. Water Supply and Irrigation Papers of the United States Geological Survey . rding to Salbach such pollution will not take place unless thecurved water table actually intersects the bed of the stream, as shownby B B B B in fig. has sometimes followed the construction of infiltra- 76 MOTIONS OF UNDERGROUND WATERS. [NO. tion works of the types described, especially in regions of low rainfallor where the underflow had not previously been subjected to carefulsurvey. The underflow cut above Dodge, Kans., is now entirelyabandoned. The supply of water was disappointing and the rive
. Water Supply and Irrigation Papers of the United States Geological Survey . rding to Salbach such pollution will not take place unless thecurved water table actually intersects the bed of the stream, as shownby B B B B in fig. has sometimes followed the construction of infiltra- 76 MOTIONS OF UNDERGROUND WATERS. [NO. tion works of the types described, especially in regions of low rainfallor where the underflow had not previously been subjected to carefulsurvey. The underflow cut above Dodge, Kans., is now entirelyabandoned. The supply of water was disappointing and the riverfloods have filled the excavation, which was dredged at great expense,with silt and sand. Mr. Willard D. Johnson lias made important suggestions for theconstruction of infiltration galleries for gravity works, as shown inthe accompanying figures (41, 42, and 43). He would use one ormore batteries of tubular wells to reach the best portions of thewater-bearing medium, as shown by a careful survey made prior tothe construction of the works. The best flow of water is usually. Fig. 43.—Longitudinal section of infiltration galleries designed by Willard D. Johnson. found at a considerble depth below the water table, and Mr. John-sons plan offers a simple way of reaching it without expensive exca-vation. SUBSURFACE DAMS. Another method of recovering the underflow of a stream is by meansof a subsurface dam. Such a dam is constructed by excavating atrench at right angles to the direction of the underflow and extendingin depth to the impervious stratum, and then filling the trench withimpervious material. If the underflow is confined within an imper-vious trough or canyon, it is obvious that such a construction mustresult in bringing it to the surface. An example of this is foundon Pacoima Creek, Los Angeles County, Cal., where a subsurfacedam was constructed in 1887-1890. It is claimed that by means ofthis dam the owners have been enabled to use the bed-rock flow ofwater for the thre
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