Updated 5/17/11

The Lower San Juan River courses through a rather forsaken landscape of clay hills and redrock plateaus in southeast Utah. At the end of a long, dusty road, there is a boat ramp at the water's edge where, at any warm time of year, vans and roof-racked Subarus bake in the sun while their owners are out on the river. This is the end of the line. A large sign stands along shore, smacked with big red letters:

DANGEROUS
WATERFALL AHEAD
DO NOT PROCEED

The sign and the waterfall mark the transition from a fast, muddy river to a bizarre and almost forgotten landscape created by upper Lake Powell, backed up behind Glen Canyon Dam a hundred miles away. Everyone gets off the river here.

In the bright and cloudless sunlight of a desert afternoon, I pump up a ducky -- a bright yellow inflatable boat that looks like a swollen banana -- and drag it down to the water past a group of river runners hauling rafts and kayaks onto the boat ramp. They pause and watch as I toss in drinking water and a drybag for my gear, and push into the current. They seem puzzled: Didn't I notice the sign? Am I just crazy? Before I can offer an explanation, the river has taken me out of sight beyond a bend in the tamarisk-thick shoreline.

River-runners generally know the San Juan by the river that lies above here. Swift and wild, it meanders through canyons where hackberries, cottonwoods and massive red boulders congregate along the shore. But few know the river just downstream, where it becomes an enormous science experiment in silt and fluid mechanics.

Lake Powell, which now starts several miles below the waterfall, is the second-largest artificial reservoir in the United States. It is the seeming endpoint for four rivers: the Colorado, Dirty Devil, Escalante and San Juan. But the reservoir isn't really an endpoint at all. These rivers aren't dead, and their persistent dynamics are slowly, steadily driving Lake Powell toward its demise.

I had come to see the San Juan in action. This confluence of moving and still water changes dramatically year by year as reservoir levels fluctuate. Mud flats grow and shrink, exotic tamarisk trees explode to the horizon, then drown underneath high water. The river itself is unstable, prone to jumping its channel, whipping across the desert in search of a new course. The waterfall that the sign warns of appeared along the river in the '90s, then vanished. A new waterfall formed about 10 years ago, this time miles downstream of the first, as if it were migrating.

This bottom stretch is legendary among river runners, although few have actually seen it. The only one I know is a dirtbag guide out of Bluff, Utah, named Paul. He told me that he caught a glimpse of the waterfall after crashing for hours through sweltering tamarisk thickets, dragging his kayak behind him. Advising me before my trip, he emphasized, "You just do not want to go over the waterfall, that's all."

Certainly not in my yellow banana.