14 



arid zone extends from the mountains to North Platte, Nebr. East 

 of North Platte some farms are irrigated, while others are not, the 

 proportion of irrigated to unirrigated farms gradually diminishing 

 to the east; that is, all the agricultural lands along the branches 

 of the Platte River in Colorado and Wyoming require irrigation. 

 Those in Nebraska, as far east as North Platte, are in the same con- 

 dition, while those east of North Platte have less need of it. 



SOUTH PLATTE AND TRIBUTARIES. 



The water supply of the South Platte Valley comes almost entirely 

 from streams fed by the snows and rains in the mountains, and is 

 largest in the early summer and smallest in the late summer. The 

 South Platte itself is a mountain stream, and receives also the dis- 

 charge of a number of other streams coming from the eastern slope 

 of the Rocky Mountains. The flow of these streams at stations near 

 the mountains where records have been kept is shown in the following 

 table : 



Mean iiiuiitlihj fioir of SoittJi PJattc Rirrr and trihiifaries. 



All these tributaries are located on the north and west side of the 

 river. There are in addition a number of torrential tril)utaries on 

 the south and east sides, which carry large volumes of water during 

 heavy storms. Below the mouth of the Cache la Poudre several tor- 

 rential streams come in on both sides of the river. These are shown 

 on the map (PI. I). These streams are of little value for irriga- 

 tion without storage, since the flow is so uncertain and lasts but a few 

 hours or days at a time; but Avith storage they become valuable 

 sources of irrigation water. Reservoirs have been built on some of 

 these streams and more will probably be built. 



There is considerable irrigation above the points of measurement 

 on the streams given in the table. There are small tributaries for 

 which no measurements are available, and the measurements for the 

 streams given cover different periods. The volume given for each 

 month for each stream is the average of whatever measurements are 



