkana," shouted Clay. "I do love it when they jump."

By dusk, when the fishing really gets hot, we'd caught so many we were both weary, so we sat on the bank munching sandwiches while the bass jumped and snatched at flies with no interference from us.

"I don't know about those other rivers," said Harry Murray, who lives on the Shenandoah's North Fork at Edinburg and teaches fly-fishing when he isn't running the drug store there, "but it's been one of the best summers we've ever had up here."

For evidence, Murray cited the success of a young student in one of his novice classes last weekend who landed three big bass in two hours -- a 3 3/4-pounder, a 3 1/2-pounder and a 2 1/2-pounder -- all on a size 8 popping bug.

"We weighed those on a digital scale," said Murray, somewhat defensively. "Now we're getting some rain to keep the {water} level up, so we should have good fishing right through October."

That suits me. What I love about wading for smallmouth in the fall here, in addition to the plentiful fish supply, is the beauty, simplicity and isolation of it.

Soon the leaves will start to turn along the thickly forested banks, attracting throngs to adjacent roadways. Yet there's so much room in the Shenandoah's North and South Forks above Front Royal, you can almost always find a spot to fish with no one in sight, and you're rarely in sight of a road.

The water should stay warm and low enough to provide ideal wet-wading conditions for another month or so, which means all the minimalist angler needs is a rod, a box of flies or lures, cutoff shorts, a pair of tennis shoes and the courage to press on.

"But watch yourself," said Clay as we forged into the 200-yard-wide South Fork. "The water is so clear you think it's only two feet deep, but it'll come right up over your shoulders."

The craggy rock ledges generally run horizontally across the river, and the water depth changes abruptly where a ledge ends and a hole forms upstream of it. The fish lie in these holes, waiting in the shade for bait to wash by.

For awhile Clay and I probed the rocky bottom between the ledges with weighted tube lures with some success. Then he tied on a floating topwater plug -- a 1 1/2-inch minnow imitation called an A.C. Shiner -- tossed it into a fishy- looking spot and let it sit there.

Ker-whomp! A bass shot up from the deep, slashed and missed, leaving foamy ripples in its wake. Ker-whomp! Another bass hit. This one stuck.

Clay's rod was still bent double, bowing to the fish, when I made it to his side, braving shoulder-deep holes and shin-barking rocks to get there. "Give me one of those," I demanded, grabbing at his lure box.

We worked our way a half-mile upriver from there fishing side by side, he casting right and I to the left. More than once we had doubleheaders on and joked about the bass-jumping competition we stirred up.

Meantime, fly fisherman Dick Blalock worked the river-wide riffle upstream where we'd left the car, enticing bass and sunfish from slackwater pools with an assortment of popping bugs and crayfish and helgrammite imitations.

"One thing about the Shenandoah," said Blalock when we reconvened: "If you see a place where there ought to be a bass, there's a bass there."

Nice thought. Nice place.

The Fishing's Fine Fishery managers in Maryland and Pennsylvania say they've heard the reports of low keeper bass populations in the upper Potomac and Susquehanna, but aren't worried yet.

Both Bob Bachman in Maryland and Rick Hoopes in Pennsylvania said the likely culprit is cold weather and flooding in the spring of 1984 and '86, which led to poor spawning success those years. Bass from those year classes would now be in the 12- to 15-inch range and are conspicuous by their absence.

Hoopes encouraged anglers to wait till next year, when bass hatched in excellent spawning conditions in 1987 and '88 begin to reach 12-inch-and-up size.

Meantime, the Shenandoah beckons with its abundance of bass. Murray recommends three stretches for autumn wading: South Fork below Bentonville. Take I-66 to Route 340 South, then a right on Route 613 to the river. Cross the bridge to parking area on the right. South Fork below Luray. Take 340 south from Front Royal, then 675 west to the river. There's parking at Bixler Bridge. North Fork near Edinburg. Take I-66 to I-81 south, exit 71 to Edinburg. Take Route 11 north two miles to Chapman's Landing.

For those who need a little help getting started, Murray has two more weekend-long fly-fishing classes scheduled this month. Call him at (703) 984-4212.

A Fish Wish List

The minimalist angler needs little gear to go smallmouth fishing in the early autumn -- just old sneakers, cutoff jeans, fly rod and flies or light spinning rod and a box of lures.

But the well-heeled equipment freak can spend a bundle gearing up. Here's what the proper smallmouth fly-fisherman wears:

Collapsible wading staff for those difficult, rocky spots; about $60.

"S.O.S.penders," a Mae West-style, horsecollar inflatable life vest; about $90.

Nine-foot or eight-weight graphite fly rod with weight-forward floating line and English-made, single-action reel; about $400.

Felt-sole wading boots with metal cleats; about $80.

Chest-pack fly box (about $50) with four trays for 250 wet and dry bass flies (about $500).

Polarized sunglasses with built-in magnifying lenses for tying on flies; about $50.

Proper fishing hat; about $25.

Laminated, wood-frame landing net with retractable tether; about $75.

Gooseneck flashlight for low-light fishing conditions ($15) and spare spools of 2X and 4X leader tippet ($5).

Grand total: $1,350. And thank heavens you don't need a boat.