File:Water Supply and Irrigation Papers of the United States Geological Survey (1902) (14586545377).jpg

Identifier: watersupplyirrig6571unit (find matches)
Title: Water Supply and Irrigation Papers of the United States Geological Survey
Year: 1902 (1900s)
Authors: United States Geological Survey
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Contributing Library: Clemson University Libraries
Digitizing Sponsor: LYRASIS Members and Sloan Foundation

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ly, the directionand the velocity of motion at various points in the pervious bed. Rain falls upon AB andescapes at AD, as in fig. 14. THE UNDERFLOW. The ground water after starting on its journey toward the rivervalley may not after all find its way immediately into the channel.Sometimes it takes a general course down the thalweg and toward thesea within the porous medium itself. This movement may be so greatas to constitute a large underground stream, scores of feet in depthand miles in breadth. The moving sheet of water beneath the bed and banks of a streamis the underflow, properly so called. This term is also extensivelyused in the West to designate the nearly stationary ground water ofthe Great Plains. It originated with those who formerly believedthat there was a great sheet of water beneath the surface of the Plains,originating in the melting snows of the Rocky Mountains and flowingrapidly through the subsoil toward Missouri River and the sea. WATER-SUPPLY PAPER NO. 67 PL. Ill
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MAP SHOWING THE DELTAS OR FANS OF DISAPPEARING STREAMS AS THEY LEAVE slkhtek) THE UNDERFLOW. 39 It is evident that a considerable underflow is impossible withmaterial as fine as is sometimes-found filling the valleys of rivers.Such material may play a very important part in the storage of water,but it can not bean important element in its transportation overgreat distances. An entirely different condition may exist where thesands and gravel beneath the stream are sufficient ly coarse, as is likelyto be the ease near the source of streams in the mountains. Here thematerial deposited by the stream is the coarser sands, gravel, andbowlders brought down by the mountain torrents. The running-water will not deposit the finer material until the stream bed reachesa gentle slope and the current has lost its high velocity. The periodicfloods sweeping down the mountain sides during the heavy rains soonfill the river canyons with coarse debris, on the surface of which theperennial stream has its

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