114 



THE IRRIGATION AGE. 



very exceptional cases, the channels of 

 streams being usually covered with a coat- 

 ing of silt, which effectually prevents per- 

 colation to any appreciable extent. 



On watersheds of diversified topography 

 containing flats, wide canyons and broken 

 and irregular ridges running in different 

 directions, the opportunity for water to 

 sink is good, and a large proportion of 

 the water falling will enter the ground 

 and become a part of the underflow, if the 

 substratum below will sustain it down to 

 and along under the channel of the stream. 



Considerable importance attaches to the 

 question of topography, and in studying a 

 watershed respecting its ability to produce 

 an underflow, an examination, as detailed 

 as possible, should be made to ascertain 

 the opportunities afforded for water to 

 sink or run off. 



SUFFICIENT AND PROPERLY DISTRIBUTED PRE- 

 CIPITATION. 



The fall of rain or snow on a basin or 

 watershed must be sufficient and properly 

 distributed throughout the year in order 

 to create a good underflow from the drain- 

 age. If the showers are too sudden and 

 violent, and partake more of the nature of 

 a 'cloudburst of great discharge but short 

 duration, the water will have but little 

 opportunity for sinking into the ground. 

 Too light rains are also undesirable as they 

 either merely saturate the ground or the 

 water is evaporated or transpired by the 

 growing vegetation. 



If the rains occur at intervals too far 

 apart, the result is also unfavorable to un- 

 derflows, as the supply will be likely to 

 become low or fail at times. The best 

 rainfall for producing a large and unvary- 

 ing underflow in a stream is that which 

 occurs in moderate showers and without 

 the intervention of too long intervals. If 

 the year is divided into a wet and a dry 

 season, the lack of moisture in the form of 

 precipitation during the dry season may 

 be compensated for by snow which has 

 fallen during the wet season and gradually 

 melts during the dry season, thus keeping 

 up the supply. 



Snow is the best form of moisture for 

 keeping up a constant and unfailing sup- 

 ply of underflow under all circumstances. 

 The melting of snow requires time and 

 gives ample opportunity for the water to 

 sink and reach the underflow. 



PROPER CHARACTER OF CHANNEL AND SURFACE 

 FORMATION. 



In order to contain an underflow the 

 channel of a stream must have a stratum 

 of loose material through which the water 

 can percolate after reaching the lowest dip 

 in the bed rock or other impervious forma- 

 tion. If the bed of a stream is directly 

 on the impervious formation extending 

 throughout the watershed or basin, no 

 underflow is possible, as the water sinking 

 where this formation is covered by loose 

 material, will again rise and become a part 

 of the surface flow upon reaching the bed 

 of the stream. It is not necessary, how- 

 ever, that the loose stratum should be con- 

 tinuous from the surface of the channel 

 down to the bed rock or underlying im- 

 pervious stratum. The underflow is often 

 separated from the surface flow by one or 

 more impervious strata; in fact, such a con- 

 dition is more frequently met with than 

 one where the loose formation is contin- 

 uous. 



Such impervious strata are deposited by 

 the water and may consist of conglomerated 

 or cemented gravel, compact clay, hardpan 

 or other solid matter deposited by the 

 water and firmly united by means of some 

 component part causing adhesion. The 

 only condition required where such strata 

 exist is that they do not touch the imper- 

 vious formation underlying the whole 

 watershed, but are separated from it by 

 some loose stratum, through which the 

 underflow can move unimpeded. Cases 

 where the two touch are very rare, owing 

 to the fact that in all large drainage basins 

 heavy floods have made deep loose depos- 

 its before the subsequent layers of the im- 

 pervious material were deposited. In 

 cases where the loose stratum is contin- 

 uous and not interrupted by intervening 

 impervious strata the whole stream may 

 sink and become underflow, unless the 

 water carries silt sufficient to line the 

 channel, or the flow of the stream has a 

 sufficiently large volume and velocity to 

 pass on even after filling all the voids in 

 the material comprising its pervious bed. 

 The surface formation of the watershed 

 tributary to the stream must be loose and 

 porous and be underlaid with an im- 

 pervious stratum practically continuous to 

 the channel of the stream in order to cre- 

 ate any considerable amount of underflow. 



A hard barren watershed, which does not 



