It isn't just psychological, though. The river corridor is different. Birdlife flourishes. Troops of Canada geese hold the ground at every bend. I catch up with a coyote swimming across the river, and she climbs to shore 10 feet away, bony with dripping fur. With a quick shake, the animal fluffs herself and trots downstream. I keep pace, paddling just off her shoulder. She squats to put a scent on the ground, with a candidness I am unaccustomed to among coyotes. I try not to stare, impolite at such close range. We continue together until she finds herself cut off by a small backwater bay. She stops and finally looks at me with dark, inquisitive eyes. Then she trots the other way, and the current carries me downstream.

That bay is the first sign of the reservoir. Water has begun backing up, slightly more penetrable to light as sediment settles out. In the next mile or so, the main current grows more elusive -- meandering through an expanding fringe of slow water. In evening light, I pull into a massive, tilted outcrop, the shore thick with soft, knee-high vegetation, weedy invasives, Russian thistle. Bats chitter around my head snatching up midges and mosquitoes. I lay my sleeping bag in the desert on top of red sand and sun-beached clamshells.

At dawn I pack up, unleash my boat from fresh spider webs, and move onto the water's gradual twist. Within a mile, I cannot count on much of a current. No more idle moments staring at the canyon: I need to pace myself with long strokes. I come upon random currents carrying sticks, seeds and little moons of rabbit pellets, but the reservoir grows. What was once a hill becomes an island. I navigate across floating cattail reeds, flower petals and Styrofoam, while grebes pop up and down, heads periscoping to watch me pass. Each time I think the river is gone, I find vestiges again. It rises and falls, as if it were tunneling deep below the surface, a submarine breaching, then descending through the murky vaults of Lake Powell.

I once spoke with a Park Service ranger who worked to recover the body of someone who drowned on this part of the river. She described floating particulate matter obliterating sunlight to a depth of 20 feet. Down deeper, the water became textured, viscous, until it felt like she was swimming through chocolate pudding.  Below 50 or 60 feet, the abyss of half-suspended mud felt like liquid concrete and it became hard to move, and for a moment she panicked, her fins trapped in the lightless tomb of mud where she had to unbuckle and extract them.

Here, the river's current may more or less disappear from the reservoir's surface, but the sediment it delivers keeps moving in the form of gravity flows, streamers of mud and silt slowly but inexorably advancing along the reservoir's floor toward the dam. In this fluctuating underworld, side-canyons dump their own sediment-deltas into the mainstem, forming subaqueous dams through which both San Juan River current and sediment carve their own channels. In the dark water far below, new canyons form within the sediment itself, and in places, walls of the original bedrock canyon have collapsed, forming dams where fine mudfalls spill over one after the next. "There are real small currents heavier than the rest of the water," explains Mark Anderson, head river ecologist for Glen Canyon National Recreation Area, who has been involved in mapping the underwater terrain of Lake Powell. "They are turbidity flows, not actually water, but they can roll down through the basin over a long period of time."