HYDROGRAPHY IN THE UNITED STATES 
147 
of the stream, in order that the central office of the district or 
that at Washington may be informed in time to issue predictions 
or warnings as to impending disaster. The operations of this 
bureau, as far as they relate to the h3ulrography of the rivers and 
of the lake and seacoast navigation, are for the exclusive purpose 
of issuing prompt notices, which shall be of immediate value to 
the farmer or other resident upon the lowlands and to the sailor. 
Coming next in the scheme of the study of the waters of the 
countiy is the work of the Geological Survey, which, taking the 
facts relating to precipitation and moisture given the Weather 
Bureau and utilizing the data as to river heights as far as possi- 
ble, expands these into a general stud\^ of the occurrence of water 
within the United States, tracing out the causes, especiall}’- those 
of topographic and geologic character, which lead to variations 
in distribution and fluctuations in supplv, and in short bringing 
together material by which the water resources of the country 
may be known as thoroughly as its mineral wealth. From the 
time, therefore, that the rain reaches the ground the Geological 
Surve}^ endeavors to trace its course on or below the surface and 
to ascertain the laws governing its circulation and its reai)pear- 
ance by seepage or through natural outlets in springs or in arti- 
ficial openings, such as artesian or other wells. 
This Surve}", as incidental to the preparation of the great map 
of the United States, examines in detail the surface of the country, 
determines the age and character of the rocks, their structure and 
position with relation to each other, their permeability or im- 
l)erviousness to water, and the probal)ilities of their l)eing able 
to _yicld a .suppl v at points not yet penetrated l^y the well-digger. 
As in all scientific work, the ultimate object is that of prediction, 
of revealing that Avhich is now unknown or but partly under- 
stood. Such extension of knowledge rests ui)on a thorough ex- 
amination and understanding of the history of the ]>ast and of 
the conditicjns in the present. Before questions can be answered 
as to what is the probable supply of water at this or that point, 
for ))Ower, f(jr irrigation, or for municipal supply, it is nece.ssary 
that long-continued and aceurat<‘ work be done. 
'I'he Work of the United States (Jeological Survey n'lating to 
water resources is carried on by the Division of 1 Ivdrograidiy. 
The field operations of this division consist of the nuaisurement 
at selected j)oints of the flowing waters of springs, erc(‘ks, and 
rivers, the estimation of the discliarge of artesian wells, and of 
the (piantities of water which can be obtained bv other metins. 
