148 
HYDROGRAPHY IN THE UNITED STATES 
Permanent river stations are estaljlished at man}" points on im- 
})ortant 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 hydrograi)hers. 
The quantities thus ascertained furnish the basis for compari- 
sons day by day, month by month, and year l)y year, throwing 
light U])on the relation Ijetween preci})itation 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 apj)earance and di.sapjiearance 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 sloj)e as oldained by 
the toi)Ographers, their volume as measured l)y the hydrogra- 
])hers, and their composition as determined h}" the chemist, are, 
however, simjfle matters in comparison with; those which relate 
to the waters immediately beneath the surface. In the fir.st case 
the ])henomena are visible and tangible; in the second, keen 
ol)servati(jn must he followetl 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 ])ressure under the influence 
of which it may rise toward the surface, are all details which 
vary with the geology of the i)articular area. To be aide to i)re- 
dict that water can be found at a given })lace, 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. 
ToAvard this end the Geological Survey is collecting and i)utting 
ui>on record all obtainable data concerning deep Avells, Avhether 
successful or not, and is making examinations of the Avater-bearing 
rocks Avherever they come to the surface or are penetrated by 
underground Avorkings. In the course of the prei)aration of the 
systematic sheets, de.signed ultimately to coA’er the AA'hole country, 
much of this AA’ork has been done, but in certain ]Aortions of the 
country, such as the subhumid, Avhere information is needed in 
advance of the completion of these atlas .sheets, the held exami- 
. nations of the hydrogra])hic division are l)eing pushed forAA'ard 
for this one object. The inve.digations of this division are thus 
seen to touch very closely the Avork of the Weather Bureau in its 
records of precipitation and in its material for flood prediction, 
