VERNAL, Utah — It is tempting to think of a river as a creature of liquid, its character deriving entirely from the water that forms the coils and loops of its serpentine body. But in truth, the character of a river depends almost entirely on the rock across which it flows. To think of it as fundamentally made of water is a bit like regarding a face as the mere arrangement of lips and skin rather than as an outward expression of the bones beneath.

Take this river, for example. It's the Green, the longest and therefore primary tributary of the great Colorado. The Green rises in the Wind River Range of Wyoming, an alpine realm where elk bugle in the meadows, pikas squeak among ice-shattered rocks, and snow clings to the crags even in late summer. Nearly 2,000 miles later, having joined the Colorado in Canyonlands National Park, it empties into the Gulf of Cortez, the desert realm of coyotes and ravens. Between source and mouth, it undergoes a hundred transformations, each determined by the underlying geology.

A few years ago, I basked in the slanting autumn sunlight and ate my lunch on the shore of Peak Lake, the first gathering of waters in the Green River drainage, a two-day hike from the nearest road and 1,840 miles from the river's outlet. I had backpacked to that spot as part of my effort to understand and describe the most important river system in the West, a key source of water and power for 30 million people.

In those first miles of its life, the Green River splashes down a cascading staircase of boulders and talus ripped from the ragged peaks. When it slows to cross broad meadows, it seems to slumber, gliding quietly between sandy banks topped by moose-browsed willows. But in the quiet of twilight, the meandering stream has a voice, a rasping whisper produced by the friction of suspended rock flour sliding across the cobbled river bed. This fine sediment, the powdered residue of rocks still being gnawed by glaciers among the Wind River summits, gives the river a curious aquamarine tint.

When it emerges from the meadows, the river widens and forms a series of lakes. But after leaving the outlet of the last lake, the Green again grows boisterous, skipping and leaping across an open landscape of sagebrush and pines as the gradient steepens and boulders again interrupt its passage.

And so it is with all rivers. The Missouri and Mississippi, meandering across the flat and loamy lands of the American plains, are powerful and muscular rivers, but they move with hardly a sound and with no disturbance in their smooth surfaces. Rivers tumbling out of California's Sierra Nevada, such as the Merced and the Tuolumne, however, must etch their way through the tougher geology of massive, erosion-resistant granite, which forces them into a series of pools and big drops. Tugboats and barges ply prairie rivers, but only kayaks and rafts can navigate the rock-governed whitewater of a mountain or foothill stream.

Now I am tracing the Green through the gorges of Dinosaur National Monument. Here in the red-stained canyon country of eastern Utah and western Colorado, the rivers sink deep into ancient layers of sandstone, the compressed remnants of beaches, dunes, bars and spits. Where the rock is particularly tough, the walls close in and the water gains speed, creating eddies and mysterious upwellings that spin boats in unpredictable ways.

Flaws and faults in the canyon walls give birth to tributary canyons, narrow slots carved by flash floods that ferry boulders and other lithic debris into the main river channel, producing falls and rapids. Where the stone is particularly soft, the channel widens and the river becomes a murky, dawdling creature, a trickster who sometimes forces boaters to clamber from their mud-grounded craft and drag them to deeper water.

But even though it is the landscape's rocky skeleton that shapes the river, it is water that makes the river a repository of magic. Stand in its flow, or buckle on a flotation vest and let the current carry you on its back, and you feel yourself connected through that liquid medium to everything above and below you in the watershed: wading moose and glacial silt on the craggy roof of North America, the dusty skeletons of Mesozoic sea creatures entombed in the Colorado Plateau's layered stone, bulrushes and wading birds in quiet lagoons at the exhausted river's desert mouth.

It is this magic that brings some of us back again and again to the river, any river; each is an elemental cord binding past to present, yoking together mountains and deserts and plains. Touch a river, and you touch time; stand in its swirling current and you join a living web that spans the continent.

— John Krist is a senior editor and Opinion page columnist for The Star. To read previous columns, visit www.johnkrist.com. His e-mail address is jkrist@VenturaCountyStar.com.