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Show Chapter 8 Surface and Ground Water WATER IS THE VITAL LINK of all things living. Fresh water is our primary self-renewing resource. Our share of this vast resource, here in the United States, is enormous. The total quantity in constant circulation, measured in precipitation, amounts to about 4,300 billions of gallons daily. This is roughly 10 times the aver- age flow of the Mississippi River. Some 3,000 billion gallons a day, on the aver- age, return to the atmosphere as vapor, through evaporation and transpiration. This leaves the annual runoff to the sea at an average of about 1,300 billion gallons a day. Our success in making the best possible use of this water depends on the way we handle the forests and other vegetation which cover the ground, the way in which we farm, the extent to which we take advantage of the surface and sub- surface storage capacity, and the skill and fore- sight with which we manage our streams. This means that the keystone of a successful national water resources policy is to treat water and soil as great assets to be conserved and used, not wasted. Proper utilization of flood waters will be found to be a part of the answer to the call for flood control. The River Basin In its principal features a river basin is the result of the weathering, eroding action of count- less drops of rainfall, freezing and thawing, gathering to form streams, then rivers of increas- ing volume as water seeks lower elevations and in part finally reaches the sea, there to be evaporated and return again to the clouds in the ever-con- tinuing hydrologic cycle. Through eons of time the folded and sharply faulted upthrust mountain masses have been eroded away by this ever-con- tinuous action. Above the timberline, the rocks receive the di- rect impact of countless raindrops traveling at more than 30 feet a second. This impact, aided by alternate freezing and thawing, as well as glacial scour, has sharply eroded the mountain peaks. At lower elevations, there is a continua- tion of the same type of erosion, in spite of the natural timber, brush, and grass which at least to some extent cover the soil. The erosion is most serious in times of high precipitation and flood runoff, when muddy streams carry materials eroded from the uplands down into the valleys. Thus is created the alluvial valley fill of geologic time, or the broad deltas which spread out over the continental shelf. In some of the western mountain areas of good natural forest and brush cover, the erosion of single major storms has been measured as high as 50,000 to 100,000 cubic yards per square mile of drainage area. With sufficient eons of geologic time a high mountain mass becomes a low pene- plain, little higher than the valley, filled by ma- terials eroded from it by the ceaseless energy and chemical action of tireless water in its hydro- logic cycle. Perhaps we can visualize the energy of the streams if we realize that the 1,800,000 109 |