FOR AN INSTANT, the raft was airborne. Then it slammed into the torrent below, buckled and pitched crazily. I lost my balance and slid to the bottom of the raft.

"Get up!" the guide commanded, and I tried, but the river was pounding us up, down, sideways. Water cascaded everywhere.

Finally, one punch rolled the raft hard to my side. I jammed my elbow deeper into the raft's side, pushed against the foot stirrup and got back up just as we slid around a boulder and headed into the next big drop.

I never stopped paddling, and I was enormously proud of that, because this was the upper Youghiogheny River.

The upper Yough (pronounced yock), in Maryland's Garrett County, is the ultimate rafting challenge on the East Coast. Fast and choked with boulders, complex and physically demanding, it is unforgiving and unforgettable.

The first mile and a half on the river is a deceptive introduction. The water flows gently over a gravel bottom, and raft guides use the time to have their customers practice responding to commands. The message is clear: "Do exactly as I say, when I say it and only then."

On the upper Yough -- so called to differentiate it from the better known and less difficult "lower Yough" in Pennsylvania -- the commercial rafting companies provide a guide in every raft because the river is so difficult to navigate. Each company also has an expert kayaker in a safety boat to quickly pick up anyone tossed from the raft.

On this day, our guide was Roger Zbell, one of the owners of Precision Rafting Expeditions of Friendsville, Maryland, who continued our schooling until we stopped before the first rapid to wait for the water. The upper Yough can be rafted year round because of Deep Creek dam, which releases water during the two hours a day that it generates electricity.

When the water had risen enough, we pulled out and headed for the first rapid, Gap Falls. On the international difficulty scale of I to VI for whitewater, with six being severe risk of life, Gap Falls is a III. It was fast and bouncy, but pretty straightforward compared to what was ahead.

The first "real" upper Yough rapid is Bastard Falls, a Class IV bank shot that we made by caroming off a huge boulder. We needed the carom in order to miss the exploding waves and the backwash at the bottom of a five-foot ledge.

We had entered a steep-sided gorge with tree-covered ridges sometimes 600 feet above us, and after Bastard Falls, the river went mad, a wild stallion on loco weed.

It plunges downhill at an average gradient of 100 feet per mile, but for 3.5 miles, the streambed pitch reaches 120 feet per mile achieved in a series of five-

(By comparison, the lower Youghiogheny below Ohiopyle, Pennsylvania -- the most popular rafting trip on the East Coast -- drops at 32 feet per mile. And the Potomac from the foot of Great Falls through Little Falls drops at an average of eight feet per mile.)

At the bottom of the ledges, the falling water creates great sucking holes where the river runs backwards and where rafts and rafters can be trapped and recycled over and over as in a washing machine.

Between the ledges, boulders choke the chutes and make the river a sieve that the rafts must try to squeeze through as a fly might try to squeeze through a screen.

Safe routes are impossible to find if you don't know this river because the normal river clues are hidden among the rocks.

The rapid called Charlie's Choice is named for a well-known and highly skilled paddler who got so confused and frustrated in it that he just took his boat off the river and walked out.

All together, there are 20 class IV and V rapids on the upper Youghiogheny, with names like Heinzerling, Rocky I, II and III, Cheeseburger and Meat Cleaver, which can make chopped meat out of anything.

After the first few, though, they seem to be one, continuous intimidating rapid.

Some of the chutes through the maze are just wide enough for a raft. Some aren't even that wide. On the latter, Zbell would have us scramble to one side of the raft so it could slide through on edge, like Kit the "Knight Rider" car on two wheels.

Then, we had to bounce, crawl and claw back into place in time to drop over a five-her thundering suckhole. Water crashed over us, saturating us, blinding us, filling the raft.

Frequently we had to pull ashore to empty the rafts. In between, we bailed with a five-gallon bucket.

I fell to the bottom of the raft several times. Most of the time, I got up without help but once I didn't get up in time. We broadsided the chute above Heinzerling and lodged between two jutting rocks just at the edge of a five-

Zbell, the miracle man, worked us free, directing us to "this side, carefully," and we plunged to the bottom paddling furiously to get past a backwash that could have recycled raft and all until the dam shut off.

Adrenaline was in flood during the whole run, but conscious fear was absent.

The big nasty drops and powerful hydraulics came as surprises, hidden by boulders and bends. We were always so busy manuevering that we didn't see them in time to get scared.

Somewhere, we passed a beautiful waterfall, and as we neared Friendsville, the river slowed down and turned peaceful.

When we hit Friendsville, we showered and changed and gathered outside for hot steak-and-cheese subs and cold beer.

We ate and joked and lied to each other about how scared or brave we had been -- lingering, remembering the upper Youghiogheny, the ultimate challenge.