Over the Rivers (An Aerial View of Geology) - Hardcover

Michael Collier has written 14 books on geological themes. In 2005, the American Geological Institute honored his work with its prestigious Outstanding Contribution to Public Understanding of Geosciences Award.

An exhibition of Michael Collier's aerial views of geology will be held at the Houston Museum of Natural Science (September 13, 2008-March 1, 2009) and the Denver Museum of Nature and Science (March 15-August 1, 2009).

Waiting to Row

Rivers flow from the mountains to the sea. Nothing could be simpler; nothing could be more complex.

To understand rivers, we should start at the beginning, in the oceans. Ours is a blue planet where water covers almost three-quarters of the surface. If seawater, all 326 million cubic miles of it, simply stayed in the oceans, there would be no rainfall, no rivers. But the sun's heat evaporates seawater, every year lifting something like 77 thousand cubic miles of water into the atmosphere. Most condenses nearby and just falls back into the oceans. But a third of that evaporated water rises high enough and is carried far enough to fall on land. And with that rain, our story begins.

Earth's atmosphere is capable of transporting prodigious quantities of water. Forty weeks of intense rainfall throughout the American Midwest set the stage for the Mississippi River's great flood of 1993 -- a volume of rainfall equivalent to twice the size of Lake Erie. On the other hand, parts of California's Death Valley can be bone dry for a year or more. On average, that region of the Mojave Desert receives only an inch and three-quarters of rain annually.

If precipitation were equally distributed, every place on earth would receive thirty-one inches of rain each year. But local conditions wring variable amounts of moisture from the air. As any frustrated farmer knows, water vapor can remain suspended within clouds as they blow on by, perhaps to fall downwind on some other farmer's field. Proximity to an ocean can increase rainfall. For instance, the Pacific aims its moisture squarely at Yakutat, an Alaskan coastal town that receives more than 150 inches of rain -- ten feet -- each year. Precipitation is also likely to be higher near mountains like the Sierra Nevada, where the atmosphere is lifted and cooled as it flows inland. Finally, rain can be wrung from the skies when air masses collide. This occurred in the 1993 Mississippi River flood when cold Canadian air repeatedly encountered moist warm Gulf air over the Midwest.

Death Valley and Yakutat highlight meteorologic extremes. Perhaps it's more useful to squint at a map of the United States and try to see generalized precipitation patterns. The first and most obvious observation is that rainfall increases from west to east across the country. Eastern seaboard states typically receive 40 to 50 inches of rain a year. Farmers across the Great Plains can reasonably hope for 30 to 40 inches each year. But west of the 100th Meridian, in Kansas, annual rainfall steadily decreases -- 14 inches in Denver, and seven or eight in Phoenix.

Rain may fall, but we still don't have a river. Once it strikes the earth, precipitation can follow one of three paths: water can evaporate and rise back into the atmosphere, it can flow along the surface, or it can sink into the ground. A surprising amount reenters the atmosphere. Evaporation occurs directly from standing bodies of water like puddles or lakes. In some areas evaporation directly from the leaves of trees or crops can be as high as 30 percent of total rainfall. Additionally, plant roots intercept water that has just begun to soak into the ground. This moisture will be carried upward and released back through leaves -- a biologically essential process known as transpiration. Taken together, evaporation and transpiration return as much as 70 percent of a region's rainfall back to the atmosphere.

The remaining fraction of rainfall will either percolate into the ground or flow across the surface. Rates of percolation are highly variable, depending on the texture of the ground: porous soil versus fractured rock, absorbent sand versus watertight clay. Once underground, water flows very slowly-maybe 100 feet a year. Eventually groundwater collects in underground reservoirs called aquifers. During a heavy storm, soil can become saturated with rain that has already fallen. When water can no longer be absorbed, it must flow above ground. Water flowing overland combines with springs bubbling up from below to create first a trickle, a brook, then a stream, and finally a river.

Rivers may be the lifeblood of a landscape, but they represent a surprisingly small fraction of the earth's water. Of all water-within the oceans, floating in the atmosphere, locked up as glacial ice, hiding in underground aquifers, or biding its time in lakes -- streams and rivers account for only 0.0001% of the total on earth. But this percentage, tiny as it may be, has shaped the world in which we live. Farms have always sprouted where water is most readily available. Cities, dependent on easy transportation, long ago sprang up on the shores of major waterways.

A river is living water. Stand on the bank, look upstream and down, and savor the sense of living water. Every river is the center of a universe where change is a constant. Is the river rising or falling? Is the surface smooth and glistening, or is it chewed into currents and chopped into waves? Is the river clear today, or still muddy from yesterday's storm? Look upstream for the tenth time this hour -- is the boat coming down yet? Look downstream -- are the salmon still running? The splash and chuckle of a living river tells a landscape's most vivid stories; a river spawns a landscape's wildest dreams.

I learned about moving water while working on the Colorado River, rowing in the 1970s and 1980s through Arizona's Grand Canyon. If that river didn't teach me everything that I know, at least it put it all into perspective. I grew out of adolescence and into manhood on the Colorado, an oar in each hand. I'd tuck those oars under my knees and lean back against the raft's load. I was intoxicated, watching sun-spangled canyon walls spin circles around an impossibly blue sky. Sometimes my passengers would self-consciously clear their throats as we bore down on a rapid -- they seemed more comfortable when I paid attention to the ominous growl of waves beating against rocks.

My pulse quickened whenever we slid into Hance or Horn, Serpentine or Sockdolager Rapids. Ninety-eight miles into the Grand Canyon, the Colorado's muscular surface flexed and freshened as it rolled over the debris fan that forms Crystal Rapid. I'd already memorized every rock and wave that loomed downstream. We swept quickly past the last point where I might have rowed to shore and reconsidered this madness. Accelerating into the turmoil, I waited. There was absolutely nothing to be done for the next ten seconds -- agonizing moments that stretched into infinity. On Crystal's tongue, the smooth surface of the river began to heave and shudder, but still I took no strokes. To start rowing even a second too soon, to strike a rock instead of sliding behind it, was to court disaster. Wait. Listen.

Wait.

NOW!

With all my strength I pulled downstream and to the right. The boat slid off the current and cut into the slack water directly behind a granite boulder. The passengers didn't know it, but we'd already run the real rapid. The titillation of waves and rough water would follow soon enough. But for me, the most breathtaking part of river running was always that moment out on a rapid's tongue, oars suspended, waiting for the precise instant when I could take that first strategic stroke.

I've since moved downstream -- studied geology, learned to fly, survived medical school. But I haven't forgotten those lessons learned on the water, those prayers offered to the river. I never nose my airplane over into stormy turbulence, never walk into a medical maelstrom without remembering how it was once to slide down the tongue, oars motionless, waiting to row.

This book examines the ways in which rivers rise and flow, the means with which they erode and shape landscapes, and the ways in which landscapes in turn mold rivers. I learned rivers from water level, but I tell their stories now from the air. From the cockpit of my plane I can sometimes see most of a river basin from a single vantage point -- its headwaters in the distance, valley below, and mouth at sea level. More o