1905 



FORESTRY AND IRRIGATION 



55 



A report has just 

 "^ k een submitted to the 



Chief Engineer of the 

 United Reclamation Service by Chas. 

 S. Slichter, engineer in charge of in- 

 vestigation on the movement of un- 

 derground waters, upon the under- 

 flow of the Rio Grande near El Paso, 

 Texas, and Las Cruces, New Mexico. 



Above the city of El Paso the Rio 

 Grande passes through a narrow pass, 

 or gorge, which the river has cut out 

 of limestone rock. The river-bed is 

 frequently dry at this point, and it be- 

 came important to the growing com- 

 munity of El Paso and adjacent set- 

 tled portions of the Rio Grande valley 

 to know how much water was passing 

 underground through the gorge of the 

 river. The distance to bed rock at 

 the gorge is about 90 feet, measured 

 from the bed of the river. To this 

 depth the channel which the river cut 

 through the rock is filled with sand 

 and gravel, through which the so- 

 called "underflow" of the river moves. 

 This term is now used to describe the 

 ground water that is slowly moving 

 downstream in the sands and gravels 

 which lie beneath the bed of a river. 

 In some river valleys the underflow 

 constitutes a second and unseen river 

 of considerable importance. In many 

 cases the quantity of water that passes 

 downstream in the gravels is very 

 small, and consequently, of little value. 



A few years ago Mr. Slichter in- 

 vented and perfected for the United 

 States Geological Survey a practical 

 method of measuring the rate at which 

 these underflow water move. This 

 method has been put in use in many 

 parts of the country, with much suc- 

 cess, by means of which a great 

 amount of valuable information has 

 been collected concerning a matter 

 about which engineers were formerly 

 very much in the dark. The measure- 

 ments are made by means of electrical 

 instruments, the principles made use 

 of in the method being quite simple. 

 Small wells of two-inch pipe are first 

 driven into the sand, one well being 

 several feet downstream from the well 



first driven. Each well contains a few 

 feet of perforated strainer at the bot- 

 tom, and both wells are driven to the 

 same depth. These small wells are 

 very quickly driven and after use are 

 pulled up and used again. After the 

 wells are put in place, electrical con- 

 nection is made from a battery to the 

 wells, one pole of the battery being 

 connected to the upstream well and 

 the other pole of the battery being- 

 connected to the downstream well. The 

 electrical current will then flow from 

 the battery to the casing of one well 

 through the water in the sand or grav- 

 el to the casing of the other well, and 

 thence back to the battery. This cur- 

 rent is at first very weak, and by put- 

 ting a suitable instrument in circuit 

 the strength of the current is automat- 

 ically recorded on a chart or piece of 

 paper that is moved by clockwork. 

 When everything is in readiness, a 

 strong solution of sal ammoniac is 

 placed in the upstream well, which 

 mixes with the groundwater around 

 the strainer at the bottom of the well, 

 and passes downstream with the mov- 

 inggroundwater. This chemical when 

 placed in water renders the water a 

 good conductor of electricity, so that 

 as the solution passes with the ground- 

 water from the upstream to the down- 

 stream well, the electrical current con- 

 tinually increases and is recorded by 

 the instrument. When the salt water 

 finally reaches the downstream well 

 and enters it through the perforated 

 screen. A very simple device per- 

 mits the current to still further in- 

 crease, which shows on the record 

 chart of the instrument the exact time 

 at which the salt water arrived at the 

 downstream well, and, hence, the time 

 required for the underground water to 

 move the distance between the up- 

 stream and the downstream well. 



By means of these instruments, the 

 rate of movement of the underflow at 

 gorge of the Rio Grande was inves- 

 tigated. It was found by this means 

 that the groundwater moved only 3 

 feet in 24 hours. The amount of un- 

 derflow passing through the gorge was 



