80 W. & L. E. GURLEY, TROY, NEW YORK 



proportion of the total run-off from a basin occurs at times of 

 flood, a inn- index <>i the flood-flow must be obtained. Condi- 

 tions stated under 3 and 4 are similar, in that flood waters 

 furnish a large proportion of the total run-off. 



5. WHERE IT is NECESSARY TO DETERMINE THE MAXIMUM 



CAGE HEIGHT OR THE MAXIMUM DAILY MEAN GAGE HEIGH I 



The necessity for a record showing maximum gage heights 

 or the maximum daily mean gage height arises principally in 

 connection with the design of power dams and bridges. If a 

 stream is subject to frequent floods during the year and the 

 maximum gage height must be determined, a continuous hydro- 

 graph for 365 days is necessary. The peak of a flood can In- 

 ascertained by observation of the driftwood at the gaging 

 station but a record of its duration would not be available unless 

 constant attention was given by the gage reader. If the maxi- 

 mum daily mean gage height is desired, the hydrograph from 

 an automatic register is even more essential. The mean 

 obtained from a staff or chain gage read twice daily might 

 indicate a different day of maximum from that shown by auto- 

 matic recording register. The day of maximum gage height is 

 not always the day of maximum daily mean gage height. Two 

 daily readings might indicate several days during a year as 

 days of maximum mean gage height, within a small per cent of 

 each other, whereas an automatic register record would doubt- 

 less show entirely different results. The accuracy required 

 for such data would, of course, play an important part in the 

 selection of the type of gage, but for a record which would be 

 beyond question an automatic register is necessary. 



6. WHERE IT is NECESSARY TO DETERMINE THE MINIMUM 



GAGE HEIGHT OR THE MINIMUM DAILY MEAN GAGE HEIGHT. 



The necessity for a record of the minimum gage height or 

 minimum daily mean gage height occurs in connection with 

 water power and irrigation practice. The minimum flow of a 

 stream in amount and duration is one of the controlling factors 

 of a water power project, and the duration of the period of flow 

 should always be determined with accuracy. In some places 

 flood water may be stored to replenish the low flow; a continuous 

 record should, therefore, be obtained to determine the amount 

 of storage required. In irrigation projects the low-water flow 

 is not so essential unless the low-water period occurs in the 



