CHAPTER XXIX. 229 



Stream observations have been maintained and expanded particu- 

 larly in the arid regions during the past five years. Discharge meas- 

 urements are made almost entirely with meters. These meters are 

 first rated so as to show the relation between the revolution of a wheel, 

 which is immersed in the water, and the velocity of the passing water. 

 By the determination of the velocity of a given stream, and its area 

 of cross-section, the volume of flow is obtained. A gauge rod is placed 

 at the point of measurement, which is called a rating station, and 

 the height of water on this gauge rod is always noted in connection 

 with the measurements. If the stream has a permanent channel, and! 

 observations are made at a number of different stages of river height, 

 the relation between the height of water on the gauge rod and the 

 volume discharged may be interpolated for intermediate river heights, 

 and corresponding volumes determined. By reading the rod daily, daily, 

 monthly and annual determinations of discharge may be obtained. 



The results of stream measurement are published in annual reports 

 by the Hydrographic Branch of the Geological Survey. There are also 

 issued "Water Supply and Irrigation Papers," the purpose of which is 

 to get quickly before the public the result of the year's work, pending 

 the final and complete publication of the annual report. These docu- 

 ments can be obtained by application of parties who are interested in 

 this subject to the director of the Geological Survey. The Twelfth, 

 Eighteenth, Nineteenth and Twentieth Annual Reports are of especial 

 interest in this connection. They describe methods and results In 

 detail. VARIATION IN STREAM FLOW. 



The relation of rainfall to run-off is very uncertain, depending upon 

 the nature of the storms, whether gentle showers or violent rains, the 

 steepness of the drainage basin and its covering, and whether the pre- 

 cpiitation is snow or rain. It has been found that in the districts where 

 the forpst cover is small the output of the basn occurs in violent floods 

 of short duration. Because these floods are violent, and of large vol- 

 ume, and owing to the fact that the soil of the drainage basins is not 

 held together by a network of roots, extensive erosions occur in these 

 barren basins, and the water carries much silt in suspension. Where 

 the basin is covered by forest the mat of twigs and leaves which covers 

 the ground is an absorbent sponge, retaining in itself large quantities 

 of water and preventing evaporation from the underlying soil. This 

 permits of a holding back of the floods and the gradual draining off 

 of the water, this largely accomplishing the purpose of regulating 

 reservoirs. 



