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National Geographic : 1896 Apr
Contents
148 HYDROGRAPHY IN THE UNITED STATES Permanent river stations are established at many points on im portant streams, usually near their headwaters, and daily records kept of the fluctuations. These fluctuations are in turn inter preted into quantities of discharge by means of measurements of area and velocity made at short intervals by the hydrographers. The quantities thus ascertained furnish the basis for compari sons day by day, month by month, and year by year, throwing light upon the relation between precipitation and discharge, and upon the modifying influences introduced by topography, geologic structure, and cultural conditions. The non-periodic fluctuation of waters, the questions of erosion, transportation, and sedimentation, the appearance and disappearance of surface streams and the minerals in solution are all matters connected more or less directly with this study of stream behavior. The surveys of the surface streams, their slope as obtained by the topographers, their volume as measured by the hydrogra phers, and their composition as determined by the chemist, are, however, simple matters in comparison with those which relate to the waters immediately beneath the surface. In the first case the phenomena are visible and tangible; in the second, keen observation must be followed by correct reasoning from well established facts and conclusions. The occurrence of under ground water in quantities sufficient to be of value, its character as regards mineral contents, and the pressure under the influence of which it may rise toward the surface, are all details which vary with the geology of the particular area. To be able to pre dict that water can be found at a given place, at a certain depth, and in quantity, it is necessary to know thoroughly all the facts which can be ascertained concerning the geology of the region. Toward this end the Geological Survey is collecting and putting upon record all obtainable data concerning deep wells, whether successful or not, and is making examinations of the water-bearing rocks wherever they come to the surface or are penetrated by underground workings. In the course of the preparation of the systematic sheets, designed ultimately to cover the whole country, much of this work has been done, but in certain portions of the country, such as the subhumid, where information is needed in advance of the completion of these atlas sheets, the field exami nations of the hydrographic division are being pushed forward for this one object. The investigations of this division are thus seen to touch very closely the work of the Weather Bureau in its records of precipitation and in its material for flood prediction,
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