We don’t expect adventure on a flatwater river, but sometimes it just happens.

An easy commercial trip on a hot morning last October started out innocently but crescendoed during a raging storm that night. Our three-raft flotilla put onto the Rio Grande at Lajitas just upstream of the Big Bend National Park boundary. The current and casual rowing carried us between low river banks thick with reeds and brush. Over river sounds, we heard the calls, twitters and trills of unseen birds.

“I’ve run a lot of rivers and spent a career looking for this river’s equal and rarely found any,” says Mike Davidson, who piloted one of the rafts. The former owner of Far Flung Outdoor Center still gets onto the river whenever he has a chance. Adana, the 28-year-old daughter of Far Flung’s current owner, Greg Hennington, was at the start of her river career, and this was to be her first overnight trip. Garry Merritt, county attorney of sparsely populated Real County, also runs the river whenever he can get away.

After a quick lunch at Terlingua Creek, we shoved off and made an almost-immediate turn directly into an opening in what looked like an impenetrable cliff where the river bored into the rock. The Rio Grande carved nine-mile Santa Elena Canyon through uplifted limestone, making the geologic equivalent of a surgically neat cut. Near-vertical walls rise to as much as 1,000 feet straight up from the water. Garry slipped his raft into that slim canyon entrance as easily a key fits into a door lock. Adana followed, and then so did Mike. The walls closed in on us, and just a ribbon of sky was visible overhead.

Santa Elena Canyon feels like a corridor of water between limestone walls that at some points are just about 50 feet apart. The canyon’s left wall is in Big Bend; the right in the Mexican state of Chihuahua. We joked that two kinds of cliff swallows flit across the canyon: American cliff swallows and Mexican cliff swallows.

In the late afternoon, we pulled out on Gourmet Beach, a small, stepped sandbar on the national park side that had somehow taken hold along the cliff. The guides set up folding tables, a charcoal grill and the groover, which is riverspeak for toilet, in a discreet spot hidden by vegetation and rocks. We set up our tents near on the highest level of the slim beach hopefully far enough from the cliff to avoid any falling rocks, ate heartily by fire’s glow and soon called it a night.

The air hung so hot and humid that I decided to sleep under the stars. I lay on top of my sleeping bag, covered just with the silk liner. I looked up at the ribbon of night sky visible between the canyon walls, then rolled over and quickly fell into a deep sleep to the lullaby of the flowing river.

First sprinkles, then fat raindrops accompanied by a stiff breeze woke me up at some dark hour. I started to pull my sleeping bag into the tent. As I stood up, a fierce wind ripped the lightweight liner from my hands. I figured it was either lost, or I would find it in the morning. I crawled into the tent and again dropped into deep sleep. It was raining much harder when Mike came around sometime later to alert the denizens of each tent that the rain-swollen river, fortified by water released from a dam upstream on the Mexican side, had risen 14 feet. The guides had already moved the rafts’ mooring lines and were keeping an eye on the water level.

Mike told us to stow all our personal gear in our dry bags and go back to sleep. If the water continued to rise, he would come around again. In that case. we would leave the tents and spend the rest of the night in the rafts, continuing downstream in daylight.

We woke up when sunlight filtered into the canyon. The rain had stopped. The river had subsided. And the groover had washed away and was on its way to the Gulf of Mexico. We wolfed down the river equivalent of a continental breakfast before we set off again. The water was swifter but still essentially flat – until we reached the fabled Rock Slide, named for huge boulders that peeled off the canyon Mexican-side wall and heaped in the river. It is a powerful and unpredictable churn of whitewater on what is otherwise a flatwater stretch.

Our boats made it though safely one by one, but someone fell off a small private raft going through at about the same time. His friends caught him, hauled him up and laughed as he clamored aboard. They kept going, and so did we. A hydraulic whirlpool caught one of our rafts for a few minutes, but Adana leaned all her strength into the long oars and bested the whirlpool to pull out. Then, as suddenly as we had entered the rough water, we were back on the smooth. The canyon widened and suddenly opened into what resembled a long, flat lake. The narrow sky became a big dome. Mountains zigzagged on the horizon.

As the night’s excitement faded, the slow timelessness of the Rio Grande took over again. We pulled over at the muddy takeout at Big Bend National Park’s Cottonwood Campground. It was the end of the trip, but not the end of the memories.