Geology of the Great Basin

14. Water

water 14 The Great Basin is a land of contrast. The outline of the driest region in the United States is defined on the basis of the flow of surface water. The longest river of the region, the Humboldt, rises within the confines of the Great Basin, flows 330 miles to the west, and finally dries up before leaving the region. In fact, all the major rivers of the Great Basin—the Humboldt, Carson , and Truckee—begin and end there. Many of the deep valleys are floored by monotonously flat mud-surfaced playas, or dry lake beds, that are usually shriveled and parched. Yet the flanks of the mountains bounding these desiccated valleys are etched by the shorelines of deep blue-water lakes that once filled most of these valleys. Such are the contrasts in water in this land of contrast. flash flood I remember a night camped under a deep blue-black sky perforated by brilliant sparkling stars. The black outline of a vertical sandstone cliff hung over me like the confining borders of a giant coffin. I chose a small alcove on the cliffside for my overnight home. It afforded a pleasant twilight view down the steep-walled canyon to the Colorado River tumbling southward through Cataract Canyon one mile distant. The night air was soft, with light, wandering , cool breezes ruffling the sleeping bag. I soon drifted off to sleep. Within a deep dream, a resonating rumble became part of a train ride over a high trestle. The sound slowly crescendoed and finally penetrated my dream with such authority that I lurched into wakefulness. The ground shook, and the parabolic walls of my alcove focused the cacophony onto my sleep-dulled Water 193 brain. It was like a continuous roll of thunder or the insane pounding of snare drums by a demoniac drummer. The sound filled everything. Through the pale starlight a surging mass plunged downcanyon. With a fluid solidarity it moved toward me. Occasionally, the falling rounded contours of its face were pierced by branches or a tree that briefly leaped ahead of the surging mass and fell to the foot of the moving wall. The sound and sight were almost beyond my reason. Fortunately, my alcove was above the flood’s crest, since I was transfixed in my sleeping-bag cocoon watching the tumult approach and fill the canyon below me. The land of water contrast. The day had been warm and clear. Far to the east, black thunderclouds had climbed. They had grumbled and slashed lightning threats all afternoon. Hours later and miles distant from me, the storm-released waters had gathered together to create the desert flash flood. By dawn, a muddy-slick surface coated everything below me, and a cocoa-brown flow still filled the central channel of the canyon. In the land of drought, the respite becomes almost gluttonously extreme. I have viewed scores of these incredible events since and am always transfixed by each as though the sensory overload saturates my mental capacity for the unreal. The lessons of flash floods are important to remember. Too many residents of the Great Basin are new to the region and familiar with only the urban environment. Sudden destruction can roar down a wash in either the desert or a city. Urban areas are faced with a difficult choice. Do you use flood-control methods sufficient to handle the usual flood, or do you build oversized structures to handle the worst possible event? Hydrologists discuss flood events in terms of their recurrence intervals. This is a convenient statistical measure of the threat of a flood. A ten-year flood is the largest flood you would expect every ten years. These are determined by studying the record of past flooding. A one-hundred-year flood is likewise the largest flood you would expect to occur during a one-hundredyear interval. Since it is a statistical measure, it does not mean that once you have had the one-hundred-year flood, you are safe for another ninety-nine years. The statistics always recycle, such that you have the same chance tomorrow as you had today. You can have two one-hundred-year floods only a few days apart. Stated another way, there is a 10 percent chance in any year for a ten-year flood and a 1 percent chance for the one-hundred-year event. It is vital to understand, however, that a one-hundred-year flood is not merely ten times larger in...